Mosfet Drain Source Resistance

[tazelikechickn]

Hey Guys,

I am trying to repair a SMPS out of a computer. I have found the Mosfet on the primary side is Kaput! Main problem is trying to get one the same. I have located a few that are similar, but I am unsure about the drain resistance. The Mosfet I have is a HD2N60 and I can get a similar one, but the drain resistance is different. the HD2N60 has 6 Ohms max and the one I am looking at (NDF02N60ZG) has a drain resistance of 4.8 Ohms. Is this enough to cause a problem?

Cheers

[TheRuler8510]

I would bet that is no problem, but what about the other specs? IV curves, breakdown, threshold voltage, etc.

At first glance I can't find a datasheet for HD2N60. I'll look again.

[theatrus]

Things to watch out for:

Often when RdsOn drops, the gate charge requirement goes up. If the power switcher can't drive the gate as fast it will dissipate quite a bit more power on each edge.

Some driver setups use the RdsOn as an (imperfect but acceptable) current limit shunt.

[tazelikechickn]

Hey Guys,

Thanks for your reply

Here is the data sheet for the HN2N60. http://www.alldatasheet.com/datasheet-pdf/pdf/18509/PHILIPS/PHD2N60E.html
and here is the one for the MOSFET I am looking at to replace it. (NDF02N60ZG) It is the closest specs I can find http://docs-asia.electrocomponents.com/webdocs/0edb/0900766b80edb286.pdf

They seem to compare OK apart from the drain resistance, which I don't know enough about.

Cheers

[TheRuler8510]

The spec I am most worried about is the threshold voltage. I think you may need to know a little more about how it is being used. If it is being driven high enough that there is margin in the current being switched, then it is not a problem. I would hope that it was designed with margin.

Bottom line, maybe a problem, maybe not. Search Digikey for a better solution first--or just roll the dice and try it.

Here are the IV plots from your datasheet, when gate voltage is X-volts, there drain currents are different:

[TheRuler8510]

One more suggestion: Try it...Then measure the temperature of the MOSFET. As long as it is a reasonable temperature, and the power supply works, then I think you have an easy fix.

[tazelikechickn]

Hey TheRuler8510,

Thanks for your good advice. That's pretty much what I had in mind. Measure the temperature and that should tell if the resistance is an issue.

It's all a learning curve!

Cheers