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 #1200
Joined: Mon Dec 31 2007, 01:43PM
Location: Kaohsiung, Taiwan
Posts: 27
Greeting
Well, I mean the one along with GDT, not the caps in the half/full bridge. I know it should be larger if possible. Just how large it should be?
I have tried replacing it with another ranging from .68uf to 3uf without problems, I'm curious if there is a way to calculate the exact value of very capacitor.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Wei-Jen,
It is simply matter of voltage drop that will occur on DC blocking cap in series with another capacitor (mosfet gates); you want it to be negligible in order for gate voltage to be close to supply voltage.
I think 20-50 times the total load capacitance (sum of all device gate capacitances) is pretty surely enough.
So for a halfbridge of IRFP450's, you're going to be fine with 100..250nF or so. You don't need anything in uF range for smaller mosfets like that.
For large IGBT's it is usual to have DC blocking caps of few uF due to their large gate capacitance and, with DRSSTC, the usual overdriving of gate to 30V (1:2:2 GDT) which doubles the amount of needed charge.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The minimum value is 'several times' the effective capacitance of the main gate capacitances x (GDT sec/pri turns ratio)squared. e.g. GDT=1:2, all four output transistors driven from one GDT with each having 3nF input capacitance. Cpri >> (4 x 3nF) x (2/1)squared = 12 nF x 4 = 48 nF.......use 0.47 uF.
The maximum value is determined by the current from the driver chip when the output transistors go short-circuit and is unimportant.
Values to avoid are those that may resonate with the effective inductance of the GDT and effective total gate capacitances.... an unlikely but possible combination
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.
How large the DC blocking cap along with GDT should be?
