Short-Circuit Protection With A MOSFET

If you have an application in which a MOSFET
is already used to switch a load, it is relatively easy to add
short-circuit or overload protection. Here we make use of the internal
resistance RDS, which produces a voltage drop that depends on the amount of current flowing through the MOSFET.
The voltage across the internal resistance can be sensed using simple
comparator or even a transistor, which switches on at a voltage of
around 0.5V. You can thus avoid the use of a sense resistor (shunt),
which usually produces an undesirable extra voltage drop. The comparator
can be monitored by a microcontroller. In case of an overload, the
software can initiate suitable countermeasures (PWM regulation, alarm, emergency stop etc.). It is also conceivable to connect the comparator output directly to the gate of the MOSFET, in order to immediately cut off the transistor in case of a short circuit.


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