Gate driver for solid state contactor

[Rooies13]

Hi

I want to use a IGBT(SKM300GA12V) for a solid state contactor.

This is for a HOBART mig welder where they used a big mechanical contactor on the secondary side of the transformer.

This contactor switches 200A DC. I want to replace it with something non mechanical because the mechanical contactor keeps on arching and this damages it up to a point where it does not want to work.

Any ideas what driver to use to switch the IGBT on and off?

Data sheets and welding machines manual attached.

Dennis

 

[chuckey]

From the spec sheet it looks as though you need >6V @12mA. So a little 6V mains PSU with its output being fed through the switch that used to control the contactor?.
Frank

 

[FlapJack]

I think you will find this will snowball in to a large project rather quickly. Voltage drop on IGBT is over 2v, x 200A current = 400 watts of heat you have to dissipate. Were are you going to put the huge heat sink.

And 6V at 12mA is way to low, power mosfet gates like this need 15vdc at two or more amps for fast switching. In this case you may luck out with turn on because you usually do not start the power with the wire touching the workpiece so you can turn on a little slower, If you want to bet that's the way it will happen every time.

Turn off is something else, you will always have to interrupt the welding current so you need a mildly fast turn off. Your gate circuit will require a lot of thought and engineering.

You may also need snubber's on the IGBT. 200 amps on long leads with an inductor already in line can mean some big spikes.

Getting all this right so it does not mysteriously burn out will be a challenge.

 

[Rooies13]

That`s my concern too, how to switch it on fast and to turn it off again. The heat sink is the least of my worries because there is sufficient room for it and the machine has a fan that runs continuous.

I think you will find this will snowball in to a large project rather quickly. Voltage drop on IGBT is over 2v, x 200A current = 400 watts of heat you have to dissipate. Were are you going to put the huge heat sink.

And 6V at 12mA is way to low, power mosfet gates like this need 15vdc at two or more amps for fast switching. In this case you may luck out with turn on because you usually do not start the power with the wire touching the workpiece so you can turn on a little slower, If you want to bet that's the way it will happen every time.

Turn off is something else, you will always have to interrupt the welding current so you need a mildly fast turn off. Your gate circuit will require a lot of thought and engineering.

You may also need snubber's on the IGBT. 200 amps on long leads with an inductor already in line can mean some big spikes.

Getting all this right so it does not mysteriously burn out will be a challenge.

 

[FlapJack]

Well the signal to turn on and off is coming from a mechanical switch so the first thing you need to do is make a De-bounce circuit. Then find a gate driving IC suitable for your IGBT. From the switch to the De-bounce, to the IC driver you may need a level changing circuit.

One more thing, from reading the Hobart manual, there is an optional circuit to delay the turn off of the contactor by 1/2 second to prevent the wire from sticking. So you may have to add a circuit for that function also.

 

[Rooies13]

Okay yes the on signal is from a relay in the wire feed. Can one not use a triac or thyristor in front of the diode bank instead, will it not be easier than after the diodes? Yes it also has a delay circuit for burnback otherwise the wire sticks to the job. that is fairly easy to do. It`s just that i am not familiar with IGBT`s that why i ask here.

Thanks for the help so far.

Well the signal to turn on and off is coming from a mechanical switch so the first thing you need to do is make a De-bounce circuit. Then find a gate driving IC suitable for your IGBT. From the switch to the De-bounce, to the IC driver you may need a level changing circuit.

One more thing, from reading the Hobart manual, there is an optional circuit to delay the turn off of the contactor by 1/2 second to prevent the wire from sticking. So you may have to add a circuit for that function also.

 

[Rooies13]

It needs +15v to turn on and -8v to turn off

From the spec sheet it looks as though you need >6V @12mA. So a little 6V mains PSU with its output being fed through the switch that used to control the contactor?.
Frank

 

[FvM]

Has it already been mentioned that the IGBT driver must provide isolation between the control voltage and the gate circuit?

It's a very slow control signal related to usual IGBT application, so anything e.g. a slow optocoupler could be used to isolate the control signal. But you also need an isolated power supply for the gate driver. Can be a small isolated DC/DC or a transformer power supply.

The problem of high IGBT on-state voltage and respective power dissipation has been already mentioned. A high current MOSFET instead of the IGBT could reduce the losses to a small fraction.

 

[FlapJack]

The gate of the mosfet on the IGBT can take up to +-20v, you should use +15 and you might be able to get away with 0v instead of -8 because of your slower switching speed.

I did not know you could by power mosfet modules so big, but here it is in stock at digikey.com

Power Mosfet
Digi-Key Part Number APTM50UM09FAG-ND
Manufacturer Part Number APTM50UM09FAG
500V 497A
$252

10 milli ohm on x 200A = 2W !!! Wow hard to believe. But i would still use a heat sink just smaller.

Opto coupler, another good idea.

Isolated power supply on gate side of opto coupler required as stated above.

The gate of the power mosfet module can take up to +-30v, but i would use the same 15v on and 0v off, with a circuit capable of driving at least a amp or 2 on and off.

 

[Rooies13]

Thanks for the info, will look into it. I spoke to someone here at work and he suggested to use three thyristor switching modules before the diode bank. Will let you all know when i have this sorted. Thanks for all the info so far.

The gate of the mosfet on the IGBT can take up to +-20v, you should use +15 and you might be able to get away with 0v instead of -8 because of your slower switching speed.

I did not know you could by power mosfet modules so big, but here it is in stock at digikey.com

Power Mosfet
Digi-Key Part Number APTM50UM09FAG-ND
Manufacturer Part Number APTM50UM09FAG
500V 497A
$252

10 milli ohm on x 200A = 2W !!! Wow hard to believe. But i would still use a heat sink just smaller.

Opto coupler, another good idea.

Isolated power supply on gate side of opto coupler required as stated above.

The gate of the power mosfet module can take up to +-30v, but i would use the same 15v on and 0v off, with a circuit capable of driving at least a amp or 2 on and off.