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MOSFET gate protection

Started by circuitmaker March 27, 2017
I have an AC 230V relay built of discrete MOSFETs.
Would a 12V 0.3W Zener diode in SOD323 coupled directly
to the gates/sources be an appropriate protection against
spurious turn-ons/oxide puncture due to high dV/dt on
DS or do I need a TVS?

c.
On Monday, March 27, 2017 at 10:34:33 AM UTC-4, circuitmaker wrote:
> I have an AC 230V relay built of discrete MOSFETs. > Would a 12V 0.3W Zener diode in SOD323 coupled directly > to the gates/sources be an appropriate protection against > spurious turn-ons/oxide puncture due to high dV/dt on > DS or do I need a TVS? > > c.
I think it's best if you post a schematic. You'll have some series R too, I trust. George H.
circuitmaker wrote:
> I have an AC 230V relay built of discrete MOSFETs. > Would a 12V 0.3W Zener diode in SOD323 coupled directly > to the gates/sources be an appropriate protection against > spurious turn-ons/oxide puncture due to high dV/dt on > DS or do I need a TVS? > > c.
Good practice indicates a use of back-to-back zener; protecting gate from over-voltage either polarity. 12V gives some safety margin as most MOSFETS seem to be rated at 15V.
Robert Baer wrote...
> > Good practice indicates a use of back-to-back zener; > protecting gate from over-voltage either polarity. > 12V gives some safety margin as most MOSFETS seem > to be rated at 15V.
Two points: First, for n-channel MOSFETs, generally we don't drive them negative, so a single ordinary zener works well. Second, most mosfets are rated at 20V, and 12V drive is often specified, so a 15 or 16V zener would be better as a rule than 12V. -- Thanks, - Win
On 31 Mar 2017 08:57:37 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>Robert Baer wrote... >> >> Good practice indicates a use of back-to-back zener; >> protecting gate from over-voltage either polarity. >> 12V gives some safety margin as most MOSFETS seem >> to be rated at 15V. > > Two points: First, for n-channel MOSFETs, generally > we don't drive them negative, so a single ordinary > zener works well. Second, most mosfets are rated > at 20V, and 12V drive is often specified, so a 15 > or 16V zener would be better as a rule than 12V.
Or a 20 volt zener, to minimize the likelihood of blowing out the zener. Most mosfets stand a lot more than the rated gate voltage. A series resistor is a good idea too, if speed allows. Of course, "good practice" is a waste unless you actually expect gate transients. -- John Larkin Highland Technology, Inc lunatic fringe electronics