MOSFET Failures

Testing N-Channel MOSFETs In-Circuit

A common cause of motor controller failure is a bad MOSFET.  Sometimes the damage is obvious — a MOSFET's package will be cracked, burned or have exploded.

More subtle damage can be detected using the diode test function of a DMM.  The diode test function measures the voltage drop across a semiconductor junction.  Testing must be performed with battery and motor disconnected.

In addition to the MOSFET's intended purpose as a transistor switch, all MOSFETs exhibit a parasitic “body diode”.  The voltage drop across the body diode in a good MOSFET will be on the order of 0.5 volts.  The exact value of the voltage drop is not critical — a shorted MOSFET will read 0 volts.

The adjacent diagram is a simplified version of the circuitry involved.  There will be other components (such as DC-link capacitors across the battery terminals and resistor voltage dividers to sense back-EMF from the motor) in any real implementation.  Fortunately, the “diode test” is still a reliable indicator despite the presence of these other circuit elements.

Note that a relay potted inside some Greenworks controllers will inhibit access to the positive side of the MOSFETs (making them untestable).