Background: I am trying to make a push pull dc-dc converter from 12vdc to 400vdc for an inverter with a center tapped transformer connected to battery of 12vdc. The MOSFET circuit is as follows

enter image description here

As can be seen in the figure, there is severe ringing in the drain waveform of the two MOSFETS. The upper blue waveshape is the drain wave, while the yellow is the gate waveform. What could be the cause, and what could I do to get a clean wave shape on the drains.

I have added a non-dissipative clamp, like so Push Pull with Non Dissipative clamp

Then I got these wave shapes, on both sides, with blue being the gate signal: Drain 1

Now my question is, If I add a passive RCD snubber between the drain and source of the mOSFETs, will that be sufficient to get a good 'square' wave shape on the secondary??

  • \$\begingroup\$ There is a similar question here: electronics.stackexchange.com/questions/80751/… \$\endgroup\$ – diverger Nov 15 '14 at 10:11
  • \$\begingroup\$ The reason I thought of asking this was because I saw that the other question shows a clean wave, I mean, the drain wave is flat for the most part, except the small period after the MOSFET shuts off, which could be removed by using a snubber. In my circuit, the entire top of the drain pulse has this ringing instead. What causes that? And how do I remove it? \$\endgroup\$ – dynamag Nov 15 '14 at 11:58

This is absolutely normal for this type of design. Leakage inductance in the transformer interacts with mosfet drain capacitance and when the mosfet open circuits there is a parallel tuned circuit with remnants of energy in the leakage inductance and you naturally get a decaying sinewave.

  • \$\begingroup\$ Thank you. The high spike that is at the beginning and end of the drain wave shape, at the start and end of the gate pulse, that I understand. That could be removed/reduced by using a snubber. But what about the big ringing with comparatively low frequency, that resides between these spikes? What is that due to? \$\endgroup\$ – dynamag Nov 15 '14 at 11:59
  • \$\begingroup\$ That "big ringing with comparatively low frequency" is what I describe in my answer and it is relatively harmless. The high spike is the dangerous one - that is due to a gap in time betweeen one mosfet turning off and the next one turning on - that spike could reach hundreds of volts and must be suppressed. The decaying slow sine is harmless. \$\endgroup\$ – Andy aka Nov 15 '14 at 12:16
  • \$\begingroup\$ So, If I understand you correctly, I need not worry myself about the slow ringing, but try and 'snub' the high spike. I am looking at a circuit which is a non-dissipative clamp, for push pull circuits. I will enclose that circuit as an edit to my original post. With that the wave shape has improved a bit, but it does not remove the spike. For that I will add a snubber. \$\endgroup\$ – dynamag Nov 15 '14 at 12:26
  • \$\begingroup\$ You sometimes have to be concerned about them both but it is usual that the high speed ringing is the one which attains the higher peak value and this is the one that can damage the MOSFET. \$\endgroup\$ – Andy aka Nov 15 '14 at 12:34
  • \$\begingroup\$ My goal is to get a 'clean' distortion and transient free wave shape on my secondary, for further work. What are the steps that I need to take, in order to achieve my goal? Once again, Thank you Andy! \$\endgroup\$ – dynamag Nov 15 '14 at 13:06

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