mosfet question

[amplifierfreak]

Ok I'm not sure if I can use an IRF540N in place of an IRF540. I did notice looking at datasheets the IRF540 has more resistance within...Just asking because I can't seem to find any IRF540's.

Thanks,
Chris

 

[Hero999]

It depends on whether the higher on resistance will be a problem.

What current are you switching?

 

[Optikon]

Ok I'm not sure if I can use an IRF540N in place of an IRF540. I did notice looking at datasheets the IRF540N has more resistance within...Just asking because I can't seem to find any IRF540's.

Thanks,
Chris

Can't find any?

https://octopart.com/search?q=IRF540

 

[mneary]

Chris,

Check the resistance again. I have only looked at the IR data sheets, but the IRF540 has a resistance of 0.077 and the IRF540N is 0.044 ohms.

regards,
Mike

 

[amplifierfreak]

Hey thanks for posting that link! mneary that's what i found. I was just curious if that resistance would play a big factor in a circuit. Hero999 I will double check that here when I get some more time I can't remember for the life of me right now. I just have a couple minutes to jump on here and post this. Thanks for the responses!

 

[mneary]

A MOSFET design shouldn't depend on the maximum resistance of the device. A lower specified maximum resistance (especially 0.077 vs 0.044 ohms) should make absolutely no difference.

The data sheets don't even say what the typical or minimum resistance is.... if the circuit depends on those values, it's a bad design.

 

[Hero999]

Whilst I agree, there is a huge difference between 0.077hm: and 0.044hm:, at 20A the MOSFET will dissipate 30.8W instead of 17.6W which is a significant difference.

 

[zevon8]

I think you will also find that depending on who makes the part, there will be a difference in the RDSon. ST Micro has a version, it is about 55mOhm. Most will be close, but there will be differences.

 

[mneary]

Whilst I agree, there is a huge difference between 0.077 and 0.044, at 20A the MOSFET will dissipate 30.8W instead of 17.6W which is a significant difference.

Since the 'N' part (the one he can get) has the lower Rdson, in most applications this would be considered 'better' than the older one which he cannot get.

Ok I'm not sure if I can use an IRF540N in place of an IRF540.

 

[Hero999]

Since the 'N' part (the one he can get) has the lower Rdson, in most applications this would be considered 'better' than the older one which he cannot get.

I would agree. The only time it would be a problem is if the circuit uses the MOSFET's resistance as a current sense resistor for overcurrent protection.

 

[Speakerguy]

Are MOSFET Rds on charactaristics that stable? I'd never heard of using them that way. I know most specs are pretty good, but there's always variability (I guess in overcurrent situations 20-50% mightbe tolerable, depending on other stuff and whatnot).

Also, you should always use the worst-case Rds on when selecting a FET. Kinda like using the worst-case beta when using a transistor in a circuit.