Re: Strange Mosfet Results

From: Peter (pmolsen_at_one.net.au)
Date: 08/04/04 

Date: 3 Aug 2004 22:48:18 -0700

Well that had a less than interesting effect!
First I tried the idea of just connecting a capacitor and resistor in
parallel between the gate and ground. Tried a few combinations but the
result was exactly the same as I was experiencing with just a cap on
the gate, namely that increasing the RC did not increase the "fade"
period. It just stretched the fully-on time then it faded off rapidly.
That seems to be indicating that the mosfet is very efficient. ie. it
is fully on over a fairly wide range then rapidly goes to fully off.

I therefore tried the idea of using the so-called Miller effect. ie. I
connected a cap from gate to drain plus the resistor from gate to
ground. Very interesting. The mosfet promptly died (internal short so
always hard on). It had gotten fairly hot so I then installed a new
mosfet with heat sink attached. Same result. Died immediately. It did
not even wait for a signal to the gate and did not bother getting hot.
Just died the moment power was applied to the drain (indicated by
coming hard on even though no gate signal). The cap I used was a 0.1uF
(104) with a 10k resistor from gate to ground.

As I said before the source is at 40v (actually rectified 38vAC). The
max rating specs for the mosfet (2SK2175 - equivalent to STP16NE06)are
as follows:
"Vdgr Drain-Gate voltage (Rgs=20k) 60V
 Vds Drain-Source voltage (Vgs=0) 60V
 Vgs Gate-Source voltage +/- 15V."

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