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
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
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.
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.
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.
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.
classd4sure said:
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.
BWRX said:
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?
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?
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.
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.
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.. 😕
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.. 😕
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 ?
How bad is it to use in class d an industry standard inexpensive mosfet? Like, say IRF540N ?
It's not bad, but you can do better 🙂 I have a really good pdf file from IR about all this stuff but it's 1.3MB. Anyone care to host it?
Edit: A little searching and I found that it's still online here: http://www.irf.com/product-info/audio/classdtutorial.pdf
Their app notes page is a very good source of info too, as Chris pointed out.
http://www.irf.com/product-info/audio/audioappnotes.html
Edit: A little searching and I found that it's still online here: http://www.irf.com/product-info/audio/classdtutorial.pdf
Their app notes page is a very good source of info too, as Chris pointed out.
http://www.irf.com/product-info/audio/audioappnotes.html
"does it make a real world difference"
Hmmmmmmm.... why do you suppose they used to say class d was only good for subwoofer amp use?
Yes it makes a world of difference. You can still use old school industrial type mosfets for low power, phase 1 testing. After you're done blowing those up, put some good ones in and go for some real efficiency/reliability.
Hmmmmmmm.... why do you suppose they used to say class d was only good for subwoofer amp use?
Yes it makes a world of difference. You can still use old school industrial type mosfets for low power, phase 1 testing. After you're done blowing those up, put some good ones in and go for some real efficiency/reliability.
BWRX said:
This app note has all the info I was looking for, thanks. Turns out I've read it already, but completely forgot about it 😀 One thing sticks out though - it's not clear in the doc whether they mean rms or peak. For example, they calculate the BV recommendation based on output power and load - but no mention if it's rms or peak. Peak makes more sense to me.
classd4sure said:
I am sorry how "low power" was your low-power amp? 1mw? 1w? 1kw?
and what is a typical amp that we build? 1mw? 1w? 1kw?
maybe if we establish a common baseline we can have a real discussion, not a conceptual one where we talk about the theoretical merits of better / best specs.
fokker said:
I don't know why you insist on having the obvious spelled out for you, but I can't hold your hand.
Gosh ... what an eye opener ...
" ... AN-9010 ... Some reading on "Cdv/dt induced turn on in buck converter" would be most useful. ..."
Actually reading the entire application notes from Fairchild on the -9010 or the others will clarify most of the definations and procedures in selection & design using MOSFETs (Metal Oxide Semiconductors / Field Effect Transistors) ... the physics is sometimes overwhelming, but Fairchild's clarity of information and diagrams of what is going on here will carry any one through the deep stuff.
A great read: http://www.fairchildsemi.com/an/AN/AN-9010.pdf
Example:
"1) Off State
(1) BVDSS: This is the maximum drain-to-source voltage where the MOSFET can endure
without the avalanche breakdown ..."
" ... AN-9010 ... Some reading on "Cdv/dt induced turn on in buck converter" would be most useful. ..."
Actually reading the entire application notes from Fairchild on the -9010 or the others will clarify most of the definations and procedures in selection & design using MOSFETs (Metal Oxide Semiconductors / Field Effect Transistors) ... the physics is sometimes overwhelming, but Fairchild's clarity of information and diagrams of what is going on here will carry any one through the deep stuff.
A great read: http://www.fairchildsemi.com/an/AN/AN-9010.pdf
Example:
"1) Off State
(1) BVDSS: This is the maximum drain-to-source voltage where the MOSFET can endure
without the avalanche breakdown ..."
OK, so Qg is important and needs to be low. How come there's no mention of Tr, Tf, and Ton and Toff? Everything I've read says that to minimize distortion, you have to keep the dead-time low, but without allowing shoot-through. Doesn't the Ton and Toff switching info dictate the dead-time?
If I am right, it takes the Ton + Trise for a fet to be fully on. Same for the on-to-off state, just Toff+Tfall. Right so far? To prevent shoot-through, the gates need to be staged properly (i.e. dead-time).
So, if (Ton + Tr) = (Toff + Tf) = x, then I need x dead-time. When x = 0, no dead-time (ideal case). But what happens when x is non-zero? Is it proper to wait the worst-case (Ton + Tr), or (Toff + Tf), which could be very long time. Or, is it ok to allow some cross-conduction between the high and low side FETs? Say, 1/2 way for example, into the (Ton + Toff). Seems it would heat up the fets.
If I am right, it takes the Ton + Trise for a fet to be fully on. Same for the on-to-off state, just Toff+Tfall. Right so far? To prevent shoot-through, the gates need to be staged properly (i.e. dead-time).
So, if (Ton + Tr) = (Toff + Tf) = x, then I need x dead-time. When x = 0, no dead-time (ideal case). But what happens when x is non-zero? Is it proper to wait the worst-case (Ton + Tr), or (Toff + Tf), which could be very long time. Or, is it ok to allow some cross-conduction between the high and low side FETs? Say, 1/2 way for example, into the (Ton + Toff). Seems it would heat up the fets.
Take a look at the old STW34NB20, that's cheap, lo Qc, lo RDSon, easy to drive and a VERY good rise time and fall time for that price, the only drawback is the slow body diode...But I'm sure that lot of people know how to overpass this...That's what I use in the 4000HVI...2Kw per side with 4 of these in full bridge configuration.
Fredos
www.d-amp.com
Fredos
www.d-amp.com
fredos said:
Not sure what you think is cheap, but 1pc price is around 4 bucks
I've seen very good specs on other devices for at least 1/2 that price. Having said that, I'm still at a loss on how to figure out the dead-time needed based on my prior post. Any ideas?
gene
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