Author Topic: Flyback feedback winding weirdness  (Read 464 times)

Offline TMaxElectronics

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Flyback feedback winding weirdness
« on: May 31, 2021, 02:23:31 AM »
I'm currently trying to build an isolated supply for some IGBT gate drivers; 24V in, 18V & 5V out, regulated via a feedback winding. But I've run into a problem...
Somehow the voltage on the feedback winding stops being proportional to the output voltage once the secondary is loaded. Looking at the scope trace there is some odd-looking overshoot on the voltage coming out of the feedback winding that isn't present on the secondary (both measured before the diode). After some pulses that seems to charge the feedback cap way over the expected voltage.

Here's a scope shot of what I'm talking about (didn't have a USB stick handy...):


Channels:
Ch1 (yellow): Mosfet drain
Ch2 (cyan):   Mosfet gate
Ch3 (purple): Transformer Secondary
Ch4 (blue):    Transformer Feedback winding

Has anybody got a clue what could be causing this? I guess leakage inductances would be the most obvious answer but I can't see how that would cause such an overshoot at turn on. The transformer is self-wound and has quite some separation between feedback and secondary... could that be causing this?

Offline klugesmith

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Re: Flyback feedback winding weirdness
« Reply #1 on: May 31, 2021, 05:33:13 PM »
Would you mind sharing a schematic, so we can see how your four (?) windings and diodes are configured?   Don't commercial flyback-based power supplies usually use an optoisolator, from a rectified and filtered output voltage?

You mentioned "before the diode" views of secondary and feedback winding.  What are the "after diode" loads?   So we can get an idea of the currents in all windings, and know when each diode is forward biased.   
« Last Edit: May 31, 2021, 07:44:57 PM by Mads Barnkob »

Offline Mads Barnkob

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Re: Flyback feedback winding weirdness
« Reply #2 on: May 31, 2021, 07:48:21 PM »
It looks like you are charging something up with a inductive kick, that then avalanches after 400ns and then has a ringdown at 6 MHz. From the frequency it sounds like parasitic components.
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Offline davekni

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Re: Flyback feedback winding weirdness
« Reply #3 on: May 31, 2021, 10:42:33 PM »
I'm presuming this is isolated flyback (boost-converter) topology.  Regulation is possible using the primary winding voltage or a separate feedback winding as you show.  The issue is parasitic inductance (non-unity coupling factor) of the transformer.  The versions I've built mask that initial voltage spike, sampling the feedback voltage during the second half of the high (primary switch off) state.

Another option is to hand-wind the transformer, with feedback coil first, then secondary coils, then primary (or visa-versa).  With the primary and feedback isolated from each other by the secondary winding(s), primary leakage inductance will have little coupling with feedback leakage inductance.  That minimizes the spike.  Still may require at least a resistor in series with the feedback diode to filter out the remaining short spike.
David Knierim

Offline TMaxElectronics

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Re: Flyback feedback winding weirdness
« Reply #4 on: May 31, 2021, 11:04:15 PM »
Sorry I completely forgot... here's the schematic (crudely hand-draw though :) )
Oh and I'm not using a bjt, it was just easier to draw than a fet lol



Quote
It looks like you are charging something up with a inductive kick
One thing I could think of would be stray fields between the primary and the feedback winding that don't go around the secondary. Then when those fields collapse they can't be compensated by the current through the secondary and induce that little bit of extra voltage on the feedback winding. I'll try to wind another transformer with the windings wound the way dave recommended tomorrow.

Quote
The versions I've built mask that initial voltage spike, sampling the feedback voltage during the second half of the high (primary switch off) state.
I saw that the flyback regulator ICs do that too using a sample and hold circuit. I was trying to use just a normal PWM regulator that I had laying around, that can't do that though :P
I think I might try using a microcontroller for the regulation if the new transformer misbehaves as badly again, they can use the PWM as the ADC sample trigger and have a 1MSPS adc.

Offline TMaxElectronics

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Re: Flyback feedback winding weirdness
« Reply #5 on: June 02, 2021, 01:08:01 AM »
Ok so the feedback is acting a lot more than I'd expect it to now that I've wound it Primary - 18V Secondary - 5V Secondary - Feedback. Regulation works accurate to 1V from no load to full load (which I consider perfectly fine for a gate driver), and that drop I could even compensate for in the microcontroller. The 5V winding is kind of useless because of the poor regulation though (seems to be a little worse than on the 18V one because I didn't wind the feedback over it), but I'll just put a buck regulator on the secondary for the finished driver i think.

Next up is the actual gate driving part, and because why not it will have a psoc on it so it can do all sorts of measuring, limiting, protecting, .... Might even port my cli over to it ;D
Thanks for the help :D

Offline davekni

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Re: Flyback feedback winding weirdness
« Reply #6 on: June 02, 2021, 04:31:45 AM »
Great to hear that it is working better!  Regulation may get a little better with a small resistor in series with the feedback diode.  Make the R*C time constant (with the capacitor filtering the feedback) short enough to charge during the flyback high time, but long enough to at least somewhat reject the initial short spike.
David Knierim

Offline johnf

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Re: Flyback feedback winding weirdness
« Reply #7 on: June 02, 2021, 10:27:50 AM »
Normally I would split the primary into two windings half each start with half primary and end with the other half with all the other windings in between.

This will reduce the leakage inductance to its lowest value. Your waveforms are a classic leakage inductance problem.

You can check your leakage inductance by measuring each windings inductance then doing it again with all the other windings shorted.
 the difference should be a ratio of about 95:1 or better ie 98:1 --this is a measure of the transformers coupling ratio --all up to build--i've seen same spec transformers made by differing people go 96 % coupling to 98.5% guess which one had the lowest switching artifacts??

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Re: Flyback feedback winding weirdness
« Reply #7 on: June 02, 2021, 10:27:50 AM »

 


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