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

"DonMack"

** The vast majority ( maybe all ) power MOSFETs have gate zeners.

IME - only small signal mosfets lack them.

Look at the specs for Gate to Source voltage - if it says something like

+/- 20 volts max - there ARE zeners.

... Phil

You mean something on the line of this?

Basically, "Auto Protected" types

Jamie

They have that spec because more than 20 volts could blow out the gate oxide.

If they had a zener, the spec would be asymmetric, lower for the negative-gate direction on n-fets, because the zener would forward bias.

The power fets I've tested seem to have essentially infinite-Z gates that blow out at, typically, 70 volts maybe.

John

"John Larkin" "Phil Allison"

** They use " zeners " plural - two of them , back to back.

Imbecile.

** Some have 'em and some do not.

.... Phil

Rarely. Stop speculating and test some real fets.

Dreamer.

John

Most always none.

Since you think most mosfets have zeners, take all your fets and drive the gates at 200 volts through a 1M resistor. They should all be OK.

John

"John Larkin" "Phil Allison"

** Most power mosfets that I see do.

BUZ901, 905 etc, 2SK176, 56 etc

Mad to say that they MUST have only 1 zener, like you did.

Fuckhead.

.... Phil

ST Micro has a number of zener-protected FETs.

Err...just because the spec states that, does NOT mean the gates are protected. Look again, carefully.

Err..some gate-protected MOSFETS use back-to-back zeners, making the protection symmetrical; see the STM Fets..

True, but gate-protected fets are the minority. Most mosfets will be destroyed by applying enough gate voltage, even if current-limited. I tried a couple yesterday, TO92 and TO220, and indeed blew them out, ballpark 70 volts.

Some of the Freescale RF laterals are rated +12, -0.5 max gate voltage, so they may have single zeners. Being RF parts, the DC specs of course suck, so it's hard to tell what's inside.

Poking around some web sites a bit, I didn't see any gate-protected IR parts, and a minority of the ON parts seem to have zeners.

Try a few.

John

Baers seem to make that "Err..." noise a lot. Perhaps just in the mating season.

John

Look...Baers must growl _someimes_ to show that they are really baers.

like

Well, lolly gag around showing your bare ass at home, and stop lolly gagging around showing it here.

No, most definitely not. Extra zeners would add extra capacitance which you don't want in a switching application. And there are other drawbacks using zeners. I recall reading an application note which warns against using zeners to protect the gate of a Mosfet. I forgot the details. Ask Google.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

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

Are you answering your own question here? The simple answer to the Q on the subject line is, it would cost more.

Hope This Helps! Rich

"Nico Coesel"

** Fraid quite a few types do.

See my other posts for actual numbers.

** A trivial amount in most cases.

** You ask and cite the link.

Piss head.

... Phil

Found it (chapter 2):

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------