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 #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a question about the maximum amount of current a mosfet can take. If I have a half-bridge using two mosfets each rated at 50A @ 25C, and the current waveform reads 50A rms, does each mosfet see 25A? Is it safe to operate the mosfet like this, or is the 50A rating with regard to the peak currents? Assume for this question that the temperature is kept at 25C.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Look carefully at the way the H bridge works. Each mosfet sees the same current, hence the series arrangement of the two fets.
But, they see less voltage than, say, single mosfet/class E. The mosfets only have to be rated for 1x (plus safety margin) the input voltage in an H bridge or halfbridge, but more like 3.3 times with class E.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The 50A rating is just that, a "rating". Which means a simplification, having taken into account many unacknowledged assumptions (like the expected lifetime, the anticipated transients the user will throw at it in their unspecified application). Bear in mind that commercial stuff is sold to commercial users, who want 99.99% of their product to work for 1000s of hours, so you can expect ratings to be conservative. If you are happy with 10s of hours service, and cool with occaisional failure, then use MOAR POWA. Often it's not the thing your are deliberately stressing that causes failure, so keep voltage transients under control whether you are pushing the current ratings or not.
Dig deeper into the data sheet to sort out the limitations due heat capacity of the conducting element and bond wires (short pulse, I2t), heat capacity of the die (longer pulse I2t), thermal conductivity of the heatsink (long term power dissipation), electromigration (long term current density), thermal balance of adjacent cells (safe operating area).
In an H bridge, devices don't conduct for 100% of the time, so already you have a distinction between peak and continuous ratings.
Or just buy more than you need, and sacrifice a few by testing in your application to destruction (use more power until it breaks, then back off a bit) in the name of knowledge.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
For a complete sinewave (with positive and negative half-cycles present) the RMS value is equal to 0.707 times the peak value. Note that it doesn't matter whether both half-cycles are different polarity (i.e. positive and negative) or if the waveform is rectified and they are of the same polarity. For a complete sinewave the RMS value is equal to the peak value devided by sqrt(2).
If you chop off one of the half-cycles, then the RMS current of this "half-wave rectified" sinewave is equal to 0.5 times the peak value. By cutting out one of the half-cycles you reduce the RMS value by another 1/sqrt(2)
So a 50A rated MOSFET would theoretically be able to carry single half-cycles of a 100A peak sinewave! (Assuming of course that you can keep the die temperature low enough that it's current handling ability doesn't need to be derated, and there are no other losses like switching losses or avalanche losses!)
It is virtually impossible to run a device right at its ratings though! This is because heatsinking requirements become un-manageable, and the slightest bit of foreign material between the device backplate and heatsink will impair thermal conductivity. As Dr Slack said, you would put a very healthy safety margin on the device's current rating or parallel up several MOSFETs to trade off cost against reliability in a commercial application.
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.
mosfet current rating and half-cycles
