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I want to measure the Rds-ON of a P-Channel MOSFET and I have setup the test circuit as follows:

enter image description here

I keep the source at constant +10V and vary the step the gate voltage up in the increments of 1V. Then I measure the resistance across the drain & source. However, the RDS values I measure are around 5-5.6 ohms vs the 0.3 ohms as per datasheet. I am using 4 wire measurement and for N-Channel MOSFETs this setup seems to work. So I wonder why it apparently doesn't work for P-Channel MOSFETs.

I tried with other P-Channel MOSFETs and also get considerably high values. So it's not just one MOSFET. For a rough comparison, I tested it on a component tester and even that gives lower RDS ~ 1-1.5 ohms. I understand that I can't trust the component tester results too much but it should be ok for indicative results.

What could be going wrong with the measurements? and is there a better way to measure RDS that would give me accurate results?

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  • \$\begingroup\$ Labelling the drain "S" and the source "D" is only going to add confusion. \$\endgroup\$ Jun 23, 2021 at 19:49
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    \$\begingroup\$ What could be going wrong with the measurements? Did you calculate how much power will be dissipated in the MOSFET when you apply \$V_{DS}\$ = 10 V and a couple of amps is flowing? Can the MOSFET handle that? The "industry standard" way of measuring \$R_{DS,on}\$ is by using a short current pulse and then measure the voltage. The idea is to measure quickly before the MOSFET heats up. Also realize that a hot MOSFET will have a higher \$R_{DS,on}\$ than a cold MOSFET. \$\endgroup\$ Jun 23, 2021 at 20:29
  • \$\begingroup\$ @Bimpelrekkie Yes, I have calc. the pd. It will be < 10W as I have CC limit of 1A. The MOSFET can handle 88W without heatsink but still I am using a decent heatsink with a fan. So it shouldn't be an issue. \$\endgroup\$
    – Zaffresky
    Jun 23, 2021 at 20:35
  • \$\begingroup\$ @user_1818839 I have updated the schematic \$\endgroup\$
    – Zaffresky
    Jun 23, 2021 at 20:35
  • \$\begingroup\$ The MOSFET can handle 88W without heatsink No, that 88 W is the maximum dissipation and assumes that the case of the MOSFET remains at 25 degrees C. That 25 degrees cannot be maintained without a heatsink unless you measure with a very short pulse. \$\endgroup\$ Jun 23, 2021 at 21:02

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A much better way to measure RDs would be to use a power supply in current mode and step the current, at each current step also record the voltage, you'll get a current to voltage plot and the slope of that is RDson. You could then do that for each gate voltage step.

It's probably better to fix the gate voltage and then vary the voltage and current.

A more comprehensive description is found here: https://goughlui.com/2020/04/20/experiment-measuring-mosfet-rds-vs-vgs-with-a-power-supply-rs-ngm202/

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  • \$\begingroup\$ Wow that seems indeed a better way. Thanks for suggesting the link. I am going to apply that. \$\endgroup\$
    – Zaffresky
    Jun 23, 2021 at 20:33

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