Author Topic: Testing MOSFET RDS or Resistance between Drain and Source  (Read 1122 times)

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Offline ThermallyFrigidTopic starter

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Testing MOSFET RDS or Resistance between Drain and Source
« on: July 24, 2021, 02:05:25 pm »
Hello Electromates,

I am needing to test a batch of Mosfets that just arrived.
I'm wanting to test the Resistance between Drain and Source.

Below is the DataSheet for one of the MOSFETS in question.

For RDS(on) it lists the following......
VGS=10v, ID=10A with a typical reading being .22 ohms and a Max ohms being 0.27 ohms.

The DataSheet does not include a schematic for testing RDS(on), only the parameters.

So, making sure I understand this, it means apply 10v between the Gate and Source Pins, with current at the Drain being 10Amps ?

This circuit is correct for this test?



DataSheet

 

Offline bdunham7

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #1 on: July 24, 2021, 02:19:11 pm »
Yes, that's the general idea, but the test is typically a short pulse (you can pulse VGs with an AWG) because the DUT will be dissipating up to 27 watts. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline David Hess

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #2 on: July 24, 2021, 04:20:12 pm »
In your case the only reason to apply 10 amps to the drain is to make a more sensitive measurement.  A lower current will be sufficient.
 
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Offline ThermallyFrigidTopic starter

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #3 on: July 24, 2021, 04:42:00 pm »
In your case the only reason to apply 10 amps to the drain is to make a more sensitive measurement.  A lower current will be sufficient.

The only thing is......

That is the specified current for the RDS test.  Wouldn't they have published a lower current if applicable?
 

Offline bdunham7

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #4 on: July 25, 2021, 12:16:28 am »
The only thing is......

That is the specified current for the RDS test.  Wouldn't they have published a lower current if applicable?

The reason they use the term RDS(on) is that it should be relatively constant over a range, within reason.  Yes, they have specified at at 10A, but as I said, you wouldn't have very long to take your reading at that level.  You should get a similar reading at 1A, for example, but with 100X less heat generated.  But if you can, try it both ways and report back on the differences. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline ThermallyFrigidTopic starter

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #5 on: July 25, 2021, 01:51:00 am »
The only thing is......

That is the specified current for the RDS test.  Wouldn't they have published a lower current if applicable?

The reason they use the term RDS(on) is that it should be relatively constant over a range, within reason.  Yes, they have specified at at 10A, but as I said, you wouldn't have very long to take your reading at that level.  You should get a similar reading at 1A, for example, but with 100X less heat generated.  But if you can, try it both ways and report back on the differences.

Will do.

I am wondering......would a circuit for testing one N channel MOSFET work for most other N channel MOSFETS ?
 

Offline graybeard

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Re: Testing MOSFET RDS or Resistance between Drain and Source
« Reply #6 on: July 26, 2021, 05:02:29 pm »
I am wondering......would a circuit for testing one N channel MOSFET work for most other N channel MOSFETS ?

Yes, so long as they are MOSFETS are big enough to not be overstressed by your test.

For the resistances you are measuring you should make Kelvin connections to the MOSFET; the voltage sense leads should be separate from your current force leads.
 
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