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DRSSTC gate signal voltage spike after burst

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loveHV

Thu Jul 19 2012, 03:32PM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

Hi,
Im currently finishing my DRSSTC i started building 2 years ago. (not enough time due to getting my masters in electronic engineering)
now its summer vacation and i have time again :p

The problem at hand:

When i tested the driver in CW mode there was no problem (driver is steve wards universal driver with some modifications).
But when i hooked up the interrupter, things got messy on my oscilloscope, this is measured between gate and emitter on one IGBT.

There is a spike right after the burst of drive square waves.
The vertical scale is 20V/div, that means that that spike ( 1div) can turn on IGBT's.
What i also noticed is that the driver started making a high pitch noise, probably from that low frequency spike entering the GDT.

Sometimes that spike isn't present but most of the times it is, ive searched everywhere for bad connections on the interrupter
but didn't find any.

Some light on this situation should certainly help :p

Sorry if the english is a little messy, im from belgium XD

Dr. Brownout

Thu Jul 19 2012, 06:06PM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

HI loveHV,
It would help if we new what modifications were made to the driver, and also did you check the driver output without the GDT connected to see if the spike was showing up ahead of the GDT.

loveHV

Thu Jul 19 2012, 07:07PM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

the modifications to steve's universal driver are the following.

The H-Bridge consisting of the STB serie mosfets, those were replaced by IRF540 and IRF9540.

Between de H-bridge output of the driver and the couple capacitor & GDT a common mode choke was placed.

The common mode choke only consists of 4 windings of twisted pair wire, each wire is connected to the 2 drains at one end,
at the other end one is connected to a pin of the couple capacitor and parallel resistor, the other wire is connected to one end of the GDT. the other end of the GDT is ofcourse connected to the other connection couple capacitor / resistor.

This common mode choke keeps the noise from the bridge away from the driver.

Also a 24V fan is connected to the regulated 24V supply. This fan cools the FET's.

here's an overview of the circuit board, the common mode choke can be seen in the middle with the gray twisted wire around it.

While I was troubleshooting the following happend. When i lifted the board up those spikes stopped.
I looked for pins that might be too long but there weren't.
To be certain I placed an Acrillic sheet under the circuit board. Between the board and the base of the box.
Problem still not fixed, im still searching what it could be.

A farfetched answer could be that there is some sort of capacitive coupling going on trough the base plate of the box.
Im really stuck here :p

HV Enthusiast

Thu Jul 19 2012, 07:10PM

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

When this is occuring i would check a few things:

1. Monitor the logic input to the gate drivers. See if that pulse shows up there.

2. Monitor all the supply voltages to ensure they aren't doing anything weird or drooping.

BTW - what does your bottom routing look like? Do you have a solid ground plane or is it traces like on the top layer?

loveHV

Thu Jul 19 2012, 07:34PM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

Thanks for the fast reply ;)

I monitored the logic's no problem there, everything as it supposed to be.

The problem starts at the H-bridge. i think it starts at the power section of the driver IC.
The regulated 15V supply wich supplies the IC has the spike visible as drop. Where the spike is the time domain,
the 15V supply drops 2V.
The 24V supply has some noise when the burst of "HF" square waves starts. but not the spike.

EDIT : all above problems dissapeare when the board is lifted a centimeter.

Both the sides use ground plane's, but not in the regular sense where every ground pin is just connected to the plane.
I use starpoint grounding, this has the advantage that no currents interfere with eachother so the supply stay's cleaner.
Then the plane is connected to the starpoint, so that the plane acts as shield for interference and such...

A thing i can't understand is why the noise on the supply line, there are 100nF SMD decoupling caps right under the regulators. And under every IC. Again this noise dissapears when the board is lifted a centimeter.

But i still can't make the link between the distance to the base of the enclosure and the voltage spike.

teravolt

Thu Jul 19 2012, 09:49PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

have you ground the case?

loveHV

Thu Jul 19 2012, 09:54PM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

Yes the case is grounded.

The supply voltages are deliverd via a transformer wich seperates the driver board from mains.

But the case is grounded to mains ground. Because 230VAC is entering the case it wouldn't be safe to leave
the case floating.

StRyKeReYe

Fri Jul 20 2012, 04:53AM

Registered Member #5777 Joined: Tue Jul 17 2012, 03:34AM
Location: Moulton Al
Posts: 3

We just had a similar issue the other day we were getting a spike on the outputs of the UD2 the GDT was sitting directly above the board and we found that for some reason the Solid metal plate beneath the UD2 used for mounting was the source of our issue, so we raised it. For your circumstance this might or might not be the issue I've seen a friend using his RLC meter to verify Congruity Between his IGBT's, measure Resistance Capacitance ETC... Pin to Pin Gate to Gate Etc... verify the IGBT isn't possibly off in some way.

You can see its only about 1inch above the board, we used a aluminium box and a 1inch piece of HDPE to lift it enough for this issue to go away.

loveHV

Fri Jul 20 2012, 08:14AM

Registered Member #1854 Joined: Wed Dec 10 2008, 06:09PM
Location: West-Flanders Belgium
Posts: 88

Interesting, i checked IGBT's no problem there.

I removed the GDT and start measuring on the output of the bridge.

The spike is still clearly visible, and its visible on both sides of the H - bridge legs. (outputs)
And its both positive with reference to ground.
But the spike on the leg with the coupling capacitor is wider then the spike on the leg without the coupling capacitor.
That means that for a brief moment the upper 2 FET's IRFP9540 conduct (gate voltage = 0V)
But on the leg with the coupling capacitor that erroneous conduction lasts longer.

So it still appears on the output.

But whats the link between this and the distance between the board and the base of the case.
Its mind boggling XD

Dr. Dark Current

Fri Jul 20 2012, 09:00AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Didn't have time to read the whole thread, but I guess your GDT primary is floating during the OFF time. The primary must be shorted during the OFF time.

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