2 Methods For Testing MOSFETS
First look up the part number for the MOSFET you want to test online and acquire the datasheet. Once you have the datasheet or have somehow gotten the specifications for the MOSFET you are testing and you know the pin configuration, set your analogue meter to the x10k range to check the MOSFET. Let’s say you are testing an N channel MOSFET, put the black test lead on the drain pin then touch the gate pin with the red test lead. This will discharge the MOSFETs internal capacitance. Next place the red test lead to the source pin while still holding the black test lead to the drain pin.
Now take a finger, while still holding the test leads in place, red on source and black on drain and use that finger to touch the gate and drain pin at the same time, the analogue meters needle should move from infinity to around the center position of the meters indicator. Taking the red test lead off the source pin and placing it back on the source pin the needle should still go back to the middle of the meters indicator. To discharge the MOSFET lift the red test lead from the source pin and touch it to the gate pin, this will discharge the internal capacitance again and if you again place the red test lead on the source pin and the black test lead to the drain pin, the needle on the indicator should not move and give an infinite ohms reading. Testing a P channel MOSFET is the same as for an N channel MOSFET only you will reverse the polarity of the test leads for each test.
Using an ohmmeter set to the x 100 ohm scale, measure the resistance between the drain and the source, then reverse the leads and take another reading between the drain and the source. Both readings should show infinity, regardless of meter lead polarity. Connect the positive lead of the ohmmeter to the gate. Using the negative lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. Disconnect the positive lead from the gate and connect the negative lead to the gate. Using the positive lead, measure the resistance between the gate and the drain and then between the gate and the source. Both readings should indicate infinity. If the MOSFET has a substrate connection, disconnect the negative lead from the gate and connect it to the substrate.
Using the positive lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both of these readings should indicate infinity. Disconnect the negative lead from the substrate and connect the positive lead to the substrate. Using the negative lead, measure the resistance between the substrate and the drain and between the substrate and the source. Both readings should indicate a low resistance (about 1,000 ohms). Most MOSFETs will fail by shorting from drain to source and sometimes they will also have a short between gate and source, between gate and drain or both.
Remember you should always try and find a data sheet for the MOSFET you are testing because you will find some MOSFETs will have different characteristics that will make the test readings slightly different. For instance the FQPF9N50CF has a body diode between source and drain, so that when testing you will get a reading in one direction between source and drain in x100 ohm setting and this is normal.
Always try to use exact replacements when replacing a bad MOSFET. If you must use a substitute always replace with the same type(N channel, P channel, enhancement mode etc…) and also replace with the same or higher BVDSS (drain to source breakdown voltage) and the same or higher ID(continuous drain current). Also if you substitute a MOSFET used in a half bridge that uses two identical MOSFETs replace both the MOSFETs in the half bridge so that both of the MOSFETs are the same, don’t just replace the bad one in this case.
Hope you enjoyed this article and don’t forget to visit my websites below for all kinds of great electronics repair information.
Testing Electronic Components Ebook By Jestine Yong
LCD TV Repair Ebook By John Preher
LCD TV Repair Case Histories By John Preher