Sizing Gate Drive Diode

[clw]

I would like to incorporate a traditional gate resistor to control ringing at the MOSFET gate and additionally a second, perhaps lower value resistor, in series with a diode to provide an alternative lower impedance path between the gate and the driver output when discharging the gate capacitance.

The question: Does the diode need to be sized to support the peak sink current of the gate driver? Intuitively it seems I could reduce the size of the diode because of the 20kHz switching frequency. The 3A SS32 fast diodes I have in the circuit now are almost as large as my 2.5A opto gate drivers.

I hate to take up so much room on the pcb when likely these extra diodes and resistors may not even be necessary. The purpose was to provide the ability to improve turn-off time of the MOSFET slightly.

 

[crutschow]

The diode does have to support the peak current value but, since it only occurs for a very short time, a smaller diode should work ok. Check the peak current rating of the diode data sheet for short pulses.

 

[clw]

The datasheet for a 0.75A BAT750 says:

Non-Repetitive Peak Forward Surge Current 8.3ms Single half sine-wave superimposed on rated load
IFSM = 5.5A

The non-repetitive part bothers me. These diodes are in the gate drive circuit of a half-bridge motor drive operating at 20kHz.

 

[smanches]

What is your switching time? Not frequency, but how fast are they turning on and off?

That's your pulse duration, and I have a feeling it's in the 100ns range. Far cry from the 8.3ms pulse they are using.

 

[clw]

Thanks - that's the part I was missing. My expected rise / fall times are indeed somewhere between 100 and 200nS. This part should work just fine.