Anyone know why mosfets do not come with zeners added on the gates?

Based on a bit of datasheet prowling, none of the IR parts seem to have gate zeners. With enough gate capacitance, the ESD hazard is small.

I've tested a few mosfets that we have in stock, and none have gate zeners. It takes essentially zero current to blow out the gates.

John

-----

You've told me so much yet so little!!

It seems the real issue is that parasitic impedance can cause ringing which can exceed the gate voltage. I suppose it's best to let the user decide what he wants to do.

"Nico Coesel"

All mid air assertions - and no proofs.

Just like YOU - s*****ad !!!!!!!!!

.... Phil

The parts that Phil mentions with gate zener diodes are in the decided minority. Most power MOSFETs I've examined don't come with zeners. It's said that their high gate-source capacitance means they protect themselves from most handling discharges.

An engineer at Supertex (they make MOSFETs with and without zeners) told me their failure analysis showed more shorts in the protection zener than the FET itself, so they decided to eliminate the zener diodes from most of their products.

BTW, MOSFET failures often show up as drain-to-gate shorts. If they have individual gate resistors, as they should, this makes it easy to find a bad MOSFET in parallel FET setups.

"Winfield Hill"

Was the engineer really a " sales engineer " ??

.... Phil

IR claims more or less the same. It makes me wonder whether the zeners are difficult to make in a MOSFET or that they get overdriven in circuits designed by -lets say- less competent engineers.

Makes sense to me. A big mosfet will have nanofarads of gate capacitance, which forms a high-ratio voltage divider when driven by charged human body capacitance, so ESD damage is unlikely. But the zener would be zapped by the *current* of an ESD event. If zeners made mosfets more reliable, more fets would have them. There must be process issues that discourage zeners, too.

I don't think I've ever blown a mosfet from ESD. It power that kills them.

I have blown mosfets by snapping the substrate diode, but that was some years back and I think the manufacturers have done things about that since.

John

I don't understand how the zener can be zapped if the gate capacitance absorbs all the charge. ISTM that the zener is just along for the ride.

John S

"John Larkin"

... Phil

The Supertex parts in question were smaller-die parts. The appliocations engineer relaying the story was not in their fab department, but presumably he heard what reason they gave (I complained that they'd discontinued some parts with zener diodes). I'd imagine one issue might be that they're not willing to devote a very large fraction of the mosfet's die area to the zener, so it ends up being a relative wimp.

That's probably right-on, Win! Zeners with sufficient energy capability are BIG. ...Jim Thompson

..at least your complaint had more weight than mine. STM had a gate protected FET with a 10uA leakage spec at 150C; they were consistently better than 40uA at 500V and 185C. Sigh!

As an aside - I remember from my monitor repair days that most UC3842 based PSUs had some form of external gate protection for the MOSFET, many used a zener but a few had a 20V shottky-barier diode.

On occasion PSU failure resulted in the MOSFET blowing all points S/C which usually blew the sourse resistor O/C and as the gate was short to drain this tended to destroy the chip.

Sometimes the SB diode protected one's didn't destroy the chip but AFAICR allways lost the chip when zener protected.