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 #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
nrhoades wrote ...
The fastest you can switch without latching-up the FET is: 100V / 5V/ns = 50ns. You will probably want to derate this value to be safe. Lets make it 100ns.
100ns is 1/20th of your switching frequency, if thats something one can live with.
The current needed to charge the gate capacitance in this short time is: Q/t = 200nC/100ns = 2A.
You want to make sure that the input current doesn't exceed 2A. So if your gate drive voltage is 15V: V/I = 15V / 2A = 7.5 ohms.
I thought the value of gate resistor is a matter of compromising between rise/fall times and ringings. Wouldn't a gate amperage limit (in your case, 2A) depend from what manufacturer says, not from dividing gate voltage by peak current needed to charge gate's capacitance?
How would you define a latch-up? I can't understand what Wikipedia says, due of my knowledge lacks.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
Have you tried scoping the gates in operation? How do they look with and without supply voltage?
Marko, on my previous gate driver (the one that was killing the FETs) I was using TC4422 before each gate. There were 3R3 gate resistors with Shottky diodes in parallel to them that provided extremely fast turn-off (effectively shorting the resistor on turn-off). I can't scope the gates in operation, my scope gets fed up by all the noise and displays rubbish, but I guess there could have not been anything wrong with them. I scoped them with power only to gate driver and of course they were perfect. Absolutely no ringing. I used 18V zeners just to be safe (gate drive was 15V).
As I said, when I removed the TC4422 "boosters", the only difference was slower gate drive - and the FETs no longer died.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Actually, the only problem with "driving" a MOSFET gate too hard would be oscillation / ringing.
Remember, the TURN-ON speed of the MOSFET is mostly determined by the reverse-recovery characteristics of the internal rectifier diode, not the "strength" of the gate drive circuit. Typically, the only parameter which can truly be controlled by the gate drive circuit is the TURN-OFF speed.
For the gate resistor, you will typically want to size it so the circuit is critically damped. Basically I shoot for a resistor value that is at least 2 times the square root of the L/C component at the gate terminal. C being the capacitance at the gate, and L being any parasitic inductance due to wiring and/or transformer etc...
Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
For the gate resistor, you will typically want to size it so the circuit is critically damped. Basically I shoot for a resistor value that is at least 2 times the square root of the L/C component at the gate terminal. C being the capacitance at the gate, and L being any parasitic inductance due to wiring and/or transformer etc...
Is it possible to add what you just said to a sticky or similar?
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
nrhoades wrote ...
For the gate resistor, you will typically want to size it so the circuit is critically damped. Basically I shoot for a resistor value that is at least 2 times the square root of the L/C component at the gate terminal. C being the capacitance at the gate, and L being any parasitic inductance due to wiring and/or transformer etc...
Is it possible to add what you just said to a sticky or similar?
Probably not necessary. Typically just use a 5-10 ohm resistor. That should do it 99 times out of 100. Plus, if you have a gate drive that is swinging from positive to negative (i.e. +15 to -15V bias), then any ringing occuring on the gate will typically be below threshold voltage anyways. The problem is more prevalent if you have a single-ended drive (i.e. 0V to 15V) where the ringing could actually turn the device back on etc...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I get it what you all are trying to say is that the gate resistor is a compromise between ringing and speed. However, with the gate driver I was using, there was NO GDT, the output leads of the TC4422 were twisted together and went straight to the FETs. I scoped the gates, there was NO ringing.
I have no explantation for this; I think I'll try it again - maybe I had a defective FET previously? Or was it because I was using 2x 3 parallel FETs? I have absolutely no idea..
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
You should scope the gates with bridge supply voltage applied, no load or dummy load - are you sure you aren't shorting anything with your scope?
You need to use some kind of isolation transformer to provide power, two small power transformer in reverse-series will do fine.
I don't like mysterious mosfet deaths either (I had one happen on my self resonant SSTC driver circuit, but in very different conditions though).
Using lots of measurement and protection equipment (CT's, fuses) can often help a lot.
When you apply supply voltage miller effect does indeed mess things up... which can be problematic with single ended drive since mosfet can easily get turned on.
When exactly did you kill mosfets? At turn on? In operation? If in operation, how hot they were?
One problem I can think off could be that your TC4422's were high impedance when unpowered, making the mosfets turn on by their will and die.
The upper driver can't get power at all until the lower mosfet is turned on, so this may be causing chain reaction.
GDT's surely fix that all - I don't know why you hate them so much.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi Marko, I apply first power to gate driver and then to the bridge. As I said, I can't scope the gates in operation- I tried with isolation transformer but there was very bad noise on the scope, the gate drive signal nearly got lost in it... The FETs died when I tried to draw an arc of my transformers, still with ~60W bulb in series with mains. The transistors were not even warm. The "failure mode" was all 3 leads shorted with a few ohm resistance beween them.
As to the schematic, the gate driver was the level shifted driver with TC4422 before each gate and the 3R3 resistor/schottky combo in series with gates as I explained earlier. It was driving a half bridge made of 6x IRF740.
I didn't say I hate GDTs I just don't like them much because they are bigger than my level shifted driver, consume more "idle" power and can't provide such clean gate waveforms with such high peak currents (which I'm not sure I need anymore )
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
As I said, I can't scope the gates in operation- I tried with isolation transformer but there was very bad noise on the scope, the gate drive signal nearly got lost in it...
You need to fix that first - use only a bulb or no load at all and then scope the gates. Make sure your ground connection is done right.
high side TC4422 always gets power only after lower mosfet is turned on, so it doesn't matter when you power the bridge. Still in that case mosfets should die as they are powered.
There are many variables there I don't know si it may help if you bothered to draw at least a quick hand-drawn schematic of current state of the circuit. :p
I just don't like them much because they are bigger than my level shifted driver, consume more "idle" power and can't provide such clean gate waveforms with such high peak currents (which I'm not sure I need anymore )
I don't think these 100mW will lead anyone into energy crisis... and gate waveforms can be better because they are bipolar. You probably just need to practice winding technique to get it perfect.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Ok, I'll post some schematic tomorrow..
My scope is not good with anything that involves HF, HV, EMF or any other kind of interference. It's a 40-year old granddady tube scope so there's no wonder. Actually it shows noise and ringing even with the probes shorted together and near the operating circuit
wrote ... high side TC4422 always gets power only after lower mosfet is turned on, so it doesn't matter when you power the bridge. Still in that case mosfets should die as they are powered.
I don't see anything wrong with this, if it doesn't get power it can't turn on... The problem was never at startup.
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.
Too fast gate drive kills FETs
I just don't like them much because they are bigger than my level shifted driver, consume more "idle" power and can't provide such clean gate waveforms with such high peak currents (which I'm not sure I need anymore 
)