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For example, a p-mosfet in a buck or buck boost topology converter. Is it working optimally or inefficiently?

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  • \$\begingroup\$ Have you tried measuring its resistance? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 28 '15 at 3:45
  • \$\begingroup\$ @IgnacioVazquez-Abrams side-question: does that work for BJTs? \$\endgroup\$ – Alexander M Nov 28 '15 at 3:47
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Measure the gate voltage, the current through the device and the voltage across the device and see where it lies on the IV curves given in the datasheet for the device.

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  • \$\begingroup\$ Note I didn't specify which switching technology (BJT or mosfet) \$\endgroup\$ – Alexander M Nov 28 '15 at 3:46
  • \$\begingroup\$ It's worth noting that we usually measure current indirectly, for example by measuring the voltage across a series resistor. \$\endgroup\$ – Houston Fortney Nov 28 '15 at 3:47
  • \$\begingroup\$ @AlexanderM my answer is specific to a FET only in the word "gate voltage". Swap that to "base current" for a BJT. \$\endgroup\$ – Houston Fortney Nov 28 '15 at 3:48
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If you want to do this properly them put the scope on the drive this means gate source .If all is well then put the other scope probe on drain source .The DS volts should come down low at switch on and stay down while gate drive is present.At turnoff there shouldnt be too much ringing .If there is ringing in gate or drain waveforms its best to do something about it now because if you dont you could fail radiated EMC and/or have your buck convertor upset other sensitive circuitry.If you are keen you can set up your test gear to plot drain current and drain source volts so you can see the switching losses .This may inspire you to reduce the switching losses because more often than not they are worse than the conduction losses.

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