How fast I should Switch off MOSFET?

Thread Starter


Joined Feb 3, 2009
I am using a P channel MOSFET switch on high side to switch +60V to a load. Current is sensed by a low side shunt resistor.

Suppose the load current shoots up by short circuit in load I need to switch off the MOSFET to save it and other circuitry.

How much time is available to me for ADC reading, processing and to switch off the MOSFET to save itself and other parts?

Hoe fast should be my software based short circuit protection?

Thanks a lot

Roy Thomas


Joined Oct 14, 2005
I don't think a software solution will be fast enough for protection purpose. Conversion and processing alone will require tens of micro second with fast hardware. Not to mention the guaranteed response time. A close-loop sense and shut-off circuit is the right way to go.

John Luciani

Joined Apr 3, 2007
The thermal mass of the die is very small so it will not take long to heat. Take a
look at the transient thermal curves in the FET datasheet to get an idea of the
time constant.

I would not do this in software.

(* jcl *)


Joined Feb 4, 2008
You need to use a sense resistor which won't allow more than the maximum peak current the MOS can conduct when the load is shorted. Note that the peak current the MOS can conduct is not forever but for a few microseconds.
It is easier to use a comparator to switch of the MOS rather a software solution.