Re: Disabling the freewheling diode

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Gavin,

At 10:51 AM 8/30/2003 +0100, you wrote:
>Hi,
>this brings up a question that I have had in the back of my mind for some
>time. If a MOSFET has a maximum current rating of say 10A, then this is, as
>I have read and understood it, is the maximum rating regardless of the
>currents duty cycle.

You have to read the data sheets to be sure.  Mine have a maximum sustained 
DC current, and then a graph of pulse currents they can take depending on 
the current, time, repetition rate...  It can be pretty messy figuring it 
all out.  Many die will also fail after several hundred thousand prolonged 
heating cooling cycles since the thermal expansion in common cheap packages 
tears things up.

>So if I switch the MOSFET on and off (across
>gate-source) with a signal that has a duty cycle of 33%, and the peak
>current amplitude of the switched current (through drain to source) is 10A,
>the device will blow even though the average of the main current passing
>through drain to source is only 3A.

There are three things that limit peak current:

1. You get the die too hot and it burns up.  As long as your "RMS" current 
is close to the DC rated current, you will be fine.

2.  The current is so high it fuses wire bonds and destroys the die just 
due to massive current.  This is highly unlikely at less then 
10X  ICmax.  Many cheap TO-220 FETs can suck 100 amp spikes easily.  I used 
to use a big TEK 371 curve tracer to blast little parts with current and it 
is amazing how hard you can hit little FETs.

3.  The current is so high the FET falls out of saturation.  If you want to 
drive a lot of current, you need a lot of gate drive voltage...

To answer your question, I think number one above is what you need to worry 
about.

>I find this strange as the drain-source
>is ohmic. Is all this correct, or can you treat the drain-source as a purely
>ohmic resistance with regard to destructive heating.

The resistance goes up when the device is hot so you have to take into 
account the resistance when the die is at say 150C.  A really hot die is 
also not going to tolerate voltage transients and such well at all.  If the 
device is being switched, there are details about how the die reacts at 
high temperature during switching.  The key is to keep the device well 
below 150C.  Personally, I like 80C for power FETs.  All kinds of problems 
simply vanish then.

>Perhaps there is
>another factor involved that makes a MOSFET blow due to peak amplitudes of
>current passing through them.

When is the "peak"?  10 amps, 100 amps, 10000 amps...  The current peak has 
to be super high really.  Maybe something else is killing the FETs?

Cheers,

         Terry



>thanks in advance,
>
>Gavin