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I'm trying to build a 16kHz single phase, full bridge PWM inverter, Vdc=380V, 2kVA rated.

When I load the inverter a little bit, the output voltage waveform is looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

I have capacitor snubbers across each half bridge as shown in the schematic.

How can I get rid of the distortion in the red circle?

Added:

Here is one of four PWMs (I only have one probe so I can't show you all four same time, but I've checked phase differences and they're ok. Other PWMs have same duty cycle and Vpp like this.): enter image description here

Here is the driving circuit (I have 2 of these for each half bridge.):

enter image description here

Here is the rising edge of one PWM, purple is pwm signal and blue is output of driver circuit.

enter image description here

Here is falling edge of same PWM above

enter image description here

Here is all four mosfet's Vgs waves

enter image description here

Here is all four PWM

enter image description here

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    \$\begingroup\$ (1) Can you edit the schematic and add some NODE symbols to show where you are taking the measurements (scope probe and ground). (2) What is the driving circuit? How are you ensuring that you don't have shoot-through when M1 and M4 or M3 and M5 are on together? (3) Where is 0 V on your scope? \$\endgroup\$
    – Transistor
    Dec 11, 2019 at 11:00
  • \$\begingroup\$ @Transistor hi, I'm taking measurement across the load with a differential probe, scope is isolated from grid, driving circuit is simple HCPL3120 driving circuit but it works well because I'm using it with different projects also. 0V is shown by arrow on middle line line (trig arrow) \$\endgroup\$
    – Das D.
    Dec 11, 2019 at 11:07
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    \$\begingroup\$ Show your HCPL3120 input waveforms. Show your full circuit from input waveforms on driver to output. \$\endgroup\$
    – Andy aka
    Dec 11, 2019 at 11:10
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    \$\begingroup\$ No, we need to see input waveforms and output waveforms on the same o-scope shot on the same timebase as the original output waveform shot. If you can't display two input waveforms and the output waveform then show the two input waveforms (M1 and M4) in same shot. I'm trying to see if you have deadband in your inputs (as you should have). \$\endgroup\$
    – Andy aka
    Dec 11, 2019 at 11:57
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    \$\begingroup\$ Your schematic V1 reads as 400vDC, but the output signal seems to be a peak differential of 750v (530vAC), centered about zero volts. Something is not adding up here... that said, it looks like cross-conduction to me. Add a 1Ω resistor to V1, scope only across that, and show us that waveform when PWMs active. \$\endgroup\$
    – rdtsc
    Dec 11, 2019 at 13:17

1 Answer 1

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You have dead band and that in itself is a desirable thing because it prevents any two MOSFETs on the same half of the H bridge conducting together and momentarily shorting the H bridge supply out (bad news and also known as shoot-through). But, it looks to me that you could probably reduce the anti-shoot-through delay and carry a little more energy to your load.

The slight glitch in rise and fall time is not a big deal and is likely caused by some driver resistance in the line that feeds each gate. Basically, as the MOSFET switches polarity an opposing voltage forces its way through the parasitic capacitance between gate and source and, momentarily halts the gate drive process. Result: you get a little glitch. It’s common enough and is probably not a cause for concern.

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