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Voltage spike on Full Bridge L6491 gate driver

Question asked by Riccardo Barbon on Jun 21, 2018

Hello,
I have a problem with a full bridge circuit that drives a high frequency transformer.
The application concerns an induction heating system that can work in the frequency range from 100 to 500kHz, with an input voltage of 320VDC and a current of 10A.
The four transistors, Infineon IPW60R190P6, are driven by 2 Gate Drivers, ST L6491, with a voltage of 12V.
When the transistors switch, oscillations occur in the supply voltage HV_DC, which affects all the other signals.

 

Already at low input voltage there are peaks on the supply voltage and on the gate signals.
In the attached image I reported the gate signal of the transistor T1 and the signal measured on the power supply (equal to HV_DC = 15V).
In this case oscillations occur at 12.5MHz with a Vpkpk signal on the power supply equal to 9Vpkpk (the signal at the bottom of the screen), while on the gate signal there is a 3V peak after the rising edge and a peak at + 3V and -2V after the falling edge (the signal at the top of the screen).

I have already inserted an RC snubber circuit (2.4nF, 33Ohm) between darin and source of each transistors. This allowed me to limit the spikes, but it is not enough.

The LC circuit, located at the secondary of the transformer, resonates at a frequency of 380kHz. I tried to drive transistors at higher frequencies and I notice a slight improvement at a frequency of 386kHz.

 

How could I proceed to resolve or limit the problem?

 

Many Thanks,

Riccardo

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