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I purchased a repair kit for a TV SMPS. The kit contained 3 identical N channel MOSFETs (K8A50D), among other components. I tested all 3 with a DMM following a number of identical instruction videos. The MOSFETs passed all the test for shorts. All 3 appeared to fail to take a charge from the positive DMM lead to gate and allow drain to source continuity. I find it hard to believe all 3 were bad components but can’t see what I am doing wrong. I assume the DMM is supplying sufficient voltage to meet the Gate to Source threshold. Any suggestions or comments?

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    \$\begingroup\$ What's your test procedure? Are you sure the DMM is providing enough voltage to exceed Vth? Are you just hoping that the charge stays on the gate, then measuring Rds on? If you touch the gate after charging it it's possible you could drain the charge off of the gate. Can you hook a power supply from gate to source to make sure the FET is on? \$\endgroup\$ – John D Oct 13 '18 at 18:34
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    \$\begingroup\$ I bet the DMM only supplies around 2v and Vth is more like 3 or 5v. \$\endgroup\$ – rdtsc Oct 13 '18 at 19:34
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    \$\begingroup\$ I believe you hold your pinky in the wrong direction. Try Bending the right knee at the same time you measure. \$\endgroup\$ – Linkyyy Oct 13 '18 at 19:42
  • \$\begingroup\$ Joke aside, we need more information. What dmm? how are you measuring it? show a drawing of how you hook it up. No one could possible know what is wrong with this sparse information. \$\endgroup\$ – Linkyyy Oct 13 '18 at 19:43
  • \$\begingroup\$ Try using something like a 9V battery to provide the gate voltage while monitoring drain-source resistance. Be very careful! ESD can quickly ruin a naked MOSFET. \$\endgroup\$ – Dave Tweed Oct 13 '18 at 21:18
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An extension of Dave Tweeds comment would be to use a 9 volt battery with an LED (your choice of color) in series with a 1K ohm resistor going from battery + to the drain pin of the MOSFET under test. Gate to source voltage should range from zero to 9 volts, almost enough to saturate the MOSFET ON 100%. The LED should get brighter with increasing Vgs. That verifies you have a good MOSFET.

If your DVM shows a voltage on the gate (about the same as the drain voltage) that you did not put there (it should read zero volts) then the MOSFET has shorted internally. A resistance reading of a few ohm from gate to drain or source is also a sign of a shorted MOSFET.

I cannot stress how sensitive to static they are when exposed. Walking up to a MOSFET laying on a table and touching the gate pin first is a good way to short it out. One should touch the metal tab or drain pin first, then put fingers across all 3 leads until your connections are done.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks so much for the advice and suggested test circuit.. \$\endgroup\$ – GIflyer Oct 14 '18 at 2:31
  • \$\begingroup\$ I built the test circuit. The short version is the MOSFETs passed. It appears these particular MOSFETs required a higher GS threshold then the other ones I tested. The data sheets didn’t indicate a difference.. live and learn. \$\endgroup\$ – GIflyer Oct 16 '18 at 2:24
  • \$\begingroup\$ You need about 10.5 volts from gate to source to saturate them on. That is where a MOSFET is so much better than a bjt. That circuit will test NPN transistors as well, including Darlingtons, but MOSFET's have a much lower ON resistance. You can run this circuit up to 15 volts but no more than that. A 12 volt maximum would be better. \$\endgroup\$ – Sparky256 Oct 16 '18 at 2:31
  • \$\begingroup\$ Any significance to a mosfet that doesn’t hold the capacitance? \$\endgroup\$ – GIflyer Oct 16 '18 at 2:46
  • \$\begingroup\$ @GIFlyer Hold the capacitance? Or do you mean hold a gate charge? The gate has very little capacitance. It would leak out into the air. \$\endgroup\$ – Sparky256 Oct 16 '18 at 4:00

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