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I’ve designed and built a 12V–360V DC-DC converter. The circuit has just been powered up and is under testing. The output is stable at 360V. I had to damp the RCD clamp with an additional RC snubber, but when the MOSFETs turn off, a fairly large spike appears at the drain. The clamp and snubber suppress it to some extent, but at higher loads it is still quite noticeable.

\$I_{out}\$=100mA enter image description here

\$I_{out}\$=200mA enter image description here

\$I_{out}\$=300mA enter image description here

\$I_{out}\$=400mA enter image description here enter image description here

The transformer, current transformer, and circuit stray inductance add up to 101 nH based on measurement. The snubber circuit schematic: enter image description here

How could the turn-off spikes be suppressed more effectively? I also checked the gate signals, they look clean, with no ringing or overshoot. Maybe I should speed up the MOSFET turn-off, e.g. by adding D7 and R27?

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  • \$\begingroup\$ Oscilloscope probing could well be a big issue. \$\endgroup\$ Commented Sep 3 at 19:28
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    \$\begingroup\$ @Andyaka I didn’t use a long GND reference lead, I used a ground spring during the measurement. I kept the probe cable away from the transformer. The ground connection was taken directly from the MOSFET source. \$\endgroup\$ Commented Sep 3 at 19:46
  • \$\begingroup\$ OK, are you worried about the spike? Did you simulate to see if you got the spike? It looks like leakage inductance and drain-source capacitance resonating to me. Not usually regarded as a problem. \$\endgroup\$ Commented Sep 3 at 20:32
  • \$\begingroup\$ "Maybe I should speed up the MOSFET turn-off" I would suspect it might make the spike, which is presumably due to leakage inductance, worse not better. \$\endgroup\$ Commented Sep 3 at 21:34
  • \$\begingroup\$ is the primary bifilar? \$\endgroup\$ Commented Sep 4 at 3:05

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