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Re: [Amps] MOSFET ratings

Subject: Re: [Amps] MOSFET ratings
From: Manfred Mornhinweg <>
Date: Tue, 19 Jul 2011 22:04:27 +0000
List-post: <">>

> Firstly, I should stress that you must not confuse me with someone 
> who understands all about these MOSFETs :-)

No problem! We are among hams here.

> I have been reading from my old (1987) IR catalogue, application note
> 966. This is describing the HEXFET III family, the first where diode
> dV/dt is specified. I have not had time to read it in full detail,
> but it refers to the diode recovery dV/dt limit applying " the
> diode recovers from conduction..." and describes a mechanism that
> occurs during the period that recombination is happening in the
> diode. If the value is too high, the bjt will conduct.

Yes, that make sense.

> I could not find AN-966 on line to give you a link - I can scan the
> article (7 x A4 pages, small print) for you if you wish.

I found an application note that says basically the same. It's AN2626
from STMicroelectronics, available on So I think that I you
don't need to bother scanning that AN. Thanks anyway!


> If the diode is for protection, it doesn't effect the operation until
> the diode/s starts to conduct.  Therefor the device has speed
> limitations when it is over driven and different speed limitations
> when it is not over driven.

It's all getting clearer today. The whole low dv/dt limitation is valid 
for applications where the diode is made to conduct, then the FET is 
driven on for a relatively short time at low current, and then it's 
switched off, and all that with an inductive load. This happens in 
zero-voltage-switching circuits. In that case, the low dv/dt rating 

In linear amplifier use, normally the drain should never get even close 
to ground, but stay at least about 5 or 7V above the source, more likely 
10V. So the body diode will never conduct, and the whole issue of diode 
recovery dv/dt is a non-issue.

But one needs to be careful to avoid any situation in which a MOSFET's 
drain could be pushed negative, for example by resonances of parasitic 
circuit elements, the low pass filter, antenna reactance or whatever. 
Such a situation could make the MOSFET smoke its way into Neverland.

Thanks to both of you, for helping clear up this matter!

Now the other mystery remains in my mind: How manufacturers can rate a 
high current MOSFET to switch in about ten nanoseconds, when the 
inductance of the source connection seems to make this impossible!


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