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SMPS control loop estability
SMPS control loop estability
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Old 10th October 2005, 07:14 AM   #41
Pierre is offline Pierre  France
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Thanks, Eva. You are always very helpful (and seem to wake up very soon as me :-(
Well, I think that going to 80KHz shouln't be very bad for the mosfets as soon as I have the gate drive setup correctly. If you recall, I started with Coilcraft SD250-4 but the waveform was not too good. You told me it was saturating and the slopes were too slow, that'w why I temporary moved to 40KHz.
Let's see what happens with my new Pulse GDT sample.

I also added a 3n3 / 30 ohm snubber, and the resistor gets quite hooot. It is very near the transformer, so perhaps it is heating it up a little bit too.

Pierre
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Old 10th October 2005, 06:02 PM   #42
Pierre is offline Pierre  France
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I have done some testing with the Pulse GDT.
It is driven by a SG3525 buffered by NPN/PNP (BD139/40) transistors on both outputs.
The outputs of the transformer are loaded with 15 ohm (gate resistor) in series with 2.2nF (simulating a mosfet gate capacitance).

Rise time is around 200nS, too slow for my taste:

What do you think of the waveform? (vertical scale is 5v/div)
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Old 10th October 2005, 06:03 PM   #43
Pierre is offline Pierre  France
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... and this is with a 1k resistor in parallel with the gate capacitor:
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Old 10th October 2005, 07:40 PM   #44
Eva is offline Eva  Spain
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They look right. This bump at the beginning is due to leakage inductance resonating a bit with gate capacitance. It confused me at first, but in these pictures it's very clear that the transformer is not saturating since the waveform ends being perfectly flat.

You should consider additional gate buffering, though. You can even use the pulse transformer to create floating auxiliary 15V supplies for each MOSFET, so that you can buffer turn-on and turn-off. I found this to be the only reliable way when I was experimenting with the now outdated IRFP460 MOSFETs, that suffered from huge capacitances.
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Old 11th October 2005, 06:21 AM   #45
Pierre is offline Pierre  France
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Then I will move that transformer to my SMPS. Let's see if I can get it to work at 80-100KHz with the same losses as I have now in the mosfets, but less in the transformer.

I think it should be ok with my current mosfets, IRFB11N50.
Thanks for your help and sharing your (huge) experience.

Best regards,
Pierre
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