Better Mosfets for Power Follower - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th September 2006, 02:58 AM   #1
drdagor is offline drdagor  United States
diyAudio Member
 
Join Date: Mar 2004
Location: Seattle
Default Better Mosfets for Power Follower

Pavel Macura's MOSFET power follower circa 2002 and updated 2005 showed an IRFP350 in the circuit. In the text Pavel talks about input capacitance as a limiting factor in the MOSFET. The IRFP350 has input capacitance above 1000 pf

I know that in the past some of the device makers have marketed MOSFETs specifically for audio with low input capacitance. Now it looks like the Class D craze has finally gotten chip makers serious about audio MOSFETs.

In looking through the International Rectifier site I cam across a couple of devices that look like the might be just the ticket for getting the last ounce of performance out of a Class A power follower. Take a look at a couple of these devices. Any of these look particularly tasty for Class A operation?

IRFB4019
http://www.irf.com/product-info/data...rfb4019pbf.pdf

IRF6665
http://www.irf.com/product-info/data...ta/irf6665.pdf

Any other new MOSFETs out there that look really good for Class A?
__________________
Dr. Dagor
  Reply With Quote
Old 28th September 2006, 03:22 AM   #2
diyAudio Member
 
Join Date: Mar 2001
Location: Auburn, WA (somewhere between Seattle and Tacoma)
Blog Entries: 1
Send a message via ICQ to Damon Hill
These look good for Class D operation, but can't take the dissipation for very much power in Class A...
  Reply With Quote
Old 28th September 2006, 12:02 PM   #3
sajti is offline sajti  Hungary
diyAudio Member
 
Join Date: Apr 2003
Location: Vác, Hungary
Check the IRFP250N, or 150N!

sajti
  Reply With Quote
Old 28th September 2006, 01:32 PM   #4
ilimzn is offline ilimzn  Croatia
diyAudio Member
 
Join Date: Feb 2005
Location: Zagreb
Input capacitance is not the only limit for a follower - rather, it's a combination fo input capacitance and gm.
This is because in a follower, the Cgs part of the input capacitance appears reduced 1+(gm*Rload) times to the signal source for the follower. The Cgd component remains constant (here I am disregarding capacitance nonlinearity which is Vds / Vgd dependant).

So, in essence, you need a MOSFET with minimum Cgs/gm + Cgd. The thing is, higher gm MOSFETs within the same basic technology, tend to have a higher maximum current, and proportionally higher C, because they are internally just more of the same kind of cell in parallel. Within a certain technology, C/gm remains roughly constant. Still, this 'constant' varies somewhat, mostly with maximum Vds, and this is where you optimize, unless you want to use different cell technology (for instance, IRFP240, IRFP240B, IRFP240N all have different cell technologies and the c/gm constant is different).
Another way to minimize the C/gm is to use a small MOSFET nearer to it's current limit, as gm increases with current, while Cin remains roughly the same. Of course, this has other repercussions, mostly to reliability, but is also one of the reasons why it is often said that MOSFETs sound best in class A or at least high bias AB.
  Reply With Quote
Old 29th September 2006, 12:15 AM   #5
drdagor is offline drdagor  United States
diyAudio Member
 
Join Date: Mar 2004
Location: Seattle
Ilimizn has illuminated the matter. And I understand. Still, a couple of these Mosfets specifically designed for class D amps seem to have much better specs in areas where it counts for source followers. The point about power handling is important... These might be limited to 10 watt head phone amps. The 6669 doesn't dissipate enough power to be useful for much more than driving a pair of 500 ohmish things.

As long as the think was kept within power ratings, the IRF6665 outperforms anything else I've seen on the source follower related capacitance. But it is a relatively low power device..
Only 42 watts.
__________________
Dr. Dagor
  Reply With Quote
Old 29th September 2006, 12:36 AM   #6
diyAudio Member
 
Join Date: Mar 2001
Location: Auburn, WA (somewhere between Seattle and Tacoma)
Blog Entries: 1
Send a message via ICQ to Damon Hill
Not being half as familiar with Class D design as I should (I used to test and repair switchmode power supplies), I assume the specs are pretty good >for< Class D operation where the devices shouldn't be operated in any other mode, else they'll cook.

I sort of suspect that any attempt to operate in Class A or even AB won't find them at their best, no matter how much heatsinking is applied. Alas, more suitable devices with higher dissipation will have to be chosen.

Don't have any Class D projects on my to-build list, though I'm interested in the subject.
  Reply With Quote
Old 29th September 2006, 02:24 AM   #7
Leolabs is offline Leolabs  Malaysia
diyAudio Member
 
Leolabs's Avatar
 
Join Date: Dec 2004
Location: Bukit Mertajam
Send a message via MSN to Leolabs
Try lateral FET from Renesas(Hitachi).
  Reply With Quote
Old 29th September 2006, 07:11 AM   #8
sajti is offline sajti  Hungary
diyAudio Member
 
Join Date: Apr 2003
Location: Vác, Hungary
I agree with ilimzn. I used IRF520 for power follower. It sounds great, better than any cheap vertical MOSFETs I ever heard. But it has very limited power ratings, so You need heavy heatsinks, or fan cooling. I killed many of them, due the very bad reliability.
Another experience: This MOSFETs sounds good only with high bias (4A at least). The realisation can be the bridged mode, with lower rails, and higher current

sajti
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Power Follower 99 barossi Solid State 45 23rd February 2014 03:35 PM
Power follower 99c Vs Zen SIMONLEE Pass Labs 32 9th March 2013 10:00 AM
How about a Power Follower with an IPT? mtlin12 Solid State 16 4th March 2006 02:04 PM
power follower renjiish Pass Labs 2 19th February 2005 03:48 PM
power follower 99 help renjiish Solid State 3 20th November 2004 09:30 PM


New To Site? Need Help?

All times are GMT. The time now is 11:34 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2