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Snubber network? Answered

Although my flyback transformer driver appears to work very well up to 18V operation, the arc length is not satisfactory. I increased the voltage to 25V which allows 2 to 3 inch stable / hot arcs. However my 250V 18A rated MOSFETs appear to fail periodically esp. When adjusting the potentiometers (for adjustments to drive frequency and duty cycle)

I Tried sticking a 1N4007 reverse biased diode across but I don't think it changed anything. I still see >300v positive voltage transients. I also tried using a neon lamp without a ballast resistor, but it did not appear to change the waveform, and glows way too brightly. I tried using 3 (really old but giant) 30V zener diodes in series but they latched up, failed, and went up in smoke lol! So clearly these HV transients have a lot of "oomph" behind them. D: 

I am reading an article on maxim electronics page about snubbers https://www.maximintegrated.com/en/app-notes/index.mvp/id/848 and I'll experament with some of the circuits presented there but I can't really calculate values needed for the components because I have not characterized my transformer with custom windings and a loose ferrite core. Those things will be subject to change anyways since I do not plan on supplying a specific flyback transformer with this kit. So I want something that is more universally going to work.

This high voltage is just  PITA to deal with. How can I reduce it to 200v without hurting performance?

Discussions

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Downunder35m

Best Answer 3 years ago

If you have a flyback with integrated diodes you might fail with all "good" solutions.
From my experiments it really helped to use proper sizes choke coils.
Main problem IMHO is the simple fact that during the switching the breakdown voltage from the flyback has to go somewhere, which can be problematic at high frequncies.
But with added choke coils you can offer "a path of least resistance".
They also act like a little storage unit giving most of the energy back in the opposite switching.
Check my induction heater circuit for the pure basics, I am quite confident it should help to keep your mosfets alive for longer.

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-max-Downunder35m

Answer 3 years ago

The difference between your induction heater and this flyback driver is that you are using some sort of modified ZVS royer oscillator driver while I am operating in the (completely different) flyback mode of operation where I store energy in the magnetic core and that energy is delivered to the secondary while the transistor is off. The flyback mode of operation is a really strange mode, and relies on a small gap between the 2 halfs of my transformer. I don't know what happens if I eliminate that gap entirely.

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Reading more about it on that link I gave, it appears that parasitic (leakage) inductance is the culprit, and that is determined by the way the primary is wound, and how tight the windings are together.

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Yes, if that energy is not delivered in the output coil, it "backfires" and comes back on the primary side. Which is what I need to prevent. I know from experimentation if I eliminate the back EMF completely the performance is severely diminished.

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Downunder35m-max-

Answer 3 years ago

Did you consider to use litz wire on the primary?

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-max-Downunder35m

Answer 3 years ago

Actually, 40KHz is only the fundamental. Assuming that the highest frequency harmonic is 1MHz, then 40AWG wire is needed, that is pretty thin! I would need a lot of it to make reasonable litz wire.

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-max-Downunder35m

Answer 3 years ago

I don't think that will make a huge difference, I'll calculate the skin depth and see if 10 strands of 30AWG wire will work.

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-max-

3 years ago

TVS diodes seem to be the magical device that I need. I read that they take basically no time at all to avalanche (in the picoseconds) and I guess they also have really fast recovery and stop conducting as soon as the transient condition passes. Would these be an effective choice? Will bombarding one with 300v transients 40 thousand times a second cause it to dissipate too much heat? I don't know how much current it takes to clamp that to 200v.

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-max--max-

Answer 3 years ago

I don't fully understand TVS diodes. Why are there so many voltage ratings, working voltage, breakdown voltage, and clamping voltage? I am guessing that the diode appears like a high impedance load at it's working voltage and below, but starts to conduct current at the breakdown voltage, and really turns on hard (low impedance and really low dynamic resistance) at the "clamping" voltage. Is this correct?

Also, what is the practical difference between avalanche and zener breakdown? (I don't really care what is happening at the silicon level, just the important things like reaction time and the IV curves.