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Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
@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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
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
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
johnf wrote ...
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
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.
Registered Member #8817
Joined: Mon Dec 17 2012, 05:16AM
Location: Australia
Posts: 110
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Fiddy wrote ...
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.
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?
[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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
[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.
Registered Member #8817
Joined: Mon Dec 17 2012, 05:16AM
Location: Australia
Posts: 110
Ash Small wrote ...
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?
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.
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Driving 'Fiddy's Flybacks' in flyback mode.
