# Mosfet controlling capacitor discharge

#### rjjenkins

Joined Apr 16, 2011
214
I posted a previous thread where I was trying to get a SCR to control a capacitor discharge through a coil. I'm now trying to use a mosfet instead. However, I seem to be destroying mosfets and I'm not sure why.

I'm charging a 330uF capacitor bank to 300V. If I remember my school physics correctly that equates to about 0.1C
If I apply the capacitor directly to the coil, it discharges in 1ms, as measured by my scope.
Which means that the current is 100A
I used four IRFP460 mosfets in parallel. Each one has a continuous maximum current of 13A and a pulsed current of 80A
I don't understand the definition of a pulse here, but I expected the mosfet bank to be able to handle 100A
The coil has a 1N4004 diode across it. Maybe this isn't sufficient protection?

#### StefanZe

Joined Nov 6, 2019
191
A schematic would be really help to find the problem.
What gate source voltage are you using, how fast are you able to turn the mosfets on?

#### rjjenkins

Joined Apr 16, 2011
214
Thanks. The turn on gate voltage is about 9V thanks to the divider. I don't know how fast the mosfets are triggering (I don't have any left at the moment)

#### StefanZe

Joined Nov 6, 2019
191
Unfortunately i cannot open the attachment.
The mosfets need a lot of current trough the gate pin to charge the gate capacitor when you turn it on. else they turn on rather slow.
While turning on the mosfet needs to dissipate a lot of power and this could destroy your mosfet.

#### rjjenkins

Joined Apr 16, 2011
214
It's a very simple circuit - a 300uF capacitor with a voltage divider across it which should produce 9V when the capacitor is charged to 300V. A pull down resistor of 100k on the gate of the mosfet. When the mosfet turns on, current flows through the coil. The coil has a diode across it in the normal way.

My understanding of mosfets was that the gate current is negligible - they turn on when the voltage reaches a threshhold. I also thought that the source-drain current rating would deal with the power dissipation issue?

#### crutschow

Joined Mar 14, 2008
34,013
My understanding of mosfets was that the gate current is negligible
True, the DC current is negligible, but there is a large gate capacitance that needs to be rapidly charged to turn-on the MOSFET and minimize power dissipation during that turn-on.
Add a 1µF or larger capacitor across R1 to provide the charge needed.

#### LowQCab

Joined Nov 6, 2012
3,935
An SCR is probably a better solution, they are extremely tough, and cheap.
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#### rjjenkins

Joined Apr 16, 2011
214
OK but SCRs rated at 100A are not cheap. Do I need a 100A rating or will a lower rated one cope for the 1ms the current is flowing?
Or should I put 5 20A rated ones in parallel?

#### crutschow

Joined Mar 14, 2008
34,013
Do I need a 100A rating or will a lower rated one cope for the 1ms the current is flowing?
Usually the SCR data sheet will show the pulse current ratings versus pulse width.

#### crutschow

Joined Mar 14, 2008
34,013
You also need to look at the safe-operating-area curves such as those below for the MOSFET you used:

#### rjjenkins

Joined Apr 16, 2011
214
You also need to look at the safe-operating-area curves such as those below for the MOSFET you used:

View attachment 307108
Ah, that is an interesting graph. Looks like 8A was about the maximum per mosfet