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DRSSTC IGBT Gate Drive Problem

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Timo

Tue Jul 22 2008, 12:23AM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

Hello everybody

Recently I ran into another problem with my DRSSTC. I tried some things to get rid of it, but nothing worked out. Now Iâ€™m somewhat stumped. Hopefully somebody can give me a useful hint.
Just to give you a general idea: I use a circuit similar to Steve Wardâ€™s, described here.

My problem is the following: The IGBTâ€™s gate voltage dose not come to zero and remains zero after the ON-Time has past. Instead, when the driver chips enable pins go low, the gate voltage starts a damped oscillation. It passes through zero and reaches about 12V again, before it stays lower than the IGBTâ€™s V_threshold. You can see it here:

IGBTâ€™s gate voltage, 60V peak-peak, ~170kHz

This little overshoot at the end turns the IGBT on again. Here is a picture of the bridge output voltage, switching into a resistive load.

Output, ~50V peak-peak, â€œHighâ€ for about 3us in this picture (170 kHz)

You see that the lower IGBT is turned on for about 250ns by this unwanted overshoot. In my setup it would have to turn of about 2/3 * I_peak after this because there is already about 250ns delay between the zero crossing of the primary current and the â€œfirstâ€ switching transition.

To clean up the gate signal, I tried to use short connections between the GDT and the IGBT, I tried different GDT designs, and I added up to 20 Ohm in series with the gate. Nothing worked. What else might be the reason? Or maybe it doesnâ€™t matter that much? Dose anybody have an idea what else I could try?

Thanks, Timo

Steve Ward

Tue Jul 22 2008, 01:13AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

To fix that oscillation, i put a 10-50 ohm resistor in parallel with the DC blocking capacitor on the primary side of the driver. Try it and see if it fixes it for you, i had the exact same problem in the past.

HV Enthusiast

Tue Jul 22 2008, 01:15AM

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

Whats your circuit look like?

What is your GDT design?

Timo

Tue Jul 22 2008, 01:52PM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

I tried 10 Ohm parallel to the DC blocking cap, but it doesnâ€™t change anything. But maybe itâ€™s because I used a film/foil type cap for this purpose. I did this because the elkoâ€™s ESR limited the maximum current and therefore the gate voltage rise time before. I will try to step back to an elko now. Anyhow, it seems you had the right idea: I made a picture of the voltage across the DC blocking cap (trace 1).

The lower trace is the gate voltage

Obviously there is a lot of ringing over the cap, too. I found this ringing to be exactly synchronous with the gate voltage overshoot.

My circuit is, more or less, the one Steve Ward uses.
=>klick<=

Here is a picture of my â€œfavouriteâ€ GDT:

Steve Conner

Tue Jul 22 2008, 02:32PM

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

Try a different GDT core, and/or more turns, looks like you might not have enough magnetizing inductance.

GeordieBoy

Tue Jul 22 2008, 04:41PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

As Dr Conner said, 6 turns is likely too few for 170kHz, depending on the Spec Ind (Al) for that core. I'd aim for about 15 turns on each winding on a high-permeability MnZn ferrite core.

I would also use a 1uF electrolytic in parallel with your film/foil DC blocking capacitor if it is less than 220nF. The suggested resistor across the DC blocking cap is also a wise addition is pulsed applications. This damps the transient response of the low-frequency resonance between the magnetising inductance and the DC blocking cap. Although that is not the problem here.

Also make sure you have a 100nF ceramic and at least 10uF electrolytic (or tant) in parallel across the supply rails right at each gate drive IC. This is important and often overlooked. When the drive IC switches on and tries to charge a power device's gate, this charge has to come from somewhere. It is best to provide a local store of charge close to the gate drive IC otherwise the local supply rail will sag and bounce due to inductance. This could possibly be causing your problem here.

-Richie,

Steve Conner

Tue Jul 22 2008, 06:10PM

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

That's true. I stick a 330uF low-ESR electrolytic right on the supply pins of each gate drive chip. I used to use 0.1uF ceramics in parallel with those, but I quit and it didn't seem to make much of a difference. (I chose 330uF because I had a load of them that I bought for something else.)

I also used 4uF worth of DC block caps with a 10 ohm resistor across them.

My gate pulses looked like this:

Notice that the line afterwards isn't quite flat: there's a hint of a damped low-frequency bounce.

Timo

Tue Jul 22 2008, 06:12PM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

Thanks for the replies so far. The GDT is 6:12 turns, sorry, I shood have mentioned this. The core is about 5cm in diameter, well suited material (mat77 at reichelt, for those who know this surpplier).

The DC blocking cap is a 4,7uF Film type. And the driver ic's have a 100nF ceramic cap each and 100uF tantal (shared by to ic's) still very close.

I was not able to change the DC blocking cap back to an electrolytic one jet. I will try this, and also a GDT with more windings, later.

Steve Conner

Tue Jul 22 2008, 06:16PM

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

Is that the same as Fair-Rite material 77? I know Fair-Rite #78 is what I used for GDTs, but #77 is very similar.

Timo

Tue Jul 22 2008, 07:23PM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

I took a closer look at the specs:
The core is a MnZn ferrit ring from AMIDON, outer diameter is 36mm (sorry, not 50) inner is 23mm.
AL: 2250 nH

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