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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Luca wrote ...
GalliumMan wrote ...
to prevent cross conduction could i not just use separate gate resistors, or is this too simplistic? - see circuit diagram below.
Well, I don't think that it can work, even it is often used to that purpose... The reason is that the turn off time reported in the datasheet is an intrinsic limit value of the devices, I mean, even if you discharge the gate with no resistor, the device takes a certain amount of time to actually interrupt the conduction (excess charges neeed time to leave the device). Moreover, usually the rise/fall time of the gate voltage is limited by the leakage inductance of the gate tansformer. I verified this performing several Pspice simulations... So, I don't think that using separate gate resistor can change in a significative way the turn on/off time, unless you use a very high value for the "on" gate resistor, but I do not think that this is a good idea...
Regards,
Luca
I don't think this is entirely correct. Using GDTs in brick DRSSTCs is very common now, using gate resistors in the 2-5 ohm range, and they seem to get on without problematic amounts of shoot-through. I can't tell you exactly how or why it works though, but if I get the time I might throw together a simulation or a test circuit and see if it supports my theory.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Anders, I think the zero current switching due to the resonant primary helps, especially if you tune in a little leading current. By the time the gate voltage goes away, the IGBT is already off :) The turn-off times in the datasheet are for hard-switching of an inductive load, which is the worst-case scenario, and it can only get better.
Of course when used with my PLL driver, my gate drivers offer adjustable deadtime to over 1us
GalliumMan: The capacitor guy is Richard Lines of Courtiestown Marine.
He advertised the caps on this forum a while ago, but I don't think anyone in Scotland was interested.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
Steve McConner wrote ...
Anders, I think the zero current switching due to the resonant primary helps, especially if you tune in a little leading current. By the time the gate voltage goes away, the IGBT is already off :)
Yes, I think that this is the point. If there is (almost) no current at the switch off the time is strongly reduced.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I think that if you drive the gates directly from two tightly-coupled secondaries with the gate resistance in the primary you will not get shoot-through because the OFF transistor will not get gate voltage until the ON transistor is off.
Sometime this year I will be testing this idea as I've just got 6x 200Adc/600Apk IGBTs.
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
I have driven CM1000 IGBTs (220nF gates) with GDTs with no problems. I've said it before and I'll say it again, GDTs are the BEST option for DRSSTCs. Discrete GD is just not necessary for this application and tends to make things much to complicated. Don't get me wrong though, it does have it's place and Steve C. has made some really nice little boards that I'm sure will work great. Steve Ward, Finn Hammer, and Chris Hooper have all run discrete GD on their large coils and all of them ended up switching back to GDTs... I have run 25KW and made 20+ foot arcs with GDTs and no provisions for dead time since the GDT creates perfect switching times by default (assuming you build the driver right). Need I say more
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BIG IGBT's
