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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
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.
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sigurthr wrote ...
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.
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?
Sigurthr wrote ...
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.
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'.
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sigurthr wrote ...
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.
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.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
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.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sulaiman wrote ...
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)
Yep, I follow this. If the energy can't get out through the secondary, it get's out through the primary.
Sulaiman wrote ...
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.
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.
Sulaiman wrote ...
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.
Not sure I completely follow this bit. Any chance of elaborating?
Sulaiman wrote ...
(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 )
I may be interested if you are considering parting with any of them
Sulaiman wrote ...
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.
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.
Sulaiman wrote ...
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.
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?
Sulaiman wrote ...
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'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.
Sulaiman wrote ...
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.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
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
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I think I get the idea, I'll re-read it later.
Sulaiman wrote ...
P.S. what is the easiest to learn / use electronics drawing software? .. free of course
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.
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.
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Driving 'Fiddy's Flybacks' in flyback mode.
