Use the unidirectional conduction characteristics of the parasitic diode inside the MOSFET to determine its health.
Step 1: Short-Circuit the Three Pins for Discharge
The purpose of this step is to discharge the parasitic capacitance inside the MOSFET to prevent any voltage difference, which could cause erroneous measurements.
Step 2: Measure the Internal Diode
Set the multimeter to the diode mode, then use the two probes to measure the drain and source of the MOSFET in both directions.
When measuring the internal diode with the multimeter, a forward measurement should show a voltage drop of around 0.3 to 0.9 volts, while the reverse should be cutoff. If not, it indicates the MOSFET is faulty.
For N-channel MOSFET:
For P-channel MOSFET:
A good MOSFET should show a voltage drop of 0.4V to 0.9V during the forward measurement (depending on the type of MOSFET). If the voltage drop is zero, the MOSFET has a defect. When the reading is “open circuit” or no reading, the MOSFET is also defective.
When measuring the internal diode in reverse, the voltage drop should be “open circuit” or show no reading. If the voltage drop is zero, the MOSFET is defective.
The following image shows a forward measurement of the diode between the source and drain of an N-channel MOSFET with a voltage drop of 0.52V:
The reverse measurement of the internal diode shows open circuit:
The following image shows a damaged N-channel MOSFET with a reverse measurement of the internal diode showing a voltage drop of 0.404V:
Step 3: Measure Other Pins
Still in diode mode, the MOSFET has three pins, and measuring them in pairs gives a total of six combinations. A good MOSFET should only show a voltage drop during the forward measurement between the drain and source; there should be no voltage drop between any other pairs of pins.
Source: Feidu Academy
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