# RDS measurement on P-Channel MOSFET

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

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?

• Labelling the drain "S" and the source "D" is only going to add confusion. Jun 23, 2021 at 19:49
• 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. Jun 23, 2021 at 20:29
• @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. Jun 23, 2021 at 20:35
• @user_1818839 I have updated the schematic Jun 23, 2021 at 20:35
• 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. Jun 23, 2021 at 21:02