Zener diode to protect gate of SMPS MOSFET

[treez]

Hello,
We are designing a 60W offline PFC’d Flyback LED driver. 240VAC input and 60V LED string.
We want our LED driver to last a long time, therefore we want to protect against transients getting coupled through the FET’s Drain_Gate capacitance and overvoltageing the gate of the FET. (IPA90R800)
We appreciate this isn’t a very common event, but we still wish to protect against it.
Therefore, we will connect a 18V zener diode at the FET gate (cathode to gate, anode to ground). The thing is, we want this zener to be as small as possible. We can pick from the following packages..SOT323, SOT23, SOD123, MELF, SMA.
Do you think that the smallest one, the SOT323, will be effective in this use? If not, which one(s) would be effective?

 

[Easy peasy]

Your scenario can never happen, if the D-G capacitance allows > 4V on your fet gate it will start to turn on and the drain volts will level off or start to reduce, by the time the gate voltage reaches 18V ( the zener voltage ) the fet will already be hard on and the drain volts ~ zero.

If you wish to keep the fet off for transients on the drain - a better gate drive is the answer.

Gate zeners usually only protect the gate drive ckt after a blow up by going short - this is really their only use. Hence 400mW or bigger ....

 

[treez]

Thanks, thats a good point. I do remember though, working at a radar power supply company. They had a high side drive to a MOSFET in one of the power supplies. (I never saw the schem as it was secret). But I remember the Engineer saying that he removed the gate drive’s shunt connected zeners once and the power supply blew up.

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I think though that those may have been for quenching transients from the gate drive pulse transformer.