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Gdt output voltage

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furnace

Tue May 05 2015, 08:11AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

How do I measure the correct output voltage on my gate drive transformer?
When my gate driver is connected to my Igbt i get a voltage reading of 120v when the multimeter is on ac but no correct reading dc.
Should I get a reading from AC or DC on the multimeter and isn't it suppose to be on maximum of 20 volts?
My gdt is a 1:1:1 turn for a half bridge and the signal is super clean, so I just want make sure about my output voltage as I'm afraid the volts is to high.

Mads Barnkob

Tue May 05 2015, 08:57AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

You first have to understand why you are using a GDT, the potential that you drive the gate with is between gate and emitter, on both IGBTs, which also places the emitter connection of the upper switch at the collector of the lower switch.

We need the GDT for galvanic isolation and we drive the gate with a "floating" voltage of 20VDC, above the voltage that is already across it.

You can no use a digital multimeter to measure these signals, you need a oscilloscope that can have the bandwidth, unless you are infact driving it at around 1kHz.

Do you mean its 1:1:1 in ratio, but with 10-15 turns for each winding, or did you infact make 3 single turn windings?

To be sure that you do not damage your oscilloscope, be careful where you place the ground clip in your driver / power circuits when trying to see signals, you risk short circuiting the circuit or you risk pulling large currents through your oscilloscope. Differential probes, isolation amplifiers and galvanic isolated/grounded negative rail supply is also a solution.

furnace

Tue May 05 2015, 12:24PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
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Posts: 108

Can you please explain what you mean driving it above the voltage that is already across it.
I have 3 single turns on the gdt.
I do use a scope and I only probe the low side of my Igbt gate with the ground connected through a resistor to main ground.

Mads Barnkob

Tue May 05 2015, 01:38PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I think it would be the best use of both your time and mine that you read this excellent guide:

furnace

Thu May 07 2015, 07:34AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Thanks for the guide, but I still don't understand it completely, it says that you have to generate a voltage greater than the supply voltage to turn your switch on. In my case I use a variac and can go from zero to 750v.
I bought a new Igbt cm600dy-24a which is rated at 1200v and 600 amps. I've been running it on 250v but feel that is getting to hot.
So I don't know if my output of my gdt is to low. There is no ringing or cross conduction on the scope.
I can really use some guidance please.

TwirlyWhirly555

Thu May 07 2015, 09:34AM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

To switch a mosfet or igbt on its gate needs to be at a voltage of 20 - 25V with respect to its source pin ( depeinding on what the data sheet says .

So in a half bridge the low side switch will have its source at 0V so when the gate is at 20V above this it will turn on .

Now With the high side switch its source will be sitting at half the supply voltage ( I think its half ) so to get it to turn on we need to drive the gate 20V above this

So if the Source pin is at for examples sake 150V , that' means that the gate need to be at 170V with respect to ground or 20V with respect to its source .

That's why a GDT is used , it provides the isolation needed but will drive the gate + and - with respect to its source .

With the GDT connected to the high side switch and not powered the gate and source will both sit at the same potential , in the exampls case 150V and the switch will be off , now if we drive a 15V square wave into the GDT primary we will get a swing of + / - 15V on the secondary side causing the gate to swing +/- 15V above and below its source pin turning the switch on and off ,

Without power to the bridge what is the voltage of the gate waveform ?

furnace

Thu May 07 2015, 10:11AM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

Thank you for good explanation.
I was told I can't measure it with a multimeter, but if I measure the secondarys of the gdt going to the gate it gives me a reading of 1.5dc and 40vac.
How else do I measure it?

Dr. Slack

Thu May 07 2015, 11:06AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

furnace wrote ...

Thank you for good explanation.
I was told I can't measure it with a multimeter, but if I measure the secondarys of the gdt going to the gate it gives me a reading of 1.5dc and 40vac.
How else do I measure it?

Normally you'd look at it with an oscilloscope. 'Measurement' sort of implies you can reduce it to one or two numbers which tell you everything relevant.

You use a scope to look at the GDT waveform to see the shape. All sorts of things can be wrong with it. Overshoot can kill your gates. Lack of 'on' voltage or sluggish transitions can cook your channels. There's much more to fixing a bad waveform that measuring the DC and AC voltages.

Normally a multimeter doesn't have the AC bandwidth to produce anything meaningful. They will all do 50/60Hz, most will do 1kHz, a few will cover to 20kHz for audio, very few will be accurate at typical GDT frequencies, though many will produce some sort of response there. The fact that it's indicating a DC voltage (the output of a GDT is a short bit of copper wire so you know there's no DC present) means that it's not measuring properly.

The only thing a DVM will tell you is if the AC is zero, then it's probably not getting any signal at all.

furnace

Thu May 07 2015, 12:41PM

Registered Member #4992 Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108

This is my waveform. it goes from 0 to 45%
but I don't know what this tells me exactly.
How would I know that its giving my gate 15 or 20volt?

1431002495 4992 FT171083 Gate Drive For Igbt

GrantX

Thu May 07 2015, 01:37PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Look at your oscilloscope settings. On the scope screen the horizontal divisions are time and the vertical is volts, thats how you use a scope to measure things. Check the "volts per division" and "time per division" settings to work out the frequency and voltage of a signal.

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