MOSFET testing question

[Scruit]

I am testing an NPD603AL (N-Channel Logic Level Enhancement Mode Field Effect Transformer) from an old Comapq Proliant 1500 CPU daughterboard. Symptoms are no post / no video, with only the psu / blackplane / cpu daughterboard and CPU in place.

First question: This is not explicitly listed in the datasheet title as a MOSFET, however in the description it states it uses D-MOS technology. I assume this means it's just a MOSFET, and the "logic level" means it only needs 5v gate-source charge instead of 10v. Is that correct?


I am testing this because when the CPU is in the socket there is no POST but this FET get VERY hot. Within a minute the heatsink will burn your finger. With the CPU removed the FET does not get hot at all.

When I tested the FET on the board I found drain-source was shorted with the power off. I removed it from the board to isolate. Motherboard showed no short between the empty source and drain pads after the FET was removed.

Measuring the FET with multimeter in diode mode:

• Source(+) and drain(-) the source I get the 0.5v diode reading as expected.
• Source(-) and drain(+) I expect to see OL but got 1.7v.

Follow-up resistance testing showed OL in both directions. So I'm a little confused. I would expect the drain-source to be fully shorted and read close to zero ohms.

Second question: Could this be a partial failure? (Not fully shorted) or is this FET allowed to read 1.7v in this direction?

Maybe it was initially reading a short on the board because the FET got so hot it shorted, and after it cooled down it is not a complete short? Or maybe there was some residual voltage at the gate? (board was out and on my desk though)

[sam_sam_sam]

Re: MOSFET testing question

I pretty sure that it is a mosfet because it S/D/G which mosfets have this labels for the pins

“ Measuring the FET with multimeter in diode mode:
Source(+) and drain(-) the source I get the 0.5v diode reading as expected.
Source(-) and drain(+) I expect to see OL but got 1.7v.”

You should have no short from the source and drain pins if this were the case it would in the on state continuously which is not what you want the gate pin is what turns the device on

Now what could have happened is that the device was still in the on position and when you removed it you might have shorted the pins together and now it is in the off position which is my guess to why you saw these readings

[Scruit]

Re: MOSFET testing question

I pretty sure that it is a mosfet because it S/D/G which mosfets have this labels for the pins

Now what could have happened is that the device was still in the on position and when you removed it you might have shorted the pins together and now it is in the off position which is my guess to why you saw these readings

Thank you for your response.

At this point the mosfet is on my bench, disconnected from anything. I still measure 1.7v from drain to source, even after shorting the legs.

Another observation: If I short all the legs and then measure drain(+) and source(-) then it will count DOWN from 1.9v to 1.7v then stop. If I touch the gate with the positive probe for half a second then move it to the drain then it counts UP from 0.5v to 1.7v.

[keeney123]

Re: MOSFET testing question

I am testing an NPD603AL (N-Channel Logic Level Enhancement Mode Field Effect Transformer) from an old Comapq Proliant 1500 CPU daughterboard. Symptoms are no post / no video, with only the psu / blackplane / cpu daughterboard and CPU in place.

First question: This is not explicitly listed in the datasheet title as a MOSFET, however in the description it states it uses D-MOS technology. I assume this means it's just a MOSFET, and the "logic level" means it only needs 5v gate-source charge instead of 10v. Is that correct?


I am testing this because when the CPU is in the socket there is no POST but this FET get VERY hot. Within a minute the heatsink will burn your finger. With the CPU removed the FET does not get hot at all.

When I tested the FET on the board I found drain-source was shorted with the power off. I removed it from the board to isolate. Motherboard showed no short between the empty source and drain pads after the FET was removed.

Measuring the FET with multimeter in diode mode:

• Source(+) and drain(-) the source I get the 0.5v diode reading as expected.
• Source(-) and drain(+) I expect to see OL but got 1.7v.

Follow-up resistance testing showed OL in both directions. So I'm a little confused. I would expect the drain-source to be fully shorted and read close to zero ohms.

Second question: Could this be a partial failure? (Not fully shorted) or is this FET allowed to read 1.7v in this direction?

Maybe it was initially reading a short on the board because the FET got so hot it shorted, and after it cooled down it is not a complete short? Or maybe there was some residual voltage at the gate? (board was out and on my desk though)

Just a question of the part number. You are saying that you are testing a "NPD"603AL but, the data sheet is for "NDP"603AL is this just and error or are they actually different numbers?