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Help with cleaning up gate drive to IGBT buck converter

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IamSmooth

Tue Nov 30 2010, 03:36PM

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

Is there a benefit to using a 330uf surface mount electrolytic and a separate 1nf capacitor in parallel?

Let's say the problem is noise on the power supply. What is the best way to clean it up? Should I put bypass capacitors across the pos and neg rails with the center tied to ground?

Patrick

Tue Nov 30 2010, 05:31PM

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

IamSmooth wrote ...

Is there a benefit to using a 330uf surface mount electrolytic and a separate 1nf capacitor in parallel?

Let's say the problem is noise on the power supply. What is the best way to clean it up? Should I put bypass capacitors across the pos and neg rails with the center tied to ground?

i dont think noise from your PS would exist at light load and disappear at heavy load, i think your FOD ic needs the cap, not your PS.

IamSmooth

Tue Nov 30 2010, 05:45PM

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

Does anyone know the mechanism of how this oscillation disappears with higher duty?

Am I correct to say that one does not use reverse recovery diodes on the IGBT for this application?

Patrick

Tue Nov 30 2010, 08:13PM

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

a internal or external diode is used as an alternate path for free wheeling current, however in this crcuit i dont think you need one. although a full schematic has not been posted.

Luca

Tue Nov 30 2010, 08:47PM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Patrick wrote ...

for power IC 10uF is too little.

Power? A gate driver IC is not a power IC! If you have to provide a typical 100-200nC as total gate charge, 10uF is enough.

Regards,

Luca

Patrick

Tue Nov 30 2010, 09:41PM

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

if you take a 10% voltage drop off of a 100uF cap you have displaced effectively 10uf, remember the voltage has to be held up, you cant discharge all 10uF and have 0 charge, or youll have zero voltage.

Luca

Tue Nov 30 2010, 10:36PM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Patrick wrote ...

if you take a 10% voltage drop off of a 100uF cap you have displaced effectively 10uf, remember the voltage has to be held up, you cant discharge all 10uF and have 0 charge, or youll have zero voltage.

If your gate driver IC has to supply a 100nQ device and you have 10uF on the power supply, you will have a voltage drop of 0.01V, not bad...

Luca

IamSmooth

Wed Dec 01 2010, 12:19AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I replaced the 0.1uf bypass cap on the FOD3180 with a 470uf leaded cap (that's all I have right now for the board, I'll have to make another that can take a smd). What I see is a totally flat top to my gate drive when the high voltage to the C-E is off. So the curved top and the small oscillations are gone. That is a step in the right direction.

However, when I increase the voltage to the IGBT, the oscillations appear again. I scoped the signal before the gate resistor - it's there. I scoped the HV supply to pins 5 and 8 of the FOD3180 - the oscillations appear there. I scoped the signal going into the FOD on pins 2 and 3 - it's there.

I scoped the pos and neg leads of EACH individual 15v supply and I can see the oscillations appearing there. I even tried putting ferrite beads on the 15v leads.

So I don't know the source. Is it something on the power supply that is getting propagated into the board?

The supplies are the same: a 16v transformer, rectified, going to a 470uf electrolytic. This smoothed signal goes to a 15v regulator that has a 0.33uf and 0.1uf ceramic on leads 1 and 3. The output goes to a 39000uf electrolytic.

What can I do to narrow this further?

EDIT:
I figured out at least why the oscillations appear at the beginning of the gate drive signal. When I scope the C-E junction, I see this is the point when the voltage potential drops to near zero as it conducts. There is a small oscillation evident across the C-E drop. This oscillation is seen on the gate-emitter potential. Right now I am using a 85ns 1200v/75A diode for the converter.

Patrick

Wed Dec 01 2010, 02:00AM

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

maybe im crazy, but i think your ground plane is where the signal comes from.

IamSmooth

Wed Dec 01 2010, 05:07AM

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

It seems that when I use a Schottky barrier diode instead of the 85ns hyperfast diode (RHRG75120) it solves the problem. The only problem is the diode is rated for 30v; the hyperfast is rated for 1200v. It seems the hyperfast diode is not switching fast enough for my topology?

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