Bob Cordell Interview: BJT vs. MOSFET - Page 10 - diyAudio
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Old 22nd November 2006, 04:21 AM   #91
mzzj is offline mzzj  Finland
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Quote:
Originally posted by ilimzn



Interestingly:
IR IRFP140: 3.1A
IR IRFP140N: 2A (it is specced 140W though).
As you may know, -N version is modern design/process with smaller chip, usually this comes with lower peak power dissipation.
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Old 22nd November 2006, 04:35 AM   #92
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Workhorse
Look at this most powerful BJT on this earth

MJ21294 SOA at 10mS at 100V= 10Amperes
The most powerfull?

Like ST's BUF460AV BJT with 30A 100V DC, 150V 200A 1ms ?

(OK, maybe not best choice for audio...)
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Old 22nd November 2006, 01:11 PM   #93
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Originally posted by mzzj


And another article, http://www.microsemi.com/micnotes/APT0002.pdf



"Power MOSFETs are generally designed as switches. Total power dissipated is the sum of "on"-state losses plus losses generated during the very short switching intervals. Desirable characteristics include the lowest possible ON resistance, high breakdown voltage, very high gain (g or Gm ), minimum switching losses, and a low gate threshold voltage VGS(th)

In those applications requiring operation in the linear region, these characteristics are not ideal. Firstly, Gm is too high and, secondly VGS(th)has a negative temperature coefficient which makes it impossible to maintain constant drain current without negative feedback. Finally, and most dangerous of all, large switchmode MOSFETs exhibit a phenomenon known as "hot spotting" or current tunnelling."

Thanks for the reference. I'll read it in detail. My initial impression is that these guys are power jocks and know little about audio amplifiers. Their comment that MOSFETs have more gm than desirable for linear applications is patently stupid.

Bob
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Old 22nd November 2006, 01:15 PM   #94
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Quote:
Originally posted by mzzj


As you may know, -N version is modern design/process with smaller chip, usually this comes with lower peak power dissipation.

I recommend you stay focused on the IRFP-240 for comparisons. My data is based on brand-new IRFP-240 devices.

Bob
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Old 22nd November 2006, 01:18 PM   #95
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Quote:
Originally posted by mzzj


The most powerfull?

Like ST's BUF460AV BJT with 30A 100V DC, 150V 200A 1ms ?

(OK, maybe not best choice for audio...)

To be fair, the comparisons should all be made at a 10 ms pulse time. The numbers you quoted are for 1 ms, which will inevitably be larger.

Bob
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Old 22nd November 2006, 03:28 PM   #96
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Bob Cordell



To be fair, the comparisons should all be made at a 10 ms pulse time. The numbers you quoted are for 1 ms, which will inevitably be larger.

Bob

Thats why I gave DC-rating also, 30A 100V.
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Old 22nd November 2006, 03:32 PM   #97
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Quote:
Originally posted by Bob Cordell



To be fair, the comparisons should all be made at a 10 ms pulse time. The numbers you quoted are for 1 ms, which will inevitably be larger.

Bob
Hello Bob,

What's your opinion about this one

APT20M22LVR N-Channel Vertical Mosfet
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Old 22nd November 2006, 04:09 PM   #98
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Bob Cordell


... these guys are power jocks and know little about audio amplifiers.

Bob
Usually I consider that as a good thing, somewhat less snake-oil business

Link that Nuvistor gave is better reading than APT's somewhat shorty explanation:

http://powerelectronics.com/mag/511PET24.pdf

(related stuff, lateral vs. trench-mos)
http://powerelectronics.com/mag/Ely%...2004%20PET.pdf
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Old 22nd November 2006, 04:20 PM   #99
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Quote:
Originally posted by mzzj



Thats why I gave DC-rating also, 30A 100V.

That would be 3000 watts of dissipation at DC. I seriously doubt it.

Bob
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Old 22nd November 2006, 04:24 PM   #100
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Quote:
Originally posted by Workhorse


Hello Bob,

What's your opinion about this one

APT20M22LVR N-Channel Vertical Mosfet

The 10 ms safe area line on this transistor is at 2500 watts - very impressive. This would appear to be 8 times the 300-watt rating for the IRFP 240. But I don't know anything else about this transistor, for example, how big is the die, how big are the capacitances, how much does it cost, does it have a p-channel complement?

Bob
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