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Short-Circuit Protection

with a MOSFET

021

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 R DS(ON) , 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 0.5 V. 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 coun-

termeasures (PWM regulation, alarm, emergency stop

etc.). It is also conceivable to connect the comparator out-

put directly to the gate of the MOSFET, in order to immedi-

ately cut off the transistor in case of a short circuit.

R1

Load

R

DSON

IC1

T1

D

G

Comparator

R2

S

014088 - 11

(014088-1)

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