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Snubber on push-pull converter

jbertrand

New Member
Hello,
I'm building a high voltage push-pull converter running at 100 kHz and I have some questions about snubbers.

Part of schematic

Transformer was build with following characteristics:
- inductance : 250 mH;
- induction : 0.2 T;
- primary Cu losses : 0.2 W;
- secondary Cu losses : 0.3 W;
- Fe losses : 0.4W.

Transformer contains two primaries :
- T-/TN ;
- T+/TN.

TN is around 28Vdc and current in primary can reach 12A.

On secondary side, I only have installed a four high voltage diodes redressor (with CLC filter).

CMD1 et CMD2 are generated by a dedicated subcircuit that runs es expected. Signal on gates are also as expected (very clean fronts between 0.2V and 14.5V). I see some ringing on MOSFET drains. Thus, I have added two snubbers (R109/C74 and R110/C75). Whith these snubbers, signals on both drains are correct but snubbers consume a lot of power (42W !).

I am quite surprised as application design show (when they deal with snubbers) very little value for C (100 to 330 pF). In my case, I measured 68 nF ! Other applications designs only show a unique snubber between T+ and T-.

I suppose I have done a mistake somewhere. Help will be welcome.

Best regards,

JB
 
Your basic premises are a little out of whack, trying to generate nearly 900V at 100kHz is not a good idea unless the power circuit is resonant and the Vout from the transformer is a sine wave - you have a push-pull power circuit feeding a cap input filter - also not a good idea - if you are able to look at the waveforms across the sec side diodes - you will see enormous spikes - try slowing the fet turn on ( 68 ohm R ) and making the turn off 33 ohm ( reverse diode and R across the 68 ohm ) this will make things a little better ( less snubbing required ) - but I doubt you will ever get to your 250 watts. You have not stated the Tx leakage ?? this makes a huge difference to snubber power - oh yes - you should really have well thought out snubbers on your output diodes !!!
 

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