Re: Disabling the freewheling diode

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

Original poster: "Gavin Dingley" <gdingley-at-ukf-dot-net> 

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

thanks in advance,

Gavin

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, August 29, 2003 6:54 PM
Subject: RE: Disabling the freewheling diode


 > Original poster: "Jan Wagner" <jwagner-at-cc.hut.fi>
 >
 > Hi!
 >
 >  > In a SSTC H-bridge, the internal freewheeling diode of the
 >  > MOSFET is disabled by a clever arrangement of external
 >  > diodes. A schottkey diode above the drain keeps the internal
 >  > diode from getting forward biased, so that an ultrafast
 >  > recovery external diode can provide the path of any reverse
 >  > EMF from the load. Any current passing trough the MOSFET must
 >  > pass trough the schottkey diode.
 >  >
 >  > In a typical design, I see MOSFETS capable of handling 20A
 >  > continous, with a 3 A schottkey diode on top.
 >
 > (the diode can be at the source or drain pin, symmetric...)
 >
 >  > Why is the schottkey diode not dimentioned for a similar 20A?
 >
 > Because luckily it conducts only for a short period. If the SSTC is
in-tune
 > and the drive signals have ~50% duty, the diode ideally
 > conducts no current at all. When the SSTC is really grossly out of tune
 > i.e. pri current draw and the drive voltage are highly out
 > of phase, almost no current will be flowing into the primary, and also the
 > diode conducts very little current. When slightly out of
 > tune, the diode will conduct the (sine shaped) current close to where the
 > sine waveform crosses zero, so, again, it's not very hard
 > on the diodes as the average current is quite low.
 >
 > Only when you're doing PWM, i.e. changing the duty cycle of the drive
 > signals so that the duty t.ex. averages at 25%, the diodes
 > will conduct a large current - in the worst case, the peak primary current
 > - and have to be rated the same as the mosfets (or, hmm,
 > probably only half the current rating of mosfets... (?))
 >
 > cheers,
 >   - Jan
 >
 > --
 > ********************************************************
 >   high voltage at http://www.hut.fi/~jwagner/tesla/
 >   Jan OH2GHR - GSM +358-41-4682893
 >   jwagner-at-cc.hut.fi - Jan.Wagner-at-cern.ch
 >
 >