Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Wed Nov 05 2014, 09:56PM

look at my high voltage bench top HV supply.
I used one 555 timer with the "diode trick" to vary the duty cycle beyond the common 555 circuit. As you suspect, the out put varies as you drive it through a duty-cycle.


EDIT: Found it ! look through all 3 pages I learned a lot as I went forward.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
dexter, Wed Nov 05 2014, 10:05PM

varying the frequency at the first 555 will limit the flexibility of the second 555. aka at some point the ON duration of the monostable will either be to low or higher than the period of first 555 giving weird output signal

so why not the good old TL494 with the CTRL pin connected to ground and output transistors paralleled?
using a pot on the timing resistor and another on the input comparators will allow frequency and 0 to 95%
duty cycle selection

here's a example:



just chose R3 R4 R5 carefuly so that pin 1 and 16 see a voltage between 0.7 to 3.3V from the 5V reference output

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Wed Nov 05 2014, 10:09PM

Sigh, here we go with the TL494 again....

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 05 2014, 10:50PM

@Patrick: Can you please elaborate on the 'diode trick'? I had a look through all three pages, but couldn't find details of this.

@ Dexter: I did consider that, but how do you vary the frequency without varying the 'on' time? (I think it's important to be able to do this).

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Wed Nov 05 2014, 11:09PM

Your right, I thought I put the full schematic for my etched board up, but I must have forgot.

Anyway, look at the second one down here : this is the one I mostly use, and used on the planar thread recently.

this one ive used long ago, cant remember why :

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 05 2014, 11:58PM


Thanks for posting this Patrick. My browser won't open the second link, but the first link explains the 'diode trick'. I don't think it's precise enough for my plans, though.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sulaiman, Thu Nov 06 2014, 07:17AM

Ash, when driving a monostable 555 the output will be high;
1) until the threshold voltage (>= 2/3 Vcc) is reached by the timing capacitor
2) while the trigger pin is low (<= 1/3 Vcc)

so you need to either
3) have a very short LOW output from the astable = low value discharge resistor
or
4) a.c. couple the astable output to the monostable input
e.g. 1nF astable output to monostable trigger and 3k3 astable trigger to +Vcc
............................................. .................................................. ...............
EDIT: as pointed out below, the 3k3 goes from MONOSTABLE trigger input to +Vcc (brain fart!)
Thanks for spotting it Ash.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Steve Conner, Thu Nov 06 2014, 10:18AM

My Dee-lux interrupter circuit is an example of how to make independently adjustable pulse repetition rate and width using two 555s.


Jan Martis' "super flyback driver" is the best circuit I know of for driving a flyback in flyback mode.

Finally, remember that the flyback needs an air gap to work in flyback mode.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 06 2014, 11:35AM

@ Sulaiman: Thanks for that info. I'll look into this some more while waiting for the flyback to arrive.

@ Steve: I've just had a look at Jan's schematic and, from what I can tell from a 'first glance', it uses the UC3844 chip which (according to the datasheet ) only allows 50-70% deadtime, and therefore doesn't allow indepandant adjustment of 'on' time and frequency. Your 'Dee-Lux' interruptor circuit is more along the lines of what I have in mind. 'Fiddy's Flybacks' are shipped with an airgap, although I may well try varying it

I guess it's time to dig out a couple of NE555's and a suitable MOSFET and start on a schematic before the flyback arrives.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Thu Nov 06 2014, 08:30PM

I was able to get 10uS on 200us off, and you can change to mS if you like. totally stable just choose a good polyester or similar low leakage cap. I was able to vary it from 0.5% to 85 or 90% i think.

this was the principle time base for the planar circuit.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 06 2014, 09:03PM


but were you able to 'set' the on-time to, say 10uS, and then vary the frequency independantly (ie set the off-time independantly), without it changing the on-time?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 06 2014, 10:07PM

Sorry about the double post, but new information.

Sulaiman, I'm assuming you made a typo, and that this is what you meant. Tying the trigger of the astable to Vcc doesn't seem to make sense, to me anyway.





EDIT: For anyone who is trying to work out how this is 'supposed' to work, R and R1 are variable resistors, R determines the 'on' time, and R1 determines the frequency, which also determines the 'off' time. The output of the second 555 (monostable) drives a MOSFET (or similar switch), which drives the primary of the flyback. For more info, see the link to the 'NE555' datasheet above, or the Wikipedia page on the NE555.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
dexter, Fri Nov 07 2014, 08:19AM

one disadvantage of the double 555 circuit is that while it offers constant ON time is not capable to offer fully adjustable duty cycle for all frequencies

at a certain frequency the range of the duty cycle would be maximum
at lower frequencies the range of the duty cycle would drop linearly with frequency
at higher frequencies the on time of the monostable would exceed the astable period giving weird output signals, the higher the frequency the weird the output wave form

lat say the maximum ON time of the monostable is 1ms then the maximum frequency of the astable should not exceed 1kHz to avoid overlaps, and as the frequency goes lower the maximum duty cycle also go down, so at 1kHz he maximum possible duty cycle would be 100% and at 500Hz the maximum possible duty cycle would be 50% and so on...

if you really need constant ON time and don't care about the reduction of the max duty cycle then this circuit is perfect

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Daedronus, Fri Nov 07 2014, 10:18AM

tl494 ?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Fri Nov 07 2014, 11:50AM

You can use a LTC6990 and LTC6993. One is a wide range VCO, and the other is a variable one-shot. Complete independent control of frequency and pulse width. I've worked with the VCO version before, it works beautifully.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 07 2014, 02:02PM


Yes, I realise that you need to keep the 'on' time shorter than the switching frequency, etc.

The optimum 'on' time for any flyback is a function of 'volt seconds', regardless of frequency, and the optimum 'off' time is dependant on several factors (load, secondary inductance, etc). Once we know the characteristics of these flybacks it should be easy to use a TL494 or whatever to 'reproduce' the optimum 'on' and 'off' times. Ideally you want to 'almost' reach saturation during the 'on' time. There are different modes for driving flybacks (continuous mode, discontinuous mode), which complicates things still further.

It's obviously possible to drive these flybacks using any flyback driver (single transistor, 50% duty cycle, etc), but to get the optimum performance I think you need to control the 'on' and 'off' times independantly, at least until you know what these are, for any given 'volts per turn', etc.

At least, that's what I've learned about flybacks during the four years I've been using this forum

@Daedronus: I'm sure you could use the TL494, but I don't think you can vary the frequency without changing the 'on' time as well, because the duty cycle is a percentage of switching period. Once the optimum 'on' and 'off' times are known, for a given application, I'm sure these could be reproduced using a 494, although I think the duty cycle is only variable within certain parameters, if I remember correctly.

@Sigurther: Thanks for those suggestions, I'll have a look later, but first I'll have a look at the 556, which, as I'm sure you are aware, is two 555's on one chip.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
..., Fri Nov 07 2014, 07:23PM

If you really want precise control over the on times, etc, you might want to consider switching to a small microprocessor (ATtiny, etc), even with an attiny85 you get a pair of 8-bit timers and plenty of A/D to connect to the necessary knobs for controlling it. If you upgrade to an atmega (which you can get pre-programmed with the arduino bootloader for $4, or an entire arduino nano clone for $10) you get a 16bit timer that will blow away anything you can hobble together using 555's. It would also give a clear upgrade path for adding things like overcurrent protection, closed loop voltage/current mode, etc.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 07 2014, 08:08PM


Interesting. I'll keep that in mind for another prohect I'm working on which requires 100nS on times, and I may look into it later for this, but I think I'll go for a single SE556 for now, with two control potentiometers, one for frequency and one for pulse length.

I think it just needs two variable resistors, two other resistors and five capacitors, plus a MOSFET and maybe a snubber/TVS. I am trying to keep this as simple as possible for now, but I do intend to take this principle further in the not too distant future

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Erlend^SE, Fri Nov 07 2014, 10:41PM

I plan to do a CC-CV controlled HV supply using micro-controllers (mine will be DC).

Not quite decided on the topologies, but I will be using current sense and voltage sense on mine.

I will be using special transformers, but they are very different from what you have.

Ash: I would suggest you have a low-side current shunt to figure out the winding current, given you are not going for isolated.

Ash: For AC transformer, what would be a good voltage rating to get? specs: 100W, round, board mounted

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Electra, Fri Nov 07 2014, 10:57PM

For one of my wacky ideas again, what about instead of driving the fixed 'on' time timer, by a variable frequency source. Have a feed-back winding on the core into some level detector or comparator that changes state at the end of the flyback pulse, use this as the trigger to start the 'on' time again. Don't think you could use the 555, as if the trigger is held low it doesn't turn off, you'd need to find a monostable that the start pulse length was irrelevant.

What you'd end up with is a self oscillating system, that the frequency would depend on the load, and the setting of the 'on' time length. I guess it would try to operate on the boundary between dis-continuous and continuous mode. I.e Soon as all the energy had been transferred from the flyback pulse it would re-trigger itself. There would be no wasted dead time when the output was doing nothing. If the output was shorted the frequency would slow to a buzz, and screech away at Max voltage and frequency under light/no load. Well that's the concept at least.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 08 2014, 11:39AM

@Erland: Can you elaborate a bit more please, I'm not sure I completely follow?

@Electra: The 'on' time will be adjustable, but as you point out, at the point betweem continuous and discontinuous is where the optimum 'on' time becomes longest.

I think it is possible to use 555's with a feedback loop, as the frequencies of the 555's can also be controlled by the 'reference voltage' (within certain parameters) as well as the RC time period, I'm not planning on implementing any sort of feedback loop at the moment, I just want independant control of frequency and 'on' time for now, for initial evaluation purposes.

Most of the big companies do produce 'flyback controllers on a chip', with feedback, etc, but these are designed for 'step down' flybacks, not ones with HV output, so would require voltage dividers, but I suppose this could be achieved with a couple of capacitors with carefully chosen values, but would reqire a DC output, I suppose.

For now I'm just after a 'simple' circuit that allows independant control of frequency and 'on' time, and using a 556 does seem to be the simplest method (one IC, two potentiometers, two resistors and five capacitors, plus a MOSFET, and maybe a snubber or TVS).

In the past, I've only used either the 'single transistor flyback driver' or a fixed 50% duty cycle with adjustable frequency, both of which work, but neither are 'ideal'. This is just the 'next step' towards implementing feedback, etc., in my opinion.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Tue Nov 11 2014, 02:32PM

Well, this is the next step in getting the layout sorted, using an SE556. Next step is to work out some values for resistors and capacitors, and decide on a suitable MOSFET. I've not considered snubbing, or TVS's, or other overvoltage protection yet.

Any comments regarding this, or anything else about this circuit will be welcomed.






EDIT: I'm updating this as I make progress by editing this post, so it may be worth checking back here to see progress. Saves double posting this way.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
johnf, Tue Nov 11 2014, 06:55PM

Ash
Snubbing is a black art but its need is directly related to your leakage inducance (coupling factor between primary and secondary.
If you have an inductance meter you can get an idea by measuring primary inductance with secondary open circuit and short circuit the difference ratio is your coupling factor. do this again the other way around ie measure secondary inductance with primary open circuit and then short circuit.
The shorted inductances now help with calculating the snubber needed after you power up the circuit and measure the ringing period this then gives the distributed capacitance of the windings. Power Integrations gives good info on their Web site with most flybacks a relatively simple dual diode CR snubber works ok unless you want super efficiency where lossless snubbing will increase your transformer count

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Tue Nov 11 2014, 08:15PM


Thanks for that advice John. I have looked at snubbers a bit in the past when I've previously played with flybacks, but the only means I have of measuring inductance is by using a sig. gen., 'scope and 'known capacitance', and finding resonant frequency, then calculating inductance from there. I suppose if I use a large enough 'known capacitor' it will get me 'in the ballpark' (self capacitance will be insignificant compared to 'large known capacitance').

I'll come back to this later. I expect to blow a few MOSFET's when I fire it up, and maybe some 556's as well.

I don't intend to incorporate 'lossless snubbing' initially, although I might look into it later.

I'm currently doing some calculations regarding RC time constants for the 556 timing, and realising I'll probably need to add a facility to switch in/out a range of different value capacitors as well as using variable resistors in order to get the ranges I want for the astable/monostable timing (or at least leave room for this option in the future).

I've also realised that, depending on the MOSFET used, I may need a 'totem pole' or similar arrangment as well, but I've not yet got to the point of considering suitable MOSFETS.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Wed Nov 12 2014, 08:06AM

I haven't driven them in flyback mode yet, ive been driving some ignition coils in flyback mode with 24V on a IRFP450 with a 10k ohm resistor across the gate and source then driving the gate with a function generator on square wave 10Vpk it works great on ignition coils, especially on the beefy race car coils.

Its pretty much the same as a 556 driver, can use the pulse width and frequency pot on the function generator to drive at what ever you want.


Heres a video i did,

You can see the arc disapear almost when i turn the pulse width down and see the higher current arcs when i turn the pulse width up.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 12 2014, 01:21PM


There are significant differences between ign coils and these flybacks. As I understand it the ign coils are designed to run at much lower frequencies (I did calculate the frequency once for a V8 at high RPM). Also, the Ohmic resistance of the ign coil primary is generally much higher, as far as I'm aware.

That is exactly the same principle as I'm using here. There are two reasons why I'm using the 556 though. Firstly my sig. gen. doesn't have variable pulse width, and secondly, I wouldn't want to risk damaging my sig. gen. with HV 'spikes' getting back to it if anything blows.

I have used my sig. gen. to drive flybacks before, with all kinds of protection between the sig. gen. and the rest of the driver circuit, but only at 50% duty cycle.

The purpose of this thread, though, is to design a cheap, simple circuit that does exactly what your expensive signal generator does

EDIT: What's the purpose of the 10k resistor across the gate and source? I think I've seen something similar on full bridge circuits in the past, Should I include one here?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Wed Nov 12 2014, 02:34PM

[quote]
EDIT: What's the purpose of the 10k resistor across the gate and source? I think I've seen something similar on full bridge circuits in the past, Should I include one here?
[/quote1415802803]

The FET gate is a capacitor, the resistor allows for the charged capacitance to discharge, which turns off the FET. You do this whenever the FET drive circuitry cannot sink current as well as source it.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 12 2014, 03:28PM

[quote]
[quote]
EDIT: What's the purpose of the 10k resistor across the gate and source? I think I've seen something similar on full bridge circuits in the past, Should I include one here?
[/quote1415802803]

The FET gate is a capacitor, the resistor allows for the charged capacitance to discharge, which turns off the FET. You do this whenever the FET drive circuitry cannot sink current as well as source it.
[/quote1415806050]

I thought it was something like that. The 556 sinks as well as sources, so I don't need it

EDIT: And if I end up using a totem pole, I still won't need it.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Wed Nov 12 2014, 04:19PM



Yes lower frequency depending on what coil your using, the MSD ignition coils dont follow OEM firing systems, they fire the spark plug multiple times during the power stroke to get a full burn, what ive found is its around 3kHz.

The resistance of a ignition coil like this isnt high ohms at all, its low, it appears to be a dead short to a DMM. The difference here is the windings ratio, core laminations and the ignition coil shares a common ground for the primary and secondary windings.


Yeah thats it, my signal generator has missed a beat driving these coils, its good to see where the arcs get their hottest Vs the current the its pulling Vs to the pulse width, like most transformers you will get to a point where the transformers core will saturate and any more current added to the primary wont increase the current on the secondary coil.

Yup, id go the 556 setup too, although i dont think you will have the range and function of my signal generator :P

As Sigurthr said, the 10k resistor discharges the gates capacitance, without it, after turning the power off and disconnecting the Gate from your driver, the Source/Drain will still be conducting so if power is still across the Source/Drain it will allow current flow even tho there is nothing controlling the gate pin, this usually causes max currents to flow and will destroy the FET.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 12 2014, 05:13PM

That is very interesting. I didn't realise that the MSD coil fired multiple times. I'm working on a multi-spark system in the thread here: (proposed circuit diagram on page 5), butv I'm waiting for MOSFET's with sub 10nS switching times to become more affordable (I need twenty or so for each coil, and last time I checked they were £40 each). Part of the reason I'm building the 556 timer is to learn more about the whole concept.

The primary resistance will be much lower if it's a multi-spark coil. (I did wonder when I saw your video how you were managing to drive an ign coil at what looks like quite a high frequency for ign coils)

Maybe I'll have another look at the MSD coil. I did look it up earlier in my other thread after JohnF mentioned it, I think, but didn't realise it was a multi-spark system. As I understand it these multi-spark systems use capacitamce, and don't fire until the voltage rises to the breakdown voltage, after several pulses, then continue to supply more current at lower voltage during the discharge. I think I posted some waveforms earlier in the other thread.

There have been several multi-spark systems in the past, which claim to fire several pulses, but as the technology of MOSFET's etc gets faster, the ability to supply more pulses per 'ignition event' is increasing, although 'state of the art' components, as used in pulse generators of the so called 'voltage adder' type, discussed in a recent thread of Patrick's on planar transformers are still very expensive.

EDIT: Just noticed this bit:


I'll have to give this some thought, too.

EDIT 2: If power is shut off at Vcc, there is no power across source/drain, so the problem doesn't occur. It did occur to me to add a capacitor and diode, so that it can shut down, without the risk of further triggering, but I don't think this is necessary, as the circuit stands, but I've had a drink. I'll look again after coffee in the morning. As it currently stands, I don't think this problem will occur, though.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Thu Nov 13 2014, 03:41AM


Yeah multiple spark discharge, thats actually what MSD the brand name stands for

Im not sure how the MSD computers fire their coils as ive never had one before, would be interesting to find out.

Sorry i wrote that part after a 12 hour night shift! Of course if the Vcc is disconnected there obviously wont be any power across S/D but if its power is applied to the S/D at a later time and the gate hasnt fully self discharged it will conduct again.

Im not sure how it will go in your situation, trial and error eh?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 13 2014, 05:00AM

I may just add the resistor between G-S anyway, to be safe, and I'll look into adding a capacitor and diode in the morning to make sure it shuts down ok. I'm still drinking at 4:45 am, can't sleep

Most of the 'multiple spark discharge' systems I've seen only supply 'several' sparks. The system I'm working on supplies around a thousand in a millisecond, although, as I mentioned above, the capacitance and inductance tends to 'smooth' it into a high voltage to initiate breakdown, followed by less voltage and greater current to give a clean burn.

It will require MOSFET's with switching times under ten nS though, and they are currently too expensive for me to buy, although I'll experiment with slower ones to start with, and see where it goes.

The 555/556 is limited to ~10uS switching times, although I have linked to much faster processors in the other thread. This exersize is just to get a 'feel' for the concept, and hopefully to get the best out of your flybacks with a simple driver.

I should learn something from it, and learn more about the flyback topology in general.

The whole concept about energy 'stored' in an air gap is something that fescinates me.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Thu Nov 13 2014, 08:34AM

I have always thought it is the same phenomenon as when you are refraining two magnets to stick together or inversely

as you are fixing the mechanical position of the two physical bounds, the other parameter is the magnetic field

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Uspring, Thu Nov 13 2014, 11:44AM

The whole concept about energy 'stored' in an air gap is something that fescinates me.

The effect of the air gap is mostly to reduce primary inductance. For a given input voltage and on time this implies larger primary currents, which will store more energy in the primary. The same effect can be achieved without a gap, when you add a small inductance parallel to the primary. Most of the energy will then be stored there.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 13 2014, 03:14PM


So how does the air gap affect the volt seconds?

Does the volt seconds remain the same, but with increased current, or is there more to it than this?

Presumably, with less inductance, it will reach saturation sooner, assuming voltage remains constant?

It's in order to get a better 'feel' for this stuff that I'm building this circuit in the first place, so I can adjust things like air gap and voltage, and number of primary turns, etc., and see what happens.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Dr. Slack, Thu Nov 13 2014, 04:13PM

An analogy that I like, and it's an analogy that gets better the harder you work at it, is to ask 'where would you prefer to store spring energy? In a length of steel wire, or a length of rubber?'

Now at the moment, before cranking the maths up, you might object how long, what cross sectional area, like with like, but just go with me for a moment. Ultimately we can equate permeability, B and H field with Young's modulus, force and elongation (not necessarily in that order, or that way up, but you get where we're headed here).

If you take a steel wire and a rubber rope which both have the same breaking stress (force/area), which will store more energy at that strain (elongation/length)? You may have to strain the steel by 1% to get it to max stress, but the rubber needs maybe 1000% strain to get it up to the same stress. So the rubber rope stores 1000x the energy of the steel wire.

Similarly, you may only need a tiny H field to get a ferrite core up to a max B of 0.2T, but a large H field is needed to get a lump of air to the same max B. As energy is stress x strain, or B x H, the air stores much more energy getting it to the same B field.

Now core volt-seconds are related to the core area and maxB, similarly ropes in series with the same breaking stress will still have the same breaking stress (by tautology) whether they are made of rubber or steel. The core has a maxB and an area. The air-gap, assuming it's short and a large area defined by a gap in the core, has exactly the same area as the core, and has a Bmax defined by the core as well, even though the air can of course support a higher B field. So it doesn't matter whether there is an airgap or not, the volt-seconds remains (to first order) the same.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 13 2014, 06:41PM



Thanks for clarifying that bit, neil. To summarise, volt seconds remains the same, but current is greater with an airgap.

Now I've read in several references that volt seconds and amp seconds are essentially interchangable, which is where things start to get confusing. If volt seconds is a function of Bmax of the ferrite, and current is essentially proportional to the length of the air gap, as the air gap increases, the amp seconds must increase as well? Or are they only interchangable when no air gap is employed? Maybe I've answered my own question here

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Dr. Slack, Thu Nov 13 2014, 08:10PM

Amp.seconds has no place in transformer design or understanding. So you have been mis-informed.

Coulombs are useful if you are trying to charge a battery

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 13 2014, 09:23PM


Fair enough. I can't find where I read about it, and can't remember the exact context. If I find any reference again I'll post it. Maybe I misunderstood the context.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 13 2014, 10:58PM

This is where I've got to now, using 1/2 Watt variable resistors this circuit gives me plenty of range, certainly to be going on with.

I probably don't need this whole range, but it does make it a very useful circuit. My only concern is that this layout could add extra inductance at higher frequencies. The layout still isn't finalised. This is somewhere between a schematic and a finished layout, though.

Shortest pulse is around 20uS and lowest frequency is around 1Hz. Longest pulse is around 0.05S, which is far more than I need in any realistic scenario. Highest frequency is somewhere around 500kHz, which is also far higher than I'll need for this flyback.





EDIT: The flyback arrived this morning, and it looks like it's possible to get a primary underneath the secondary, rather than on the opposite leg, which is always the best place to put it. I'm now thinking of using a foil primary on the same leg as the secondary.

EDIT 2: I've started looking at MOSFET's now. First one I'm looking at is the IRF450, as it seems to be quite polular, and from first impressions looks to be acceptable, but any comments or suggestions will be welcome. Datasheet here:

EDIT 3: corrected 20nS to 20uS.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Fri Nov 14 2014, 01:39AM


Because of how voltage is reflected back at the primary, and thus the switch, and due to the high turns ratio of these transformers, I always advocate using a VERY beefy transistor.

My go-to FET for power switching is always the FDL100N50F. I've used these in everything from DC switching to multi-kW CW inverters in the MHz range. When the internal diode is blockaded and external schottky's are used they're essentially bulletproof, even when switching capacitive loads at the Vds rating. I can put up a datasheet if you need.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 02:06AM

Looks pretty amazing, Sig.

£10 each from RS, but it looks worth it.

Very low RDS and 100A.

Is there any particular external Shottky that you recommend?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Fri Nov 14 2014, 02:08AM

foil with paper or overhead projection film is a good choice for skin and proximity effect reasons.
copper foil is not found in my kitchen, and aluminum is hard/impossible to solder, so i dont know what youll do for connections. maybe crimps to copper wire?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 02:18AM


Maybe copper foil, or maybe experiment with crimped aluminium.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Fri Nov 14 2014, 07:55AM

if I recall well enough 3M sells some copper foil adhesive which could be used for this kind of thing

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Dr. Slack, Fri Nov 14 2014, 09:10AM

and aluminum is hard/impossible to solder,

nah! You just have to use the right flux. I bought some 'alli solder' from farnell two decades ago, and I'm only halfway through 1m of it. The trick is to heat the foil from the back, hotter than usual, and dab the multi-core on it so the flux gets heated by the foil. If you use the iron bit on the solder side, it sizzles the flux away while the foil is still cold and doesn't do its stuff. The flux is a bit nuclear in behaviour, so take the opportuntity to clean the foil at this point. Now solder the wire onto the tinned bit.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 11:03AM


You can solder it using an agressive flux, but you also need the right solder. tin-zinc solders are not good. tin-lead-silver is much better. The datasheet for the 'multicore' product available from Farnell is here:

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 12:33PM

Is this Shottky diode suitable for protecting the MOSFET?

600v and 114A pulsed @25C for 10mS repetitive.

Looks like it's two devices in one package, each rated for 57A pulsed, but I'm assuming it will share the load evenly between the two legs.

These are under £5 from RS.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Wolfram, Fri Nov 14 2014, 01:06PM


In a regular flyback topology circuit, the MOSFET reverse diode will never be forward-biased as far as I understand, so bypassing it with a schottky diode will not make any difference, aside from increasing switching losses slightly due to the increased drain-source capacitance.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 02:58PM


I'm glad you raised this point, Wolfram. I'm aware that this has been discussed here before.

My present opinion goes something like this: When the MOSFET switches off, current still tries to flow, resulting in a forward voltage spike, which, as you point out, will not forward bias the reverse diode.

However, due to the 'transformer effect' during switch off (or is it due to leakage inductance), there is a 'back EMF' which results in a spike in the opposite direction (I'm not completely sure about this, I'm currently reading up on all of this stuff), which WILL forward bias it. (Im not certain about these points)

I think my 'scope may be too slow for me to see exactly what happens when I've played with flybacks in the past, but due to the fact that there is definitely 'ringing', which I have seen and not been able to completely damp in the past, this suggests that the voltage across the MOSFET and reverse diode goes positive and negative.

The bottom line is that it can't do much harm, and 'may' do some good, although, as I said, I'm not certain about this either.

I'm working on snubbers and protection at the moment, and will update the schematic later with the ideas I've arrived at, although I do welcome input regarding this subject.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
hen918, Fri Nov 14 2014, 04:47PM

What you are referring to is the flyback topology: what gives flyback transformers their name (even though they are not actually transformers, but coupled inductors) and this is what gives them their unusual characteristics.
This is where the ringing comes from: when the MOSFET switches off, collapsing the magnetic field in the flyback transformer, the voltage reverses and causes a high voltage spike which, in a boost converter (which is very similar but with the "primary" and "secondary" as one winding), would be tapped off with a diode. Stray capacitance creates a little LC circuit that continues the high frequency sine wave until it dies away due to stray resistance.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 14 2014, 05:15PM


Yep, I'm familiar with the 'coupled inductor' bit, and with the 'stray capacitance' and 'leakage inductance' bit, but it also acts as a 'conventional transformer' while the MOSFET' is 'switching off', as both primary and secondary are conducting during this period.

During this, the 'turns ratio' comes into play, inducing a voltage in the primary that is equal to the secondary voltage/turns ratio. Once current starts flowing in the secondary, the voltage drops, but current can't start flowing instantaneously, due to the inductance of the secondary.

I think this is the 'back EMF' bit, and that this bit would 'forward bias' the reverse diode, but I'm not sure about this.

EDIT: I think there are three different factors involved here, firstly, the primary wants to continue conducting, creating a forward voltage spike, then there in the 'stray capacitance-leakage inductance' bit, then there is the 'transformer action' creating a 'back EMF', but I'm not certain.

I do know you get ringing and voltage spikes, which, I think, means you get both positive and negative spikes, but I don't think my 'scope is fast enough to ascertain exactly what's going on.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Sat Nov 15 2014, 12:54AM

My scope is too slow as well to really SEE the reflected spikes, but I have run tests with and without body-diode-isolation, and the fet always runs cooler with isolation. Usually I can spare the wasted power from a schottky, so I just add it in anyway now as default. Remember it takes TWO diodes to blockade a body-diode. One forward biased in series with the DS junction, and one antiparallel to the combined DS-Schottky series string.

I will say that the reflected spikes can be surprisingly bad in some configurations. I had been working on a flyback driver circuit for a friend a few months back and spent hours blowing fets (not the FDLs though, cheap 100V ones) as I expected about 65V reflected. Instead I was seeing (on my slow ass 40MHz CRO scope!) spikes over 390V which were causing DS avalanche and massive heating. The culprit turned out to be that the core had a chip missing from it where the air gap spacer should have been, resulting in a HUGE gap (compared to as normal). I sanded the core down to be flush on both halves and reassembled with a new gap spacer and the reflected spikes settled back down to the <75V I was expecting, and the heating went away. It ran happy for hundreds of hours after that.

For Schottky diodes I typically use MUR1560s, because they're inexpensive and I have tons. I haven't had any projects where >15A continuous is exceeded, and I've used them for ~22A pulsed, so if you want more current capacity you'll have to worry about balancing parallel diodes and the inherent flaws, or find a beefier schottky.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 15 2014, 01:44AM

Sig, can you elaborate on this bit please:


I pretty much follow/agree with the rest, but I'm not clear about the 'antiparallel' thing.

Thanks for the input.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Sat Nov 15 2014, 01:49AM


Not sure where it was coined, but I picked the term up. It means "reverse biased, and in parallel to". I.e. Anode to Source, Cathode to Anode of the series diode.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 15 2014, 02:24AM

It was the 'it takes two diodes' bit that I don't follow.

I can put a Shottky in parallel with the 'freewheeling diode' that forms part of the MOSFET, but where does the second diode go?

Or have I misunderstood something?

EDIT: I agree with adding the Shottky, and I agree that it can make the MOSFET run cooler, but I don't understand the bit about a second diode, unless I've misunderstood something.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Sat Nov 15 2014, 04:02AM

The losses in the body diode aren't mostly from conduction, but from charge accumulation (and charge neutralization), so simply putting a schottky in parallel doesn't really prevent the body diode junction from charging as there will be a voltage developed across the schottky which will also be seen by the body diode. You have to totally isolate the body diode by preventing it from seeing that reverse voltage at all. See snipped schematic:




The image is from my Half Bridge schematic, but you can see the two-diode-to-one-fet arrangement.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Andy, Sat Nov 15 2014, 06:27AM

What about 1 analog to digital and 1 digital to analog resistor network, with a resistor/ cap time delay. The first ADC reads the input voltage, and triggers a mosfet to open, when more voltage in, more voltage is sent to the R/C, the R/C has a fixed 3.3 hex inverter(10-15v) that powers the DAC, which adjust the the ADC by mosfet/resistor voltage divider.

To get it to work, you increase the voltage with something, which adjust the on time, you send a control signal to the DAC to control the off time.

Hope it makes sense

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 15 2014, 12:07PM


Yep, that makes sense now, the second Shottky goes in series with the MOSFET. Thanks for posting.

@Andy, I think you'll need to post a schematic for me to follow.

EDIT: I've had a look at the MUR1560 datasheet and, while they are pretty fast, instict suggests that Shottkys may be better here (lower forward voltage drop and instantaneous reverse recovery), although the MUR1560 is a lot cheaper than the Shottkys I was previously looking at.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 15 2014, 02:57PM

Ok, I've added a snubber circuit and Shottky diode protection to protect the MOSFET from spikes, etc. (in red)





Not sure if I need to add a capacitor from Vcc to GND, to smooth out anything that gets back into the driver circuit, though.

I think I can do this without risk of ringing between the snubber capacitor and the inductance of the 'smoothing capacitor' (the two Shottkys, one in the snubber and the other in anti-parallel with the MOSFET should block this). Maybe it's not needed, but I think I'll leave room for it, plus some room for anything else I decide to try later.

I'm not sure what values to use for the snubber capacitor and resistor, I think this will be 'trial and error' (experimental procedure), although the capacitor needs to be low ESR and low inductance. I'm guessing something like 0.47uF maybe a good starting point, but just guessing.

If I start with a very short 'on' time, I can gradually increase 'on' time, and see what is happening, as far as ringing is concerned.

I suppose next step is to design the board layout, etc., even though a few components haven't been finalised yet.

As usual, any comments/suggestions will be welcomed, and thanks to everyone who has contributed thus far.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Sat Nov 15 2014, 03:50PM

You may want to add a gate resistor to avoid surge currents, remember that the 556 isn't a super low output impedance device. Capacitor rate of change formula dictates peak currents drawn by the fet gate for charging and discharging.

You'll need some kind of filter capacitance to decouple the power supply, no question about it. I'd start with 100uF minimum. It increases with frequency but nearly all 555 circuits need this for a clean output waveform.

And yeah I agree about the schottkys you found, the 1560s are just cheaper, poorer substitutes.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 15 2014, 04:27PM


Yes, this did occur to me. Gate charge for that MOSFET is 238nC@10V. I'm planning on running whe whole thing at 12V initially, presumably gate charge will be under 300nC. Output current for the 556 is +/-225mA. V/I=R, which works out at 53.34 Ohms. So presumably something like 60-100 Ohms wiil be in the ballpark?


I suspected this could well be the case. Again, I assume low ESR and low inductance is important, so maybe a few in parallel, scattered about at different points between Vcc and GND?

EDIT: I'll presumably need these between pins 4-7, 10-7 and 14-7 for optimum results, with others elsewhere, maybe?

Also, this setup is supposed to decrease turn off time, which is generally what's required with flyback circuits:





Taken from here:

I'm now wondering whether the above setup or this one would be best for this application:





The application note linked to above states that when driving the gate directly, with just a gate drive resistor between the driver IC and the MOSFET gate, that Shottkys are requiredbetween the output pin and Vcc and GND, so I assume these are also required for the top circuit here (PNP switch off). It says they aren't required for the totem pole setup.

EDIT: Unless this is what is referred to as 'does not cause ground bounce problems'.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Sat Nov 15 2014, 11:51PM

A single large electrolytic on the board and then a single smaller low ESR low inductance cap near the chip is all that is needed, it's basic IC decoupling. I usually use 100uF on board and then 0.1uF at chip.

It's been a few years since I've had the 555 committed to memory so forgive my not commenting on which pins might need caps on them for what. I pretty much moved away from the 555 when I found that I needed independant PRF and PW across a wide range.

A totem pole works fine of course but you must remember that the gate won't see the full voltage swing because of the bipolar voltage drops. If feeding with 12V you'll probably still get >9.5V at the gate though. Honestly I don't think it is needed in this case, you're not pushing near-MHz into the gate, but scoping will tell you for sure.

What I've done in the past is two NPN pull down transistor stages for current/voltage amplification. You still don't get the full voltage swing, but you only get one bipolar voltage drop, plus whatever voltage drop the pull-up resistor causes at the current of your load. You need two stages because a single one inverts the signal (useful in its own right). It's simply a 50 ohm resistor from Vcc to Collector, Emitter to Gnd, signal in to Base. Take output signal from Collector. Feed output into next stage's Base. The second stage's resistor is sized to determine max available current to the end load, and of course the ratings of the chosen NPN. Again, the output is taken from the Collector. I do this when using logic chips that can only source a few mA.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sun Nov 16 2014, 12:21AM


Thanks for the imput, Sig. I like the look of the PNP turn off thingy. Fast switch off is something most people say is important for 'flyback topology', so this may be worth employing, although, from memory, PNP devices have greater forward voltage drops. I think I'll leave room on the board for it, it might be interesting to experiment. The 'two NPN system' has some disadvantages, according to the 'app. note' I posted, but I might try it, I'l certainly make sure I leave enough space on the board.

I'm not sure about electrolytics here, I think ceramic or MPK, or whatever, might be better. I have loads, so if I sprinkle a few about I don't see it doing any harm, as long as they 'add up to' ~100uF, or so.

I'll give these points some thought, and post again in the morning.

Once again, thanks for the input.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sulaiman, Sun Nov 16 2014, 07:55AM

I have built a few 555 based flyback drivers, some of my mental notes;

If you have no output current sensing/limiting/control
then for a given supply voltage/frequency/duty the input POWER to the coil is constant
when there is no load ... where does this power go to?
(answer: usually it goes to heating or over-volting/destroying the switching transistor)

I've seen IGBTs intended for flyback that have continuous avalanche ratings but I don't rely on that.

I would not use an RCD snubber as it absorbs/wastes energy on every cycle
but gives negligible over-voltage protection to the switching transistor,
the most robust design that I made without output current control
had a circuit like the snubber shown above, BUT
instead of a resistor I used 1.5KE350CP TVS diodes,
three parallel strings of three series TVS for 45W 1050V.
This wasted little power when there was an output load
and clamped the flyback well to protect the transistor.
Although rated at 5W each the TVSs got too hot for my liking at this power level.

The simplest output current sensing is to put a resistor between the low voltage end of the output winding, and 0v.
For a negative output eht you get a positive voltage on the sense resistor
which can turn on an npn to stop output cycles,
or feed the 1v threshold current limit pin of a UCxxxx type smpsu ICs.
You could use a current transformer but I haven't tried yet so I don't know how easy it is.

(I have 10x HR6549 35kV flyback transformers and 5x UC2825 smpsu ICs that have been waiting a very long time for me to use them )

Turn-on speed of the switching transistor is not critical but you definitely want really quick turn-off,
my prefered scheme is a resistor from +Vcc to gate for turn ON
and an npn from gate to 0v for turn OFF,
but a 555 with a 10R resistor from pin3 to gate is usually sufficient.

Sometimes when switching transistors are killed they send current from the HVDC through the gate to the driver electroics causing massive damage.
For 555 type drivers I put;
A 15V TVS and large electrolytic across 0V and Vcc
A fast 1A diode fom 0V to pin3 (cathode to pin3)
and one fast rectifier from pin3 to +Vcc (anode to pin3)
A resistor from pin3 to switching transistor gate (e.g. 10R 1W)
so far, no dead drivers.

Personally I don't think that Fiddy's transformers are suited to flyback operation
because the primary and secondary are on separate limbs of the ferrite
and there are airgaps, so there will be significant leakage inductance.
I expect that Fiddy's transformers would be excellent for CFPR/zvs/royer use
as the leakage inductance gives some short-circuit output protection.

In general, try to avoid having your windings too close to the airgap
as the fringing/leakage magnetic flux DOES cause the wires to overheat in this area.



I've posted most of this info. in various posts before, but I thought it worth reiterating here whilst you all are experimenting, appologies if this post is too long-winded/boring.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sun Nov 16 2014, 03:20PM


Yep, I follow this. If the energy can't get out through the secondary, it get's out through the primary.


I'll consider adding TVS's as well as the resistor. I think I need to look into this some more. I won't be operating this without a 'safety gap', at least.


Not sure I completely follow this bit. Any chance of elaborating?


I may be interested if you are considering parting with any of them


I'd need an 'inverting NPN' as well with the 555/556. I am considering this, though. I think I need to read up on characteristics of NPN and PNP.


The App. Note linked to above says to do this when driving the main switch directly from the 'output' pin. It actually says to use Shottkys.

By 'switching transistor' do you mean the main MOSFET, or a BJT switching it?


I'm pretty certain there is room for a foil primary under the secondary on these flybacks, which should circumvent both of these issues (well, it should help with cooling a bit, and reduce leakage inductance). I've read in the past that you want the windings over the airgap. I have one flyback that has three gaps in the leg with the windings on.


I appreciate the input, Sulaiman.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sulaiman, Sun Nov 16 2014, 11:44PM

If you put a 10k resistor between the 555 RESET (pin4) and +Vcc (pin8),
the 555 will operate normally with pin4 held high by the resistor.

If you put a small npn (or n-channel mosfet) transistor, emitter to 0v, collector to pin4
when the npn transistor turns on it will pull down pin4 and RESET the 555
,,, pin3 will go low, removing drive from the main switching transistor (mosfet or igbt)
... pin7 will go low and discharge the timing capacitor.
....................................... .................................................. ............................................
IF you connect the inner wire (less insulated, near the core) of the secondary to 0v,
and use the outer (hot/eht insulated) wire for negative eht
and arrange your primary winding connections so that when the switching transistor is ON
the output hot wire will go positive to about vcc x (Nsec/Npri)
until the core saturates, or the switching transistor is turned OFF (basis for a forward converter)
When the switching transistor is turned OFF then you get a positive flyback voltage on the primary
and a negative eht flyback on the hot secondary wire, Vsec = Vpri x(Nsec/Npri) still applies.
When there is a load on the eht secondary flyback pulse (e.g. arc, spark gap, multiplier etc.)
current will flow into the hot wire, through the secondary winding back to Vcc/ 0v
............................................... .................................................. ....................................
If you now put a resistor in the link between 0v and the secondary inner wire (e.g. 100R)
a negative eht output current of say 10mA peak would give +1v across the (100R) resistor.
Most of the UCxxx type smpsu ICs have a current feedback pin that switches at +1v
so connecting the 1v across the (100R) sense resistor to the current sense pin of a UCxxxx
(via say 1k in series to handle occasional extremes, and a cap to 0V to take out 'spikes')
puts it into current-limiting mode (and you don't damage your eht transformer).
OR
a resistor from the 1v could be connected (via say 1k ) to the base of the npn mentioned above
and limit the 555 output power when the eht secondary current is too high.

P.S. what is the easiest to learn / use electronics drawing software? .. free of course

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 17 2014, 12:51AM

I think I get the idea, I'll re-read it later.


I just use any old cad package (doesn't need to be 3D or have rendering or anything), just 2D CAD, draw my own symbols (sometimes just 'copy and paste' symbols from Google, if I can't be bothered to draw them. The beauty of CAD is you only ever need to draw things once, then 'copy and paste'. The CAD package I use came with a full 'symbol library', but I prefer to draw my own.

Most simple CAD packages will save a drawing (or part of it) in several different formats, but the 20 year old one I find easiest to use doesn't do .jpg, so again I just use 'print screen' to copy to the clipboard, and paste into 'Paint', then 'clean it up' (once I've saved it, the next bit doesn't work before you save it) using the little blue buttons that 'Paint' has at the left hand side and bottom of the screen, then use 'rotate 180' to do the top and right hand side. It takes a lot longer to type this than to actually do. (you don't need to use Paint, you probably have other image editing software that saves as 'jpg so you can 'upload' here)

You only need a really basic CAD package for circuit diagrams. and the only things you need to learn really, if you've never used CAD is how to turn the 'grid' on and off, and how to use the 'snap' command. Everything else is completely straightforward. Once you either draw your own symbol library, or use the library that comes with it, it's just a matter of 'copy and paste'.

Some of the schematics I've posted above aren't completely 'clean', not all the lines are dead straight, for example, but that's due to constant 'editing', and not always taking the time to clean everything up and align everything, especially when I just want to post 'work in progress'.

You can just 'copy' symbols from the internet, either using 'print screen' or 'save as', then 'clean them up' using 'Paint', and then put the lines in connecting them together using 'paint', but I find the old CAD program is more flexible, although even the '556' and 'mosfet' symbols I used above were just 'grabbed' from Google images, and cleaned up a bit using 'paint', then 'copied and pasted' into the CAD environment. Whatever's most expedient at the time.

The other CAD functions I use a lot are 'create group' and 'explode', when drawing the symbols from scratch.

Then there are things like 'line width', which can help with clarity, etc. Setting the 'paper size' to A4 and using a 5mm grid spacing and 1mm line thickness usually works for most things.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Mon Nov 17 2014, 03:27AM

If you only need a Schematic tool I recommend ExpressPCB, which has ExpressSCH as part of it. It's totally free, easy, and intuitive to use.

I think on Fiddy's transformers both secondary ends are equally insulated and equidistant from the core. It's an AC flyback designed without a primary under the secondary.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Mon Nov 17 2014, 08:01AM

I use eagle cad for the boards i make, it has a schematic drawing part to it

this is what eagle cad schematic looks like:

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 17 2014, 08:48AM

I really like the look of this setup, it's so simple, and gives a REALLY fast turn off because the PNP bipolar transistor never saturates, but I do have a few questions about it.





Firstly, most of the diagrams in the App note it came from are pretty much complete, so I have to assume that this is, however, pretty much all the PNP datasheets I've looked at say max voltage between Emitter and Base can't exceed five volts, yet most MOSFET data sheets say they need more than this, so the question is this:

As it is, assuming the output from the 556 is 12V, when the MOSFET is on, Veb will be equal to the diode forward voltage drop, but when the MOSFET turns off, at first impression, it looks like Veb is 12V. Is this actually the case, OR, once the PNP BJT starts to conduct, does the voltage on the Gate of the MOSFET drop to an acceptable voltage OR due to the fact that there is only ~250nC of charge on the Gate, does this mean it doesn't matter OR is there some other explanation?

I've been up all night puzzling over this, but can find very little information on the internet regarding PNP 'switch off' transistors.

EDIT: Does the Gate 'mesh' resistance play a part here, and keep Veb under 5V? (I'm just guessing here)

EDIT: the other advantage of this setup is it reduces the current flowing in and out of the 555 by half.

Another advantage is it eliminates 'ground bounce', whatever that is

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Dr. Dark Current, Mon Nov 17 2014, 09:45AM

At the frequencies of interest (few tens of kHz), the 555/556 has more than enough current to drive a powerful MOSFET. No additional circuitry needed.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 17 2014, 11:31AM


But the sbove setup gives faster 'turn off', according to the app note I posted above, as the MOSFET doesn't need to discharge through the gate resistor, and everything I've read says you want as fast a turn off as possible for 'flyback topology'.

It's also the same 'parts count' as just discharging through the gate resistor, because that entails adding two Shottkys, one from GND to 'out' and one from 'out' to Vcc, to protect the output transistors in the 555.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Steve Conner, Mon Nov 17 2014, 12:37PM

The local turnoff circuit doesn't protect the 555 output transistors in any way, so if you needed protection diodes before, you would still need them.

Veb of the turnoff transistor can never be exceeded. In the forward direction, it will clamp the voltage itself by conducting through its own base-emitter junction. In the reverse direction, the diode conducts and clamps Veb to 0.7V.

Too fast turnoff can be as bad as too slow sometimes. Search the archives for a thread called "Goldilocks Gate Drive"

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 17 2014, 01:23PM


The application note linked to above states that the local turn off circuit eliminates 'ground bounce', which, according to Wikipedia is

"In electronic engineering, ground bounce is a phenomenon associated with transistor switching where the gate voltage can appear to be less than the local ground potential, causing the unstable operation of a logic gate.

Ground bounce is usually seen on high density VLSI where insufficient precautions have been taken to supply a logic gate with a sufficiently low resistance connection (or sufficiently high capacitance) to ground. In this phenomenon, when the gate is turned on, enough current flows through the emitter-collector circuit that the silicon in the immediate vicinity of the emitter is pulled high, sometimes by several volts, thus raising the local ground, as perceived by the transistor, to a value significantly above true ground. Relative to this local ground, the BASE voltage can go negative, thus shutting off the transistor. As the excess local charge dissipates, the transistor turns back on, possibly causing a repeat of the phenomenon, sometimes up to a half-dozen bounces."

Which appears to be, from what I can make out, the reason why the extra protection is required in other setups, eg just using a gate resistor, but I will re-read it.

I'm no expert, though, hence the reason for the question.

Thanks for the other info. I'll look it up.

I have been doing a lot of Googling about gate drive systems. I do want to 'keep it simple' though, but leave room on the board for possible modification in the future.

EDIT: The discharge rate is still limited by the Gate 'mesh' resistance. It's this that defines the 'switching time' in the first place, along with the gate capacitance (RC time constant).

EDIT: This Fairchild App note on 'Ground Bounce' states that

" In order to change the output from a HIGH to a LOW, current
must flow to discharge the load capacitance. This current,
as it changes, causes a voltage to be generated
across the inductances in the circuit. The formula for the
voltage across an inductor is V = L • (ΔI/Δt). This induced
voltage creates what is known as ground bounce."



As the current doesn't flow back to the driver chip, but goes to ground instead via the PNP BJT, the above circuit supposedly eliminates it. I'm still reading the full article, though.

EDIT: The 'Fairchild App note' also says this

"Although this discussion is limited to ground bounce generated
during HIGH-to-LOW transitions, it should be noted
that the ground bounce is also generated during LOW-to-
HIGH transitions. This ground bounce is created by the
large gate capacitances associated with the output transistors
on the die. Because these gate capacitances are
larger than the gate capacitances of earlier-stage transistors,
more current is generated when they switch. The output
buffer stages of CMOS devices are inverters; thus their
inputs are switching HIGH-to-LOW when their outputs are
switching LOW-to-HIGH. It is the currents associated with
switching these inputs to the output transistors that generate
ground bounce when the outputs switch LOW-to-HIGH.
This LOW-to-HIGH ground bounce has a much smaller
amplitude and therefore does not present the same concern."

The 'TI' App note from which the 'local PNP turnoff' diagram was taken suggests using Shottkys, one from GND to 'out' and one fron 'out' to Vcc to deal with ground bounce, and implies that they aren't needed for the 'local PNP turnoff' setup, although it still recommends using capacitors.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Patrick, Mon Nov 17 2014, 10:19PM

Is this the only reason, or one of several for why we see capacitor infantry lined up near CPUs ?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 17 2014, 10:24PM


There are other reasons for the capacitors. I've been reading up on DIY reflowi soldering ( google 'Skillet reflow'), and looking at SMD's, but I'm not sure I need the Schottkys that the TI App note recommends for 'ground bounce'.

EDIT: just to qualify the above statement, if using the 'local PNP switch off' setup, which is supposed to eliminate ground bounce, the schottkys may not be required, but for other setups, eg just using an output resistor or totem pole then the it would be advisable to use the shottkys.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Tue Nov 18 2014, 08:20AM

I guess that those schottkys are inside the silicon for most IO now, and capacitors taking much more space, they haven't been able to integrate them yet

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Tue Nov 18 2014, 04:51PM

There's a lot more to it than miniaturization. A lot of it is balancing high frequency impedances with parasitics.

For the task at hand I still think it isn't really needed to have secondary active turn off switching. If you're that concerned just run the 555 on 15v and use a BJT totem pole to switch the fet gate. You'll get fast turn off and still have 10v on the gate. I'm working on debugging a buddy's board now that uses this setup and was hard switching a power fet at 2.2MHz. The drive section didn't break a sweat.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 19 2014, 08:26PM

I've been giving the board layout quite a bit of thought, and, as I mentioned above, it really does seem that using as many SMD components as possible is by far the best way to reduce parasitic inductance, as this facilitates the shortest distances between other conponents, thus keeping wire and track lengths as short as possible.

Having read this article, along with the comments below it, it would appear that 'reflow soldering' isn't beyond my ability, so the current plan is to use Maplin stripbpard, with SMD components mounted on the side with the copper strips, and all the non SMD components mounted on the reverse side of the board.

Then I can mount the SMD components in one go, and then flip the board over and mount the non-SMD components, soldering these by hand.

It could take a while to sort out the board layout especially as I haven't finalised all the components yet. The other thing I still need to give some thought to is the foil primary. My initial thought on this are aluminium foil with paper insulation and crimped connections, with twisted wires from the board to the flyback to reduce inductance.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Thu Nov 20 2014, 10:25AM

what about a custom lamination tool to flatten classic copper wire?

I guess it would be relatively easy to construct a set of successive steel rollers which would be geared to a motor... no?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 20 2014, 12:25PM


Copper foil is easy enough to obtain. The simplest 'custom tool to flatten wire' is a hammer and anvil

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Thu Nov 20 2014, 12:45PM

yeah, but I meant "in a reliable, repeatable and consistent way" ;)

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 22 2014, 07:55PM

Well, I'e started thinking about board layout, and come up with the sketch below:





(This is 'looking down onto the 556', with pin 1 at top left, although the SMD components are on the reverse of the board. It's just drawn this way, to show the relative positions of everything.)

The IC socket and MOSFET (not shown) are on one side of the board, the SMD components are on the other, with the copper strips.

Blue for SMD ceramic capacitors, red for thick film SMD resistors and green for SMD Shottky diodes.

There are currently ten x 10uF ceramic capacitors between Vcc and GND, equalling 100uF right at the 556. I've gone with the concensus and used Shottkys for ground bounce protection, with just a gate resistor between the OUT and GATE pins.

I've not added the potentiometers or switched capacitors yet and I've not added the components in red on the schematic.

I've included the schematic in the sketch, so it's easier to follow.

Once again, any comments/suggestions will be welcome

EDIT: I've just noticed I've left a capacitor off pin 11, and maybe I should add capacitors to pins 4 and 10, with one side connected to Vcc and the other connected to GND.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sulaiman, Sun Nov 23 2014, 01:55PM

Although schottky diodes are excellent, high voltage high current units aren't cheap,
at the frequencies used for flybacks you can use UF400x, UF540x etc. (x = 8 for 1kV)

I still think that a bunch of TVSs is better than a resistor for the snubber
(try TVSs for MkII)

Where does the +Vcc come from?
if you are using transfromer/bridge rectifier/reservoir capacitor/regulator
you could take the +ve supply for the transformer primary from the reservoir capacitor
this allows a simple low power regulator (7812 etc.) for the 555/556
and max power available to the primary.
Higher primary dc supply usually gives better efficiency
due to 'lost' voltages across transistor, diodes etc. and lower current for a given power.

Last but not least, be sure to use a traditional 555/556 and not the low power CMOS types,
that often do not have the required current output.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sun Nov 23 2014, 03:46PM


Do you mean the MOSFET protection Shottkys or the 'ground bounce' protection Shottkys on the output of the 556?

I understood that the ground bounce Shottkys don't need to be anywhere near 1kV. I was actually planning to use 600V devices for the MOSFET protection.


I probably will add some TVS's, I've not given the snubber circuit a lot of thought yet.


Initially I'll run everything from 12V, although it will be fairly simple to increase coil voltage at a later date. As I understand it, the 'reflected voltage' is a product of 'turns ratio' and output voltage, so I should be able to increase primary voltage without needing higher voltage components, this should just result in shorter 'on' times, in order to maintain the same 'volt seconds'.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Erlend^SE, Sun Nov 23 2014, 07:08PM

Quick idea: add a extra winding to return the excess power to the supply. (winding + diode).

Have less windings than the primary, so it's still possible to build a bigger flyback voltage.
Non-delivered energy is returned to the supply, for use on the next pulse)

It may able be possible to have a winding to make the controller cut back the duty when it starts to generate more than a given voltage (feedback)

None of this apply, if a SMPS controller is used, given it's used with secondary feedback.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sun Nov 23 2014, 07:17PM


Yes, I've seen this idea before, thanks for reminding me about it. It's certainly something I intend to experiment with once it's up and running.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Nov 24 2014, 08:33PM

Ok, so this is the board layout for all the components on the schematic except those in red.

SMD capacitors are blue, SMD Schottky diodes are green and SMD thick film resistors are red.

Potentiometers are 22 turn cermet that I happened to have lying around. The switches in the schematic have become 'jumpers' in the board layout, as these take up less space (I think) and are cheap.

I'm currently working on the layout of the MOSFET and schottky diodes, all of which need separate heatsinks, I think. (EDIT: I think one Schottky diode can go on the same heatsink as the MOSFET, so three heatsinks in all, I suppose.)

I'll then look at snubber placement.

The number of smoothing capacitors has increased to 18 at the moment, which is 180uF. I expect it will end up with more. I'm just adding them where it's convenient, can't do any harm, can it?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Tue Nov 25 2014, 08:32AM

no ground plane ?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Tue Nov 25 2014, 01:21PM


I'm going to try to deal with the 'ground plane' issue once I've got the component layout sorted. I imagine loads of de-coupling capacitors sprinkled all over the place will help here, but my current thoughts are to have two 'ground planes' connected by a thin trace. One for the 556 and associated components, and one for the MOSFET and associated components.

The routing of the trace from the 'output' of the 556 to the MOSFET is the bit I'm currently working on, and the layout of the heatsinks.

Is it worth putting a resistor in the trace that connects the two ground planes? This would allow them to both remain at the same potential, but might improve stability at the 556?

I will be adding plenty more capacitors around the MOSFET, etc. A bag of 100 SMD ceramic capacitors costs next to nothing

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Tue Nov 25 2014, 03:01PM

I'd use a small choke to connect both ground planes, it's the way I have seen it done in many designs

if you don't want a ground plane yet, you can still place your PCB over a blank copper clad board which would be grounded

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Wed Nov 26 2014, 03:20PM

Before you go into adding bells and whistles of layout, I'd work on getting to a stable prototype. Simply working on a grounded metal surface is usually good enough up until the 500KHz level from what I've seen first hand, and even then some things will do ok even up to ~3MHz. It depends on a lot of factors. Still, there's no reason to break your back on layout until you know whether the schematic you're using is even working.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Wed Nov 26 2014, 03:45PM


It's just the layout of the heatsinks for the MOSFET and three associated Schottky diodes that I'm working on at the moment.

I need at least three heatsinks, unless I use insulating pads, which I don't really want to do.

The bigger the heatsinks, the further apart the components are. Using smaller heatsinks presumably means using fans as well.

This does warrant a bit of thought at least

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Wed Nov 26 2014, 04:58PM

I was referring to the ground plane discussion going on =).

Yes, component layout is of course important to consider for the process of getting a working prototype. I suggest a single large heatsink and using either 1mil kapton tape (ala Steve Ward) or even high grade sil-pads (as I tend to do). Yes, there is quite a difference in thermal performance, but if the electronics and design are solid then getting a an acceptable stable thermal state shouldn't be too much of an issue.

To reference an appropriate build; my 1.5kW CW SSTC is an entirely hard switched half bridge with all bridge components on a single 4" x 4" x 1/4" aluminium heatsink that has 3/4" x 1/8" fins on about 60% of the radiating surface. It runs continuous duty with only a ~25CFM fan blowing over it. I used relatively inexpensive Sil-Pads for it too.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Thu Nov 27 2014, 04:54PM


Well, I've given this some thought, and the optimum setup, with the four components requiring heatsinks to be closest together on the board, and for the maximum amount of heatsinking, is to use four heatsinks, two on each side of the board.

I'm trying to keep this as simple as possible, so I may want to compromise here, and either use three heatsinks mounted reasonably close together on one side, or one heatsink, with insulating pads for the Schottky diodes, and mount the MOSFET directly onto the heatsink, as this will probably run hotter than the Schottkys.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 28 2014, 01:19PM

I've given this quite a bit of thought, and decided the best way forward from here is to not put everything on one board, but go for a 'modular' setup, with the driver on one board, and the power components on a separate, stacked, board.

Apart frm aiding swapping bits in and out, it also provides the shortest distance from the 'output' pin and gate resistor to the gate of the MOSFET, assuming these are directly above and below each other once the boards are stacked.

The sketch below, showing two heatsinks and fans, is what convinced me that a modular, stacked approach makes a lot more sense than just using one board for everything.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
hen918, Fri Nov 28 2014, 05:58PM

You will need a low pass filter/smoother/snubber between the power components and the driver. I made that mistake last year - had a load of SMPS boards made; ended up having to cut the power trace to the driver and then bridge it with a 100uH inductor. I also put a little electrolytic on the driver side to form an LC low pass filter. Works now: I was getting basically random noise coming from the H-Bridge before!

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 28 2014, 06:36PM


So the capacitor is in series with the inductor?

What value capacitor should I start with, assuming I use a 100uF inductor?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Shrad, Fri Nov 28 2014, 06:42PM

you want the shortest current path between switches, diodes and caps, and no loops, so use star or "T" topology in the switching zone for power

ground planes are cheap and spare your etch solution, and you'll eventually need to make more than one or two prototypes

switching ground plane and command ground plane can be separate and connected via a wire passing through a ferrite bead, or choke

this is theory, as you'll easily understand ;)

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Nov 28 2014, 08:45PM


Thanks for the pointers, Shrad.

I'm going to divide this up into three boards now, driver, switch and Schottkys, and snubber.

At the moment it's looking like the connections to the primary will be on the 'snubber' board.

The 'driver' board and the 'snubber' board will be ~ half the size of the 'switch' board, and will be mounted above the 'switch' board. The heatsinks will be mounted on the underside of the 'switch' board, along with the MOSFET and Schottkys.

I will switch to 3D CAD once I finalise the layout of each board.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
hen918, Sat Nov 29 2014, 06:11PM


No, the cap is between the load side of the inductor and ground. I used a 33uf capactior, but the bigger the more smoothed, as I'm sure you'll guess! Because the load drawn by the driver circuitry isn't massive I only used a small one.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Sat Nov 29 2014, 07:31PM


Ok, I already have plenty of capacitance between driver +ve rails and ground for precisely this reason, so I just need to add inductors/ferrite beads to the feed to the driver board, and between the driver groundplane and switch groundplane?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
hen918, Sun Nov 30 2014, 10:59AM


Yep, that's it

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Rod-on, Thu Dec 18 2014, 11:29PM

any updates on this?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Dec 19 2014, 10:58AM


Should be soon, just got a bit side tracked with helicopter blades and laminar flow, and going to collect a transformer today for another project. Should see some more progress with this over Christmas, though

Re: Driving 'Fiddy's Flybacks' in flyback mode.
dexter, Fri Dec 26 2014, 12:59PM


a bit unrelated but this circuit has any usefulness or even works with bipolar gate signal from a GDT?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Dec 26 2014, 02:44PM


I think the idea of this circuit is that it doesn't discharge through the driver chip, thus speeding up 'switch off'.

I'm not sure if there is any advantage to be gained using this with a GDT, and I think a GDT would saturate due to DC component, unless maybe if it was gapped, but the main advantage of this circuit (without GDT) is that discharge isn't limitued by driver chip current limit.

I do intend to try this, and I'm leaving provision to add it later.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Dr. Dark Current, Fri Dec 26 2014, 03:05PM

This circuit, with an additional diode in series with the input and a resistor from the PNP transistor base to ground, is very commonly used for gate driving using a GDT driven in "single ended" mode (duty<50%). The advantage is that you can drive the GDT primary using just a single transistor.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Fri Dec 26 2014, 11:45PM


Do you mean something like this?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
dexter, Sat Dec 27 2014, 03:52PM

i think is something like this:



The advantage come from the fact that it requires only one transistor to drive the GDT primary and it provides faster local turn OFF for the IGBT.
Also i'm not entirely sure where is the best place to put the 4R7 resistor - as is shown in the picture or right after the GDT (red arrows)?

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Ash Small, Mon Dec 29 2014, 05:00PM


I've studied this for a day or so, and I think the gate resistor should go on the output of the GDT. This ensures a faster turn off. The original schematic also has the PNP switch off transistor connected directly to the gate of the MOSFET.

The gate does have a 'mesh resistance'.

EDIT: Do I need to use a core with a distributed gap to deal with a DC component, or is this something that I don't need to worry about?

EDIT: I still haven't fully understood the two diodes and resistors.....

EDIT: I think I've worked it out now, there is a 'return path' for the current through the GDT secondary, through the resistors (1k and 470R), so I don't need to worry about a DC component, I think.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Thu Jan 15 2015, 06:06AM

Finally got around to building the Single Ended driver I designed for Fiddy and testing his AC flyback on it; works great!

13.8V Audio test:
6turns on primary
1cm spark gap
Roughly 4A current draw at 93KHz, which is just below resonance.
Best audio around 41KHz, ~3A draw.

Sorry for the crap video, it's late and I spent my evening putting the board through its paces.


Second video;


Inquire to Fiddy about buying this Driver kit off him!

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Thu Jan 15 2015, 11:05PM

Some nice vids from Sigurthur on this board getting some impressive audio arcs:

Re: Driving 'Fiddy's Flybacks' in flyback mode.
GrantX, Fri Jan 16 2015, 05:14AM


That's a fantastic little driver circuit, well done guys. Are there any circuit details available online (I understand you probably don't want to release the whole schematic)? Looks like a fun kit, I'd be interested in getting one to play with.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Fri Jan 16 2015, 06:01AM

Nothing online at the moment, since Fiddy wanted to sell these as kits I kept the development closed source. 'Tis his ball to throw as he sees fit. I might put out another revision too, but he has a decent stock of the displayed version on hand. I can say it is single ended, variable input voltage from 12-26V, with headphone level audio input - single channel mono, and low modulation distortion at the mid and upper range of the tuning band (below 30KHz you're gonna get distortion, simple physics). Future revision would include onboard stereo to mono conversion, and easy end-user modifiable frequency range adjustment for if your flyback needs higher than 90KHz.

What's funny is that I designed this circuit years ago, and literally had it sitting on a shelf covered in dust when he asked me if I knew of such a circuit or could design one, haha. There's something poetic in being too poor to produce your own designs. I'm just glad I found such a stand up fellow to take them to market for me. He even puts my name on the PCBs, haha! XD

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Fri Jan 16 2015, 07:25AM


Thanks dude I can sell you this driver in kit form with the board and components for $60 AUD and $10 express post being in Australia already! PM if your interested. Can supply a DC flyback too if required.



Hehe yeah, we did a lot of design work before the boards was even manufactured I had 10 boards made.

Hats off to Sig who has the brains to design the circuit, disregarding the LED resistors, theres only 1 resistor on the board in the circuit! Amazing

Re: Driving 'Fiddy's Flybacks' in flyback mode.
GrantX, Tue Jan 20 2015, 12:47PM

PM sent.

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Mon Mar 02 2015, 05:33AM

So! I've finally had some time to get around to my long list of things to do, and I whipped up another one of my driver circuits to show off Fiddy's transformer and driver.

One of my home made electrodeless tubes; ~55Torr air.


13.8V input @ ~6A.


High Pressure Argon, made by Ron Soyland

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Fiddy, Mon Mar 02 2015, 09:12AM

very nice, I love the engineered piece of metal to hold the fan down

Re: Driving 'Fiddy's Flybacks' in flyback mode.
Sigurthr, Mon Mar 02 2015, 01:20PM

Haha, 50g cast Bismuth! I'm an EE, not a ME!

Re: Driving 'Fiddy's Flybacks' in flyback mode.
paris, Thu Sept 15 2016, 11:14PM

I know old thread but I was wondering if there is an update on this Fiddy flyback mode circuit ???

Is the circuit discontinuous mode ??