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MOSFET plus electromechanical switch

I'm using an IRF540 MOSFET plus a chunky electromechanical slide switch and am encountering some problems. I have 12V connected to one contact of the switch, that when it is switched on, connects the 12V to the drain of the FET. The source of the FET is connected to ground. The gate is off (low) when the electromechanical switch is flipped on. I believe the FET is getting damaged by a high transient voltage on the drain when the e-m switch is flipped.

The VDSS (max drain-source voltage) is 100V. This seems pretty high for such a transient, but in another circuit configuration (where the e-m switch was controlling the gate of the FET, without a series resistor) it was even more reliably and thoroughly destroying the FET, presumably by exceeding the even lower VGS (max gate-source voltage) of 20V. This was fixed by adding a

1k series resistor between the switch and gate. In the current configuration, the series resistance of the load (between the 12V from the switch, and the drain of the FET) is only a few ohms.

I have tried to view the switch-on transient on a scope, but it seems to be faster than the resolution of my scope.

For a solution, I'm thinking of putting a small general-purpose diode across the e-m switch -- basically with the anode connected to the drain of the FET, and the cathode connected to the 12V on the other side of the switch -- so that when the e-m switch closes, any resulting spike on the drain would be clamped and conducted back to the 12V supply rail.

Does anyone have any other thoughts on what is happening, or if this is a good or bad solution for any reason? I'm also not sure how to size the diode -- maybe 50 or 100 mA?

Thanks for any help...

Reply to
The Dude
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What is the load? Is it inductive? If so, you need a diode across it to handle the back-e.m.f.

You said you previously controlled the gate using a switch. You should never leave the gate floating / open-circuit. One solution would be to connect a high value resistor between the gate and ground.

Reply to
Andrew Holme 

--
1. Why are you using a switch?

2. What is your load?
Reply to
John Fields

So your circuit is a mosfet connected from 12 volts directly to ground???

Just what is that about... killing mosfets, crowbarring power supplies???

Luhan

Reply to
Luhan

Sorry, my description wasn't clear. I'm attaching a representative schematic of this section of the circuit. There is 12V through a resistive load switched onto the high side (drain) of the (n-channel) MOSFET. Whenever I flip the e-m switch to on (ie., going from 2-3 and 6-7 to 1-2 and

7-8), the FET is off, so when everything gets to steady-state, the drain is at 12V, and there is no current flowing across the resistive load. However, I believe there is a brief transient when the e-m switch is first turned on, such that the drain shoots up above 12V, maybe even past the 100V VDSS maximum. The question is about this transient voltage on the drain of the FET and the best way to deal with it.

Just to be clear, when I turn the switch on and then the FET on, the drain is close to ground, and the 12V is across the resistive load (which is about

3 Ohms, so I'm pulling about 4 Amps), so it's not like the FET is crowbarring the 12V and ground rails.

To answer John and Andrew's questions:

For the purposes of this problem, just treat the load as a pure resistor -- virtually no inductance. I'm thinking of using a diode across the switch (anode to pin 8, cathode to pin 1) because there seems to be some kind of inrush transient voltage on the FET's drain when flipping the switch on.

I'm using an e-m switch because the device requires user input corresponding with switching the load onto a different circuit (not shown in this schematic), and it's the cheapest way to get it done.

Thanks, The Dude

From John Fields:

--
1. Why are you using a switch?

2. What is your load?

From Andrew Holme:

What is the load?  Is it inductive?  If so, you need a diode across it to
handle the back-e.m.f.

You said you previously controlled the gate using a switch.  You should
never leave the gate floating / open-circuit.  One solution would be to
connect a high value resistor between the gate and ground.



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Reply to
The Dude

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