output device protection w/zeners

[pprasse]

Hello all!

In some MOSFET amplifiers there is a zener btw source and gate of the output devices.
I suppose they are there to limit the maximum gate voltage and by doing this they limit the maximum current flow thru the output stage. (correct me if I'm wrong)
And if the value of the zener is choosen right a short circuit accross ground and output should not
damage the output stage.

My question is now:
How do I calculate the value of the zener?

I suppose:
If I know the maximum current thru one output mosfet, I
multiplie it with the number of output mosfets.
Now I look up this current in the transfer characteristics diagram(gate-source voltage vs. drain current).
I suppose the value of the gs-voltage at this positon minus 0.7V should be the value of the zener.

Am I right?

Patrick

 

[jam]

Mosfet Protection

Patrick,

The main reason for the zener is to prevent the the gate to source voltage from exceeding 20volts approx. At this voltage the mosfet will break down and the device will be destroyed.

The choice of zener will limit the drive to output devices but will not limit the current when you short the output to ground.
The value of the zener cannot be less than the turn on voltage of the device and values between 8 and 12 volts are often chosen.

If you plan to current limit the output stage a better solution would be to monitor the voltage across the source resistor of an output device and clamp the drive to the gates of the output devices(traditional V/I limiter).
There is a sonic penelty when using such circuits.
My approach would be to overbuild the output stage and fuse the supply rails.

Jam

 

[pprasse]

Well, I don't want to use a vi-limiter circuit due to the sonic penelties.

Fuses would be good but I want something which is electronically resetable.
Should be no problem to build such a circuit.

How slow should the fuse be?