What do you look for in a MOSFET to replace with another?

[orientalsniper]

I have this motherboard I'm working with, MOSFET failed test, but I have some boards laying around and I would like to use one of those.

For example, the schematic says it's using IRF8113PbF and AO4704 is installed, I checked the ratings, none matches but the V (30V) and type channel.

So what criteria do you use to look for equivalent MOSFET? Thanks.

[orientalsniper]

Re: What do you look for in a MOSFET to replace with another?

I think I found one possible answer from this post:

I had to deal with just such a repair job...


So the voltage can be higher, it's just a maximum rating, but the most important thing is that the RdsON matches or exceeds the spec. So it should be lower at equivalent draw.

The AO4710 has a typical RdsON of 11.7 mOhms @ 4.5V 11A, while the Vishay si4712dy has a rating of 13 mohms @ 4.5V 10A. So the Vishay part is actually slightly inferior. If you tell me which site your ordering from, I might be able to find you something better.

But I'm still not exactly sure how he could tell the lower rating? Did he calculate 11A/11.7 against 10A/13?

[spleenharvester]

Re: What do you look for in a MOSFET to replace with another?

Lower resistance = better, and I=V/R so perhaps the lower resistance means it can take a higher current?

Just a wild stab in the dark, I know almost nothing about mosfets myself.

Though those specs look pretty close, I imagine the difference is probably negligable unless they're being loaded quite highly.

[orientalsniper]

Re: What do you look for in a MOSFET to replace with another?

Thanks for the input, I'm going to be experimenting in the meantime.

[mariushm]

Re: What do you look for in a MOSFET to replace with another?

Input capacitance is somewhat important.
Vgs is very important, should be close to the others.

Another thing that matters is the efficiency at 90% and at a particular power dissipation (usually around 10w) - doesn't matter that a mosfet can do 11.7 mOhms @ 4.5V 11A if it dissipates 20w to do those 11A, because the board may only handle 7-10w of power dissipation.

See this video, hope it clears some things:

Motherboard VRM Explanations Part 3 IR3550 versus other MOSFETs.mp4
http://www.youtube.com/watch?v=6Fl1iFtOLKU

parts 1 and 2 are also worth watching.

[mmartell]

Re: What do you look for in a MOSFET to replace with another?

This was posted in a thread I started (cheers to that person..)

http://alltransistors.com/

[mariushm]

Re: What do you look for in a MOSFET to replace with another?

And what does it have to do with mosfets? It's like linking to youtube ... yeah it's a great site but very unrelated to this thread.

[mmartell]

Re: What do you look for in a MOSFET to replace with another?

What do you mean what does it have to do with mosfets ?? If he's looking to use an equivalent device then he can use the site and see if he has any other matching devices he can cull from another board.

If this is a purely technical discussion then excuse me for trying to help.

[mariushm]

Re: What do you look for in a MOSFET to replace with another?

That site does have a section about mosfets and finding others with similar parameters, but it's somewhat stupid and you really can't use it to find VRM mosfets.

For example, the form there shows you this:

Struct =

Polarity =

Pd > W
Uds > V
Ugs > V
Id > A
Tj > C
tr < nS
Cd < pF
Rds < Ohm
Caps =

But here's for example a common VRM mosfet: http://www.digikey.com/product-detai...SCT-ND/2705206

Drain to Source Voltage (Vdss) 30V
Current - Continuous Drain (Id) @ 25°C 8.8A (Ta), 58.5A (Tc)
Rds On (Max) @ Id, Vgs 6.95 mOhm @ 30A, 10V
Vgs(th) (Max) @ Id 2.5V @ 250µA
Gate Charge (Qg) @ Vgs 25nC @ 11.5V
Input Capacitance (Ciss) @ Vds 1400pF @ 12V
Power - Max 870mW

Now what Id do you think I should put on the site? 8.8A at ambient, 56.5A at who the fuck knows what temperature? The 56a on this part is on datasheet at a specific unrealistic temperature and power dissipation level, the site uses values in the database extracted from datasheets, with other unrealistic, stupid use cases.
Rdson at what voltage and current? It just says Rdson < x Ohms
How much current at a reasonable power dissipation (this mosfet says 58.5A but that's at 20-30w power dissipation, unrealistic on a surface mount component... in practice the motherboard will push it to 20-25A... but I can't just enter in the for 60A because I may get mosfets that do 60A at 40w power dissipation, or just 10-15a at the same power dissipation as this mosfet)

You get hundreds of results that won't be suitable (i got 1715 results after trying to fill the form with values above as best as possible)... it's basically too dumb to make your life easy, to find compatible mosfets.

[selldoor]

Re: What do you look for in a MOSFET to replace with another?

I generally google "AO?????? Equivalent" or substitute or replacement
for a quick hit.

Today I spotted this which may help many

http://www.google.co.uk/url?sa=t&rct...60157871,d.ZGU