measuring mosfet's Rds(on)

Thread Starter

electron_jj

Joined Aug 28, 2021
14
Hello everyone, I have been testing a mosfet which has been desoldered out of circuit and the readings are generally okay and in line with expectations. But I wasnt able to get the expected reading for the Drain-Source On-State Resistance. The steps that I have followed are as follows (which are based on the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17 ):-

(i) Trigger the MOSFET by setting the multimeter at diode mode then fix the meter black (negative) probe to the drain and touch the red one momentarily to the gate. This should trigger the gate (as shown in figure below). The MOSFET must turns ON by this.

(ii) Take an ohmmeter and check the drain to source resistance, Rds(on). This time the reading should be very low (zero or approximately zero) than the previous Rds(off) reading. This will confirm that the MOSFET is in good condition. Refer to the datasheet of the MOSFET to check the value of drain to source resistance in on condition, Rds(on) and compare it with the observed value. If the observed value has much variation with that specified by the datasheet, the MOSFET is faulty.

I have tested this many times and the Rds(on) is measuring several 10s of thousands ohms instead of the 0.47 ohms stated in the datasheet. I am sure that the MOSFET has been turned on by step (i) because when I check under diode mode there is voltage difference in both directions between the gate and drain which only happens when the MOSFET is turned on. Once I discharged the gate pin, the MOSFET is turned off and diode test will only show voltage difference in one direction only. So, I am sure the MOSFET is working in terms of turning on and off. But despite that, I dont know why I am still measuring 10s of thousands of ohms resistance between drain and source while it is in an "on" state.

Is there something wrong with the MOSFET or the tests does not work with every MOSFET? The full set of steps are in the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17/

Thanks!
 

Papabravo

Joined Feb 24, 2006
17,318
Most MOSFETS will require a larger value of Vgs to turn the device on. This value and other test conditions for measuring rds(on) are given in the datasheet. You probably need to create a test jig to do this testing reliably.
 

crutschow

Joined Mar 14, 2008
28,227
(i) Trigger the MOSFET by setting the multimeter at diode mode then fix the meter black (negative) probe to the drain and touch the red one momentarily to the gate. This should trigger the gate (as shown in figure below). The MOSFET must turns ON by this.
That test procedure may work to see if the MOSFET works, but is not correct for measuring Rds(on).
The typical multimeter diode-mode voltage of a few volts would likely just barely turn on a standard MOSFET and is generally not enough to fully turn on a MOSFET and give an accurate Rds(on) measurement.
You need to look at the MOSFET data sheet and see what Vgs they use when they measure Rds(on), (that's not the Vgs threshold voltage, which is much lower).
This is typically 10V for standard MOSFETs and 5V or less for logic-level type MOSFETs.
That is the Vgs value you must use.
If you don't have info on the MOSFET, then use 10V.
 
Last edited:

dl324

Joined Mar 30, 2015
13,574
Is there something wrong with the MOSFET or the tests does not work with every MOSFET? The full set of steps are in the webpage https://www.engineersgarage.com/testing-mosfet-part-16-17/
What's wrong is the procedure given for testing the MOSFET on resistance.

What is the threshold voltage for the MOSFET? A part number would have been nice...

What is the voltage your meter applies to the gate? Is it high enough for the MOSFET to have a low on resistance? What is the maximum gate voltage tolerated by the device? It's unlikely that 9V could damage it, but it might be possible with logic level MOSFETs (e.g. NTA4151P that only tolerate 6V).
 

LowQCab

Joined Nov 6, 2012
1,379
You say You have a Data-Sheet .......
Find the Graph that shows the Rds-On vs Gate-Voltage.
believe what the Graph says, it will be very close.

Remember that Rds-On goes up as the temperature goes up.
( there's usually a Graph for that too )
.
.
.
 

crutschow

Joined Mar 14, 2008
28,227
Find the Graph that shows the Rds-On vs Gate-Voltage.
believe what the Graph says, it will be very close.
Why are you referring to that, when the TS is interested in the minimum Rds value?
The graph value may not be close, as it shows a nominal value and that will vary significantly between MOSFETs of the same type due to their large difference in Vgs threshold voltage.

The minimum Rds(on) value is measured with a fixed voltage as stated in the data sheet.
 

LowQCab

Joined Nov 6, 2012
1,379
I may have been sloppy and missed an important point in the dialog.

The absolute minimum Rds-On is seldom, if ever, achieved in practice,
and all the dynamic Factors that affect it are all but impossible to nail-down
with any significant repeatable precision.
.
.
.
 

MrSalts

Joined Apr 2, 2020
742
I may have been sloppy and missed an important point in the dialog.

The absolute minimum Rds-On is seldom, if ever, achieved in practice,
and all the dynamic Factors that affect it are all but impossible to nail-down
with any significant repeatable precision.
.
.
.
But the typical listed on most datasheets is commonly achieved on about 50% of parts.
 

LowQCab

Joined Nov 6, 2012
1,379
It "could be" important to know what the percentage of variation is.
I would hazard a guess and say ~2 to ~3% is a passing grade.

If hairs need to be split, I would suggest going to a more "over-rated" part,
rather than hoping that the questionable part will survive in a particular application.

This is something that I find hard to appreciate in many SMD designs where
many parts may be running on the ragged-edge of overheating.
.
.
.
 

crutschow

Joined Mar 14, 2008
28,227
Many parameters, such as the on-resistance are given as typical, and maximum and/or minimum (below for a BUZ10):
Note that this gives the on-resistance at a Vgs of 10V.

1631656305771.png
 

Audioguru again

Joined Oct 21, 2019
3,873
You do not use "diode test" to turn on or measure Vds of a Mosfet.

The article wrongly shows triggering the Mosfet on with the meter between the gate and the drain instead of correctly applying a needed voltage (usually 10V) between gate and source. The diode built in the Mosfet will reduce the actual Vgs by 0.6V.
My meter (Fluke) set to diode test does not produce enough voltage to light a white LED so it is less than 3V which will not turn on many Mosfets.
 

ronsimpson

Joined Oct 7, 2019
1,680
A real MOSFET tester, puts the device in the condition on the data sheet. (Vds=10V, ID=14A) for 10 to 100uS then off before the part heats up. I know because there are several with in arms reach of me right now. A home made test needs good Gate voltage and the resistance could be measured at a very low current. The Rds(on) might read differently but will give reasonable answers.

You can test for Gate turn on voltage by changing the Gate voltage while watching the resistance. Who can afford a curve tracer or a double pulse tester?
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
Thanks everyone for your replies! The MOSFET in question is FMV16N60E and the datasheet is in the link below:-
https://datasheetspdf.com/pdf-file/714069/FujiElectric/FMV16N60E/1

According to the datasheet, the Rds(on) reading should be between 0.4 to 0.47 ohms and this is based on ID=8A, VGS=10V.

But as mentioned in my post, the diode test between the drain and the gate did work as expected. The exact steps are as follows:-

negative probe -> Source
positive probe-> Drain
Reads OL (which is normal result)

move positive probe to Gate (just for a few seconds)
Reads OL (which is normal result)

move positive probe back to Source
Reads around 1V (instead of OL before)

The above works because the MOSFET was partially turned on but not fully on?

One more strange thing I noticed is that after trying to measure resistance across Source and Drain a few times, the above steps doesnt work any more and the Source <-> Gate diode test reads OL in both directions despite after touching the positive prove to the Gate (as per above). That seems very strange. How could a resistance test affect the MOSFET in such a way. After about half an hour or so, the above diode test would work again. The above MOSFET diode tests had been repeated many times and worked consistently every single time for this MOSFET and the only time the test fails and shows OL in both directions was after the resistance test between Gate and Drain. So, I am sure that the cause of the problem is the act of doing the resistance test itself. I repeated this effect several times.

Does anyone know of any reason behind the above behaviour in the MOSFET? It seems very strange though.

Thanks!
 

Papabravo

Joined Feb 24, 2006
17,318
Are you aware that besides being a semiconductor, the various node combinations consist of actual and parasitic capacitors. Question: what happens when your charge up a capacitor and then remove or eliminate any possible low impedance discharge path?

One more thing. Your method doesn't work for making measurements and I'd recommend not doing it anymore before it damages or (worse) compromises a part.
 
Last edited:

MrSalts

Joined Apr 2, 2020
742
Thanks everyone for your replies! The MOSFET in question is FMV16N60E and the datasheet is in the link below:-
https://datasheetspdf.com/pdf-file/714069/FujiElectric/FMV16N60E/1

According to the datasheet, the Rds(on) reading should be between 0.4 to 0.47 ohms and this is based on ID=8A, VGS=10V.

But as mentioned in my post, the diode test between the drain and the gate did work as expected. The exact steps are as follows:-

negative probe -> Source
positive probe-> Drain
Reads OL (which is normal result)

move positive probe to Gate (just for a few seconds)
Reads OL (which is normal result)

move positive probe back to Source
Reads around 1V (instead of OL before)

The above works because the MOSFET was partially turned on but not fully on?

One more strange thing I noticed is that after trying to measure resistance across Source and Drain a few times, the above steps doesnt work any more and the Source <-> Gate diode test reads OL in both directions despite after touching the positive prove to the Gate (as per above). That seems very strange. How could a resistance test affect the MOSFET in such a way. After about half an hour or so, the above diode test would work again. The above MOSFET diode tests had been repeated many times and worked consistently every single time for this MOSFET and the only time the test fails and shows OL in both directions was after the resistance test between Gate and Drain. So, I am sure that the cause of the problem is the act of doing the resistance test itself. I repeated this effect several times.

Does anyone know of any reason behind the above behaviour in the MOSFET? It seems very strange though.

Thanks!
your "method" is highly dependent on the meter used. The diode function on some meters attempt to apply a constant current (10uA for example) and report the voltage when the constant current is achieved (Fluke 87 IV). Others apply 1.5v through a resistor and report the voltage if different than 1.5v (many) and many more models of meters have their own methods which may or may not be described in detail in the datasheet. My point, if you are charging the gate "capacitor" with a Fluke 87 (or similar operating meter) you may be applying up to 4v and turning the mosfet on somewhat but not necessarily all the way. Other meters may apply only 1.5v to the gate (gate capacitor) and not turn it on at all or only to the "threshold level".
 

Audioguru again

Joined Oct 21, 2019
3,873
You cannot and should not measure the on-resistance of a Mosfet with a "diode test".

I tried that wrong method with a 3.3k ohms resistor and my Fluke multimeter. The diode test put 0.0003A through the resistor and showed the same 1V that your Mosfet drain to source voltage showed. How much is your Mosfet turned on when its on resistance is 3300 ohms?? What voltage is used in the "diode test" for your multimeter? Probably less than 3V so the Mosfet is only barely turned on since it needs a Vgs of 10V for it to be fully turned on.

Why are you using a 600V/16A power Mosfet and testing it with only 1V and 0.0003mA?

Why do you think that the reading of OL was wrong when diode-testing the gate-source of the Mosfet? The gate-source is not a diode and it has infinite resistance of its capacitance.
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
You cannot and should not measure the on-resistance of a Mosfet with a "diode test".

I tried that wrong method with a 3.3k ohms resistor and my Fluke multimeter. The diode test put 0.0003A through the resistor and showed the same 1V that your Mosfet drain to source voltage showed. How much is your Mosfet turned on when its on resistance is 3300 ohms?? What voltage is used in the "diode test" for your multimeter? Probably less than 3V so the Mosfet is only barely turned on since it needs a Vgs of 10V for it to be fully turned on.

Why are you using a 600V/16A power Mosfet and testing it with only 1V and 0.0003mA?

Why do you think that the reading of OL was wrong when diode-testing the gate-source of the Mosfet? The gate-source is not a diode and it has infinite resistance of its capacitance.
I was following the steps in https://www.engineersgarage.com/testing-mosfet-part-16-17/ to test the MOSFET. There are similar test found on the web also using diode mode. From everyone's replies, I understand that this test may work sometime but the Rds(on) cannot be measured in this test. Thanks.
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
your "method" is highly dependent on the meter used. The diode function on some meters attempt to apply a constant current (10uA for example) and report the voltage when the constant current is achieved (Fluke 87 IV). Others apply 1.5v through a resistor and report the voltage if different than 1.5v (many) and many more models of meters have their own methods which may or may not be described in detail in the datasheet. My point, if you are charging the gate "capacitor" with a Fluke 87 (or similar operating meter) you may be applying up to 4v and turning the mosfet on somewhat but not necessarily all the way. Other meters may apply only 1.5v to the gate (gate capacitor) and not turn it on at all or only to the "threshold level".
I was using Fluke 107 and the manual states that "Typically, the open circuit test voltage is 2.0 V and the short circuit current is < 0.6 mA" for diode mode. I think 2V is not enough and the mosfet is not turned on completely. Thanks.
 

Thread Starter

electron_jj

Joined Aug 28, 2021
14
As i do not have the necessary tools and equipment to fully test the mosfet, I do not want to risk putting it back to the circuit fearing that a damaged or faulty mosfet may do further harm to the circuits when turned on. The mosfet was from a smps board in a LED TV and it is a used as a high frequency switcher to feed the output to a transformer.

I have searched and found a potential replacement (TK25E60X) for this mosfet (FMV16N60E). The parameters I believe are as close as possible to existing mosfet FMV16N60E, they both have the same Vds and Vgs, but a higher Id and Pd and a reduced Rds which I understand are fine. The minimum and maximum gate thresholds are also the same. Are there any other parameters that also needs to match the old mosfet? The two datasheets are attached. Thanks!
 

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