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Driving a Igbt

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Andy

Sat Dec 20 2014, 08:42PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Hi, i've been looking at discharging a capacitor quickly, and would like 600V at roughly 400A max values, with a on-offtime in 1000th/ms and a rep rate of 1ms.

Found this on Mouser

Would that fit the requirements.

I don't know how to drive them, are they like Mosfets or transistors, if I was going to discharge a cap, can I have one on the forward side and reverse of the capacitor?

Cheers

BigBad

Sat Dec 20 2014, 09:53PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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You drive IGBTs very much like a mosfet, they have the insulated gate/base but they have transistor-like transfer characteristics.

Andy

Tue Dec 23 2014, 03:49AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Thanks, I don't were I picked it up, but I thought the gate had(in the eg) to be 20volt above the source.

For Duel modules, do you double the specs, or is the specs take that into account(I'm assuming its just two wired in parallel)

If you are quickly discharging a capacitor, do you have to turn off the switch thats from the source to the cap, before you open the switch through the load, or can they be at the same time(aiming for longlife).

Cheers
Andy

hen918

Tue Dec 23 2014, 05:37AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The double ones are often two seperate IGBTs in the same module. In some of them, the drain of one is connected internally to the source of the other.

The IGBTs are usually designed to operate with a MAX voltage of 20v. 18v is a safe voltage to go for, in that case.

Andy

Tue Dec 23 2014, 08:16PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Thanks hen918
I've drawing up a schemtic of the setup, that I think will work, is there any parts that need adding or subtracting.
Cheers

Kolas

Sat Jan 10 2015, 11:27AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

The modules are usually in fact a half bridge (in series) and would be totally useless for this application unless you needed the V stand off. IGBT's typically have diodes and fast ones anti parallel within the switch it's self. I don't believe this one does! Not great to start with. Paralleling IGBT's for this application is probably going to end badly. I recommend some shunting resitance, or inductance to help assure that one doesn't momentarily attempt to conduct all of the current due to turn on time difference.
If your mot's are wired for 240V and their primaries are in series you still run a big risk of overvolting these igbt's as they do not tolerate voltages over 600 (rarely if at all)

In short: SCR's are more ideal for this application because they are well known for being able to withstand much higher peak currents for the amount of time you're looking at. they can also be seen as a drop in replacement for the IGBT in this case.
Finally it would be a good idea to indeed switch off the charging current before discharging. In this case it probably wouldn't be a cause of failure, but it would be a waste of power.

Andy

Sun Jan 11 2015, 03:54AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Thanks kolas, looked at a Scr that is in spec, but it says holding current, does that mean I need to source say 200mA but when it detect zero crossing, I can then disable the holding current or does it act like a Fet but I need to supply the amps to keep it on/off?
Thanks

GrantX

Mon Jan 12 2015, 05:53AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Andy wrote ...

Thanks kolas, looked at a Scr that is in spec, but it says holding current, does that mean I need to source say 200mA but when it detect zero crossing, I can then disable the holding current or does it act like a Fet but I need to supply the amps to keep it on/off?
Thanks

Once you trigger an SCR it will keep conducting until the current drops below it's holding current. If the holding current is 200mA, then the SCR will stay on until the current drops below this point, even if the trigger pulse applied to the gate is very short. When discharging a capacitor the current will ring back and forth (if there's ANY inductance), crossing the zero volt line. If you place a powerful diode in anti-parallel with the SCR this will conduct the reverse voltage and allow the SCR to turn off (so your trigger pulse needs to end before the current from the capacitor reverses).

I don't have much experience with SCR's, so there might be some flaws in this description. Hopefully someone with more experience can correct any mistakes ;)

Kolas

Mon Jan 12 2015, 05:15PM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

Everything you said is correct grant. I personally have very little practical experience using SCRs in this particular fasion, but the theory remains valid. Once you remove the current to the gate the SCR will stay on until the latching current drops below the specified value. I would suggest getting two identical SCRs and and wiring them in anti parallel. You can easily find SCRs in one package that meets this description (triac). Then, you have reverse voltage spike protection while still getting the functionality you desire. If it were me, I'd leave the gate current on until the ringing is over so that neither device ever has to see a large spike stress. This should be pretty desirable unless your caps cannot handle some voltage reversal(which may be the case) OR you wish to attempt to recover the remaining energy (which would be tough.)

Andy

Mon Jan 12 2015, 05:57PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

Thanks Grant and Kolas.
Do you think if I have a Scr on the postive side of a cap, when it gets allmostcharged, the scr switchs off, allowing the cap to be discharge?
Im thinking that if the leakage current of the ccapacitor is less than the holding current it might work.

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