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gdt problems and suggestions for drsstc driver?

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LAN_403

HV Enthusiast

Wed Apr 12 2006, 11:40AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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I personally would use (2) UCC3732x chips in a push-pull fashion to drive your GDT. When you turn off the IGBT, you effectively have -15V across gate-emitter on the IGBT ensuring that is stays off. With a single UCC3732x chip, your off state will be around 0V gate-to-emitter and noise could inadvertently turn it on.

Desmogod

Wed Apr 12 2006, 12:08PM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

So use an inverting and non inverting UCC to drive it, ala Steve W's miniSSTC?

HV Enthusiast

Wed Apr 12 2006, 02:13PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Dr Desmo wrote ...

So use an inverting and non inverting UCC to drive it, ala Steve W's miniSSTC?

Yes, thats a basic push-pull GDT drive.

Dr. Shark

Wed Apr 12 2006, 02:27PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

EastVoltResearch wrote ...

I personally would use (2) UCC3732x chips in a push-pull fashion to drive your GDT. When you turn off the IGBT, you effectively have -15V across gate-emitter on the IGBT ensuring that is stays off. With a single UCC3732x chip, your off state will be around 0V gate-to-emitter and noise could inadvertently turn it on.

Are you sure about that? The gate drive does not pass DC, so it should swing from +7.5 to -7.5V, unless you use a 1:2 turns ratio (which I always do).

Steve Conner

Wed Apr 12 2006, 02:35PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, it should swing from -7.5 to +7.5 (and asplode horribly unless you use a DC blocking capacitor in series with the GDT primary)

That only applies in the steady state. With pulsed drive like from a DRSSTC the result will be a rather odd transient, unless you take steps to make sure the DC blocking cap stays charged to 7.5V between bursts.

HV Enthusiast

Wed Apr 12 2006, 05:06PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Actually, it will swing +15/-15V, not +7.5/-7.5V.

Remember, you have a non-inverting and inverting gate driver IC driving a single winding of a GDT. When one driver chip is ON, the other is OFF. When the other is ON, the first one is OFF. This puts an effective +/-15V across the primary of the gate winding.

the_anomaly

Wed Apr 12 2006, 08:10PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

I did not order any inverting ucc chips so is it possible to use 2 of the same non inverted chips to do the same job. When you say "swing it +/-" do you mean the first square wave goes positive 15 volts and then the next goes negative 15 volts? I found vasils drsstc driver which looked good for me but is he using one inverted one not ucc chip? I havenâ€™t added a dc blocking cap to my gdt so would something rated around 3uf be good or should I just play with the values until I get the clearest signal?

I made some progress on the rest of it here are some more pics.

I started work on an plexi-glass case. unfortunately I did not compensate for where the sides come together.

The top

The new gdt I made.

Trying to put everything into the case to see where to put connections. It is a little cramped but the wires should be tidied when Iâ€™m done. Ive also left some space for the ucc chips.

vasil

Wed Apr 12 2006, 10:01PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

I modified the schema picture. I am using one UCC37321 and one UCC37322 (invert, non-invert).

HV Enthusiast

Wed Apr 12 2006, 10:17PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

wrote ...

When you say "swing it +/-" do you mean the first square wave goes positive 15 volts and then the next goes negative 15 volts? I found vasils drsstc driver which looked good for me but is he using one inverted one not ucc chip? I havenâ€™t added a dc blocking cap to my gdt so would something rated around 3uf be good or should I just play with the values until I get the clearest signal?

The waveform as viewed across the GDT primary will swing from +15 to -15V (measured differentially).

the DC blocking capacitor must be sized in accordance to the load (gate) your GDT is driving. When you reflect the load (gate) back to the primary side of the GDT, it forms a capacitive divider with the blocking capacitor. For the Fairchild 40N60 IGBTs, i found 10uF to be a good value for DC blocking capacitor. It will vary with device. I would try different values until you maximize the output voltage seen at the gate.

Steve Conner

Thu Apr 13 2006, 10:07AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

EVR, when I said +/-7.5V, I was referring to what another poster suggested, driving a GDT with a single driver chip used as a half-bridge.

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