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I am trying to design a circuit with the following specifications:

  • Topology: Push-Pull
  • Input Voltage (DC): 24 V
  • Output Voltage (DC): 400 V
  • Maximum Output Current: 2.5 A
  • Output Power: 1000 W
  • Operating Frequency: 50 kHz

The schematic of the circuit I have designed to test the transformer needed for this circuit is as follows:

Note: While testing the transformers, no load was connected to the circuit. The secondary side was left completely open circuit. If the problems described here were not encountered, the light bulb load would be connected directly.

Schematic of designed transformer test circuit

For the circuit whose specifications I provided above, I have had four different transformers designed by manufacturers. These transformers do not have detailed and well-organized datasheets, but the table I can provide regarding the transformers is as follows:

Transformer specifications

In this circuit, when I only connect the 12 V power supply for the integrated circuits (ICs), the signals at the gates of the MOSFETs are as follows:

Mosfet gate signals when only 12 V power supply is connected

When I later connect the 24 V supply to the circuit, the gate and drain-source signals of the MOSFETs are as follows:

Mosfet gate signals when 12 V and 24 V power supplies are connected

Mosfet drain-source signals when 12 V and 24 V power supplies are connected

Note: The tests performed to obtain oscilloscope images were performed on the same circuit. Only the transformers were changed.

Note: (to remember) While testing the transformers, no load was connected to the circuit. The secondary side was left completely open circuit. If the problems described here were not encountered, the light bulb load would be connected directly.

When I want to test Transformer - 3 and Transformer - 4, my mosfets and driver are exploding. Do you think this is because wrong wire selection in primary (litz)?

In my opinion, due to the high leakage inductance of transformers, oscillations occur in drain-source voltages. On the other hand, I could not understand why the situation I have circled in red in the drain-source voltage of Transformer - 1 occurs. However, it may be related to the magnetizing inductance.

  • I may be wrong, do you know how to fix this problem?
  • Or is there anything you are seeing as a problem?
  • Do you have any advice for me to test designed transformers before I go through the actual circuit?
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    \$\begingroup\$ Please add photo showing how scope probes were connected when the waveforms were captured. The negative spikes on Vgs & Vds are particularly concerning. \$\endgroup\$ Commented Mar 19 at 20:17
  • \$\begingroup\$ Thank you so much for your comment @FabioBarone . I did not use ground spring clips while testing it, I used crocodile instead. Maybe this affected my measurements but even if I had used spring one, I am sure same problem would continue. maybe with less oscillated version. What do you think? \$\endgroup\$ Commented Mar 21 at 6:12
  • \$\begingroup\$ We don't need to see new waveforms, and you do not have to power up the gate drivers again. We only need to see the physical set-up of how the waveforms were obtained (where the scopes probes were connected to the circuit being tested), but with the bench de-energised. How quickly do the gate drivers explode after the gate drivers are powered up? \$\endgroup\$ Commented Mar 21 at 6:51
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    \$\begingroup\$ This previous answer may assist: electronics.stackexchange.com/a/714111/341959 \$\endgroup\$ Commented Mar 23 at 1:03
  • \$\begingroup\$ @FabioBarone Yes, I saw it before I asked my question here. Thank you so much for your help! After I did my research deeply about the snubber circuits for the situation im currently in, I decided to try the one you already suggested in the link. Hope to see you in my next question - heheheh. \$\endgroup\$ Commented Mar 31 at 21:33

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Leakage inductance holds leakage energy. Leakage energy does not couple to the output so, it can give rise to massive ringing artefacts (leakage inductance and drain-source capacitance resonating) seen on the primary. Double check your scoping technique is good (spring clips on the probes and no crocodile clip wires).

It's likely that you need to use a snubber between the two MOSFET drains to suppress the ringing to a level that it doesn't produce voltages that destroy the MOSFETs. Ringing gets back to the MOSFET gate via the drain-gate parasitic capacitance.

However, it may be related to the magnetizing inductance.

It's likely not related to magnetization inductance because magnetization inductance couples to the secondary 100% when you have no secondary load and, secondary leakage is ineffective due to it being open circuit.

Your drive circuits like good but, your PCB layout could be suspect. If you haven't used a PCB then it's no surprise that this happens.

Do you have any advice for me to test designed transformers before I go through the actual circuit?

Get a decent handheld inductance/impedance meter and perform open secondary and shorted secondary primary inductance measurements. The numbers you get can be used to determine the leakage and magnetization inductances.

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  • \$\begingroup\$ Thank you so much for your detailed, valuable comment @Andy. What do you exactly mean by "snubber between mosfet drains" ? I was thinking of adding a RCD snubber, which resistor and capacitor in series and diode parallel to them, between drain and source for each mosfet. Any more advise about snubbers or at least a key word in order for me to research better? \$\endgroup\$ Commented Mar 21 at 6:23
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    \$\begingroup\$ @NafiCanEreli a snubber is one of these: en.wikipedia.org/wiki/Snubber ||| an RCD won't work because they're intended for flyback circuits and yours is push-pull. if we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$ Commented Mar 21 at 8:11

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