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Old 28th September 2006, 11:53 PM   #1
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Default How to select MOSFET's

What is the best way to weed through all the MOSFET selections available? Seems to me that turn-on/off time is important because if it's short then the dead-time can be programmed short giving decent distortion (hey, is that a run-on sentence?) Anyway, if true, then how short is good?

I'm sure that Rds is important too, with low being better for minimal losses and heat generation. But, with Rds in the range of sub 100milliOhms or so, how low is really important?

Seems that gate charge is important too. I think it affects the turn on time, but am not certain how to use this.

Can someone put me on the right path?

thanks
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Old 29th September 2006, 12:05 AM   #2
BWRX is offline BWRX  United States
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Here are some of the important characteristics to look for: on state resistance, gate threshold voltage, total gate charge, turn on and rise time, turn off and fall time, reverse recovery charge, and reverse recovery time. Of course don't forget about the drain source voltage and drain current ratings. Pay careful attention to conditions used to generate some specs. Not all manufacturers use the same conditions and thus some characteristics between mosfets can't always be compared directly.

A good mosfet to check out is the STP14N10FP. It's the one used in the UCD180s and has some very good specs. While there are others that excel in certain areas, it's not terribly easy to find other mosfets that have such good overall numbers.
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Old 29th September 2006, 12:44 AM   #3
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I dont' know if I'd recommend that as being a great mosfet, maybe a generation or two ago.

RDSon is pretty high!

Best you can do is start hitting app notes and figuring it out the hard way, it's not an easy thing to answer.

What BRWX mentioned isn't all encompassing.

Start digging for some power mosfet application notes I say, buck converter info is most relevant here.
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Old 29th September 2006, 12:50 AM   #4
BWRX is offline BWRX  United States
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Quote:
Originally posted by classd4sure
I dont' know if I'd recommend that as being a great mosfet, maybe a generation or two ago.

RDSon is pretty high!
I never said it was great, just that it had some very nice specs. Rdson is a little high, but that's all part of the trade off Post filter feedback enables you to pick mosfets with higher Rdson and better switching characteristics.
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Old 29th September 2006, 01:15 AM   #5
fokker is offline fokker  China
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Quote:
Originally posted by BWRX
Here are some of the important characteristics to look for: on state resistance, gate threshold voltage, total gate charge, turn on and rise time, turn off and fall time, reverse recovery charge, and reverse recovery time. .
I have no doubt that those attributes are important. However, are they important enough to make a real life difference?

Alternatively, are the run-off-the-mill type mosfets good enough for switching applications in audio class D amps?
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Old 29th September 2006, 01:39 AM   #6
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I'm sure a book could be written on this, but how do you use this information? I will start looking for app notes, but maybe someone can point me at some good ones?
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Old 29th September 2006, 01:49 AM   #7
BWRX is offline BWRX  United States
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There's no simple answer to that question. As with anything, you have to look at the whole picture when selecting components. What you look for ultimately depends on your design goals and budget (unfortunately, this is true for most of us).

Post filter feedback can correct for some things like higher Rdson but it certainly wouldn't hurt if the STP14N10FP had lower Rdson!

If you want an amp with a lot of power you'll probably want to use mosfets with very low Rdson. The downside is that they usually have a higher gate threshold voltage and much more gate charge which means they need a more capabale gate drive circuit to switch them fast.

These compromises are a result of the way the devices are fabricated. A little research into semiconductor physics will shed some light onto that topic.
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Old 29th September 2006, 02:07 AM   #8
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Quote:
Originally posted by BWRX
A little research into semiconductor physics will shed some light onto that topic.
ugh, not semi physics

I had a hard time with that one in college!!!!
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Old 29th September 2006, 02:09 AM   #9
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I have a whole folder reserved for such app notes....

AN-936
AN-937
AN-9010

Some reading on "Cdv/dt induced turn on in buck converter" would be most useful. Search google.

Also research the reverse recovery effects in buck converters

AN7019 is very interesting

AN1005 deals with avalanche breakdown as does
AN10273_1

Then you'll be familiar with alot of the problems in design with them for such an application.

Other papers like perhaps the class d basics tutorial from IR might be a quick list of a few factors that need to be optimal, and its' a good read anyway.

Minimal output capacitance is important, Cdv/dt induced turn on immunity ..... by evaluating the gate charge ratio's around Vth, could prove worthwhile, if you research that you'll find differing views on it though, some are more accurate than others. Fully avalanche rated... its a topic worthy of a mean headache but when it's all said and done take your best educated guess order up some free samples and give it a shot.

Just going with a good recommendation can get you startequickly, but if something goes wrong.. it pays to be aware of the above.

Sorry I can't just give you the links, but like I said I save them all to a folder so..
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Old 29th September 2006, 02:21 AM   #10
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I second fokker's question, or rather paraphrase:
How bad is it to use in class d an industry standard inexpensive mosfet? Like, say IRF540N ?
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