Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Thermal instability in MOSFETs
Thermal instability in MOSFETs
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
Thread Tools Search this Thread
Old 16th September 2019, 12:04 AM   #1
wg_ski is offline wg_ski  United States
diyAudio Member
Join Date: Oct 2007
Default Thermal instability in MOSFETs

A few years ago, when looking up data sheets for mosfets specifically for class D amps I noticed that a lot of newer types have a region of thermal instability at high (or even not very high) Vds that is not unlike BJT second breakdown. I figured that it was just a consequence of making them “better faster cheaper”, and that optimizing for low Qg was probably responsible. No big deal - they’re intended for switching, right? But I looked at data sheets recently for some devices I had traditionally used for lower-speed switching duties - and found their recent data sheets showed the same thermal instability regions in their SOA graphs. Went back And looked at the PDFs I downloaded years ago, and they show the SOA to be strictly TJ limited - like you expect a mosfet to be. What gives? Is it something that manufacturers recently realized whe their newer flavors started having problems when used as linear amplifiers, has it always been this way, or has something fundamentally changed in the way they process them? Is an older IRFP2907 or IRF1405 better than a new one? Similar Fairchild types still show the old SOA graphs - are they also susceptible to this issue? Newer types i’m Using at high voltage as linear amps (tube voltage regulators and source followers) still show TJ-limited SOA on the data sheets - but can this be trusted? Or is it something you just have to watch type by type like you do bipolars? I wouldn’t be as concerned if newer data sheets on older types weren’t showing poor thermal performance.
  Reply With Quote
Old 16th September 2019, 06:17 AM   #2
davidsrsb is offline davidsrsb  Malaysia
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur
New production of older types are still using modern processes and cell structures, so not in detail the same as the original parts.
My suspicion is that the really early Siliconix VFETs were nearly as SOA robust as laterals, but almost anything after that has some SOA issues.
  Reply With Quote
Old 16th September 2019, 07:48 AM   #3
sesebe is offline sesebe  Romania
diyAudio Member
Join Date: Jan 2011
Location: Iasi
"Second breakdown" is present on most of the MOS-FET-s but is not documented by suppliers.
  Reply With Quote
Old 16th September 2019, 01:58 PM   #4
davidsrsb is offline davidsrsb  Malaysia
diyAudio Member
Join Date: Dec 2005
Location: Kuala Lumpur
The original VFETs 40 years ago like the VN66 were a single cell, fairly high Rds and had a low temperature coefficient
something I have noticed is that recent generation parts have a very high threshold temperature coefficient, so they get runaway.
This is a problem with class AB amplifier designs like the SKA
  Reply With Quote


Thermal instability in MOSFETsHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Biasing/thermal compensation of Thermal Trak transistors Bob Cordell Solid State 163 23rd March 2019 10:53 PM
heatisnk copper shims, thermal paste or thermal pad? vishalk Solid State 11 12th March 2018 05:30 PM
help with Thermal Impdeance vs. Thermal Conductivity Eric Pass Labs 4 22nd July 2005 07:23 PM
Zen instability? peted Pass Labs 10 27th August 2001 09:21 AM

New To Site? Need Help?

All times are GMT. The time now is 03:21 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio