Is this setup for measuring MOSFET RDS_On correct?

Question

I ordered some MOSFETS from AliExpress and I think I've been duped.

The diagram below shows my setup:

If I'm not mistake, this means that the R_DS(On) of this N-channel MOSFET is 0.042/0.62 = 67 milliohms, at a V_GS of 3.83V.

The datasheet for this AO3416 says that even at a V_GS of 2.5V the R_DS(On) should be less than 26 milliohms. The MOSFET doesn't warm up, so its temperature is near ambient 25C, like the datasheet.

Have I been duped, or am I measuring R_DS(On) incorrectly?

edit 1: At V_GS of 4.83V under a new load I measure 0.58A and 0.030V voltage drop, so RDS_On is 51.7mOhms, still higher than what it should be at <22mOhms...

edit 2: At V_GS of 3.96V under another load I measure 0.28A and 0.0165V voltage drop, so RDS_On is 58.9mOhms...

I think it's fair to say that the product does not meet the specifications?

Answer 1

Have I been duped, or am I measuring R_DS(On) incorrectly?

The method is correct, although R_DS-on may or may not include the lead resistances but let's ignore them for now as I don't think the SOT-23-3 lead resistances can be as high as 10 mΩ for less then 1 Amp of current. Also the R_DS-on may change with drain current, especially when the gate-source voltage is closer to the threshold voltage, V_GS-th. Judging from the characteristic curves given on p.3 (e.g. Fig.3) I can say this pretty much not the case in your setup, so we can ignore this as well.

I personally prefer switching instead of applying continuous DC, just to prevent the junction temperature rising because it has significant effect on R_DS-on. The speed and sensitivity of the equipment becomes important here, also it may require using a data logging capable equipment.

We can expect some deviation from the datasheet values as the conditions do not perfectly match. But this deviation is not normally +100%.

Anyway... You should think twice before buying electronic components from unknown sources. And you've learnt this in a harder way.

Answer 2

With these numbers I would question the test equipment and the actual connections. The measurement for voltage DS should be at the leads of the package, not the PCB. Also are you using at least a 5.5 digit meter that is in calibration?

When doing this for qualification we placed them in a temperature controlled oil bath and let them stabilize for about 1/2 hour. Measurements were pulsed, micro seconds if my memory is correct. The current source was extremely accurate good to at least 0.0001 mA. I also used a current meter in series with it. All metering connections were kelvin.