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Help with cleaning up gate drive to IGBT buck converter

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IamSmooth

Mon Nov 29 2010, 03:36AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

The buck converter I am working on is working, but not as well as I would like. I notice that as I increase the voltage potential across the common-emitter of the igbt the upstroke of the gate signal starts to develop an increasing oscillation. Instead of it being a clean step-up, it goes up and oscillates before settling on the final value. As I increase the duty, the oscillations diminish, until they disappear above 50% duty. This is a problem as it will misfire the igbt.

I have the drive circuit, which is a tl494 driving a fod3180. The fod3180 has a separate 15v supply. The supply is 16vac transformer that goes through a 470uf capacitor, then a 15v regulator (0.33uf and 0.1uf caps attached to the in and out). The final output goes to a 39000uf capacitor before going to the fod3180.

The igbt has a snubber 15R and 2000pf surface mount capacitor. The connections are wide and as short as they can possibly get.

All scope tracing are across the gate and emitter using two probes with the invert/add.

So, how do I clean this up? Do I try a different IGBT with a lower gate capacitance? Since this is for a buck converter, is it correct that I do not want a built-in recovery diode?

50v input

zoomed out from above picture

100v input - notice oscillations on upstroke of gate

zoomed out from above picture

Patrick

Mon Nov 29 2010, 04:13AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i am suspicicous of your rc snubber, let me get my calc...

EDIT: when i multiply RC i get (2nF x 15R = 33.3MHz) or 0.3uS time constant

i may be wrong but i think your RC snubber is raising/lowering your emitter above and below ground thus causing the unwanted oscillation, maybe try less cap, like 1nF and more resistence like 30R Try that, and lets see if the oscillation goes away or matches the new RC constant.

IamSmooth

Mon Nov 29 2010, 04:49AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

You mean 0.03uS.

I will try this tomorrow. So too much snubbing is no good? I used the CDE website for the design. A quick method was to double the output capacitance. Figuring about 370pf, I got 740pf. I was then going to go with 1000pf, but chose 2000pf as I thought a little more is better to reduce turn-off ringing. I did not realize it could increase the turn-on ringing.

How is this?

Patrick

Mon Nov 29 2010, 04:54AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

snubbers are kind of circuit/circumstance dependent, rules of thumb are fine, but you have to change values up and down, to see how your components/circuit layout reacts. and yes, you can make things work if you botch it.

and of course this presumes the snubber is actually at fault it could be some thing else, can you post a pic of your board?

IamSmooth

Mon Nov 29 2010, 05:14AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Well, I had to see what happens. I dropped the capacitance to 1000pf. There may be a slight improvement with the upstroke ringing, but I begin to see some overshoot on the downstroke at higher voltages. I added a diode across the resistor, but this did not do much.

I also notice at very low duty, the ringing is at its worst.

Patrick

Mon Nov 29 2010, 05:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

hmm, it may not be the snubber then,
that diode was a good idea try it agian in series just for shits and giggles.
also did you follow good layout design rules?

IamSmooth

Mon Nov 29 2010, 05:28AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Maybe if I alter the turnon and turnoff speeds using a larger gate resistor and a parallel diode. I can also try using one resistor for turnon and another for turnoff with series diodes.

Here is the layout. Everything is tight. If there is something I am doing that is wrong someone can comment on how I can improve the layout.

Patrick

Mon Nov 29 2010, 05:50AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yes about the gate resistor, try independent paths with different values, also what frequency are you switching at?

and i must admit im not familiar with the fod3810.

EDIT: you should also read the unitrode pdfs on gate drive

before i can say if your design is good or poor i need to know more about freq, current, rise and fall, load type conditions. you do have alot of traces overlapping around the transistor, some are narrow, im still figureing those out.

IamSmooth

Mon Nov 29 2010, 06:08AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I lowered the gate resistor to 3.3R. Higher values did not help.

I noticed that when I connected the emitter to earth ground it cleaned up almost all of the oscillations. However, this is not an option that I want because I don't want the voltage to have a ground return path from the positive rail. I also found that when I had the earth ground on the emitter and I increased the common-emitter voltage to 140 with a low duty, oscillations just popped up, causing the gate to go crazy.

Patrick

Mon Nov 29 2010, 06:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

oh,oh, oh .... are you feeding the fod-thing with +15v, 0v, - 15v with the transistor emitter grounded to the -15v supply ?!?

i need sleep!

the fact that it almost goes away at high DC, and is worst at low DC, has my brain thinking stuff.

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