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'nother dumb SISG question: Firing voltage?

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J. Aaron Holmes

Sun Oct 22 2006, 03:23AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

S'pose this is mostly directed at Terry, but seemed generally interesting, so...

...what dictates the choice of firing voltage for a given IGBT? For example, in the "Terry Original" SISG, we've got a 1200V IGBT firing at only 900V. I assume that narrowing the gap carries the risk of overvolting the IGBT, the badness of which is pretty obvious.

What if I used 1400, 1700...6500V IGBTs? What would be considered a "safe" firing voltage? I'm guessing it has something to do with the total number of SIDACs required to approximate (but not exceed) the IGBT's Vce rating and their tolerances? Or maybe some rule-of-thumb factor? 75% seems like a pretty big "fudge factor" if you're gonna pay big $$ for the HV IGBTs, though!

Regards,
Aaron, N7OE

Terry Fritz

Sun Oct 22 2006, 04:24AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

I assume that narrowing the gap carries the risk of overvolting the IGBT, the badness of which is pretty obvious.

The badness is extreme! IR tends to over estimate the breakdown voltage. APT under...

There are timing issues about how fast things can turn on in an emergency...

In general, one needs to really "work out" the "timing numbers" to be sure... This odd program may help.

Cheers,

Terry

J. Aaron Holmes

Sun Oct 22 2006, 07:24PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Terry Fritz wrote ...

There are timing issues about how fast things can turn on in an emergency...

In general, one needs to really "work out" the "timing numbers" to be sure... This odd program may help.

So, when you say "emergency", are you mostly referring to the possibility of failure in "stacking" sections (e.g., one section "fries")? In that case, perhaps one can push things a bit more in the single-section scenario, where such stacking failures are obviously a non-issue? In such cases, where you have arranged for the transformer peak voltage to fall well short of the IGBT's rated voltage, it seems like you should be "safe" (except in the case of, say, a primary strike, where you're probably a goner in many cases regardless).

BTW, I really dig that program! I've been plugging in input capacitances from all my various IGBT datasheets for the last hour

Seems like you can stray a little from the IRF figures without screwing things up too much... I like the idea of building a driver board that can be made to work with a few different IGBTs.

Thanks, Terry!

Regards,
Aaron, N7OE

Terry Fritz

Mon Oct 23 2006, 12:16AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

By "emergency" I was trying to think of bad or odd situations that might damage the IGBTs (like explosion

) But so far I don't think I have seen such a situation.

I have never "tested" A primary strike (too chicken). But the IGBTs are locked full ON at such a time so maybe it would not be a bad thing. Unlike the DRSSTC, the SISG has no buss caps or cross conduction issues. A hit to the electronics might just go safely through one of the many transorbs too.

Glad you like the program! Let me know if I should change or fix anything.

Cheers,

Terry

Finn Hammer

Tue Oct 24 2006, 12:18PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Terry,

Thank you for a wonderfull program. (sisgtiming.exe)
Plugging in numbers to get a feel for the interaction of values, I focused on Rgate, Cmain and Cgate. It appears to me, that the IGBT should take over as soon as possible after the TVZ fires.
At that time, Vgate is 10V.
Any further increase in Gate voltage has to come from Cgate sharng the charge stored in Cmain.
This means, that to get Vgate up to 24Volts, either:
Cmain has to be much larger than Cgate, or:
TVZ breakdown voltage has to be "enough" higher than the intended final Vgate.

Am I far off already?

Suppose that "someone" had 12pcs. Cm600`s to spare with their 120nF gate capacitance and wanted them to function in a SISG, This could be a starting point:

To keep charging time of Cmain short, and avoid more than 100A through the Sidac`s, I stuck with 100nF Cmain, and opted for a 35V TVZ.

SISG Timing Program V-1.00 July 30, 2006 Terry Fritz

Cprimary = 1.000000e-006
Lprimary = 2.000000e-005
Rprimary = 2.500000e-001
Vfire = 1.200000e+004
Cmain = 1.000000e-007
Cgate = 1.200000e-007
Rgate = 1.800000e+000
Rdrain = 8.000000e+002
Rsisg = 4.000000e+006
Vsisg = 1.000000e+003
Vlimit = 3.500000e+001
TakeOverVoltage = 1.000000e+001
TimeStep = 1.000000e-010
StopTime = 0.500000e-006

Main Capacitor Charge Time = 118.0 nS
IGBT TakeOver Time = 150.2 nS
Primary TakeOver Current = 90.0 Amps
Primary Peak Current = 2683.3 Amps
Primary Resonant Frequency = 35588.1 Hz

With 72J of bangenergy it would pull 22kW of the wall at 300BPS.

Question:

Rpri appears to take part in the calculation of Ipri in the dataout.csv, but not in "Primary Peak Current" in "output.txt"

Is that Right?

Cheers, Finn Hammer

Terry Fritz

Mon Oct 30 2006, 08:31PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Finn,

Typically, Cmain is like 20X Cgate. C gain should charge up completetly to 24V before Cgate reaches the IGBT turn-on voltage.

Rpri does not effect the peak primary current since the peak happens far before Rpri has a change to significantly use up any power. The peak current is governed by the primay circuit impedance at the instant of firing.

My interent has been down, so that's why I am replying so late.

Cheers,

Terry

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