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I'm currently making a pure sine inverter using a MOSFET H bridge with following specs.

Vin=360VDC

Vout=230VAC at 50Hz

Output Power=750W

Switching frequency=16kHz

enter image description here

I use Sine modulated PWM(SPWM) as shown below.

enter image description here

At this stage I'm still operating in 100VDC.Earlier I had problems with voltage spikes appearing in the DC bus line which was solved by moving C11 & C12 closer to the H bridge in the PCB layout.when i connect a resistive load directly to the bridge, the waveform of Q2 are almost perfect as shown below with no spikes when Q2 is off and Q4 is switching as in the previous question.

Vgs:cyan Vds:Purple Id:Green enter image description here

However when I connect the LC filter, a ringing appear in Vds.

enter image description here

What is the reason for this?

How can i minimized or avoid this ringing?

How this affects the MOSFETS?

Thanks in advance...

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    \$\begingroup\$ Possible duplicate of Voltage Spikes in MOSFET H Bridge \$\endgroup\$
    – winny
    Commented Jan 8, 2018 at 20:50
  • \$\begingroup\$ Solved that problem with spikes. This is the next problem I faced in the same design. \$\endgroup\$
    – Chinthaka
    Commented Jan 9, 2018 at 3:39

2 Answers 2

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Avoid this design method which has these issues with open circuit resonance and dead time shoot thru.

Instead only use a centre tapped transformer and with 3 primary voltage levels (V+,0,V-) each at low impedance (DCR+RdsOn).

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  • \$\begingroup\$ How should i filter PWM frequency? \$\endgroup\$
    – Chinthaka
    Commented Jan 9, 2018 at 7:34
  • \$\begingroup\$ Read how professional designs and papers that describe them work, before you try to reinvent the wheel or improve it \$\endgroup\$ Commented Jan 9, 2018 at 14:11
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What is the reason for this?

When Q2's Vgs goes low, the MOSFET turns off and stored energy in the inductors (L1 and L2) pass through the body diode of Q1 (the upper transistor) to the positive DC rail. That is normal and it reclaims energy.

This stored energy depletes to a point where the body diode of Q1 no longer is forward biased enough to pass the energy to the DC rail. At this point, the inductors form an LC circuit with the open circuit drain-source capacitance of Q2 (and Q1) and you get a damped oscillatory waveform until Q2 reactivates (Vgs goes high).

How can i minimized or avoid this ringing?

I don't think it should be regarded as needing a solution - it is what it is and it represents a minor loss of efficiency and probably nothing to worry about. It's a signature effect of this type of circuit - would you complain that a picture you had bought was spoiled by the presence of a signature?

How this affects the MOSFETS?

There is no real detriment to the MOSFET from what I can tell of your circuit. This sort of thing happens in plenty of switching circuits.

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