If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I have been tinkering with SSTC`s for quite a while now, and have observed something odd. In some of the SSTC half bridges I have built, I have noticed that one MOSFET becomes significantly hotter than the other. I have even observed this in commercial Half bridges, specifically those in plasma lamps. Is there some reason for this? (bad design perhaps), and how can it be prevented? I was thinking they are possibly resistive losses, from a MOSFET not turning hard on? Maybe a couple of extra turns on the GDT?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I experience this too, once in a HV transformer driver, everything was perfectly symmetrical even the drive waveform, and one FET was no matter what still getting hotter...
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
Have you put the two on the same heatsink with mica? I was have that problem, because I forgot to insulate the fets from heatsink. That shorts one fet (that doesnt get any heat) and stay the other switching all current (gets hotter)
Registered Member #76
Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458
i think this problem is due to the small differences of each mosfet. its a mass product and none of the mosfets even the same type have the same specs. you can measure the gate capacitance of a halfbridge brick and sometimes you have 2 or 3 nanofarads difference between each igbt gate. the rds on of the mosfet may be different too or the gate capacitance. if the gate cap is different but the drive of each mosfet is equal, one is switching on/off faster or slower and so on, causing shootthroo or more switching losses on turn on/off. thats the only explanation i can think of.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
I think the better think to do, its buy the two fets of the same lot of production. For example, I sampled two IGBT's from ST that have same gate capacitance and Emitter-Collector capacitance. But I think that is not a easy think to do, because you cant see the lot number from the factory.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Well so far I havent had any catastrophic failures, its just one of those little irritations. My heastsinks are good, the noticeable heat difference is on the MOSFET body itself. Maybe I worry too much, it probably is just differences in manufacture. So long as my Silicon isnt in danger of exploding, I dont suppose there is much point demanding matching pairs from the manufacturer
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> ...everything was perfectly symmetrical even the drive waveform, and one FET was no matter what still getting hotter...
The explanation for this behaviour is quite simple... Switched Mode power MOSFETs have two parameters that are influenced by die temperature. These are the Vgs(th) threshold voltage (where the device just starts to turn-on,) and the Rds(on) resistance (AKA The minimum device resistance when the channel is fully enhanced.)
As die temperature rises the gate-source threshold voltage falls and the Rds(on) resistance rises. If the load current is a fixed current, then the first phenomenon makes the load current flow in the hot device for a longer time because this device turns on earlier and turns off later.
The second phenomenon directly increases conduction losses due to the increased channel resistance. This effect usually dominates and can lead to thermal runaway if thermal managment is bad in a design.
A final thing worth considering is that the performance of co-packaged fast-diodes can also be adversely affected by increases in die temperature. Forward voltage usually drops, decreasing conduction losses with increasing temperature. Conversely reverse recovery time can increase with temperature, increasing switching losses. Thermal runaway of fast diodes is possible if the diode dissipation is switching loss dominated.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.