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IGBT Circuit Problems

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Bdrt

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Hello everyone,

I have some doubts on a result i've obtained.

I'm using a circuit, as shown in the attached picture, to drive one IGBT of a module.

CircuitoDeAccionamento.jpg
The wave forms shown are the voltage at the Load( Yellow ) and the current at the load ( blue).

Result.jpg

The load is a 28Ohm Resistor.

Since the load is a resistor i was expecting the current to follow the voltage. Instead it appears to have some delay (as if there was some inductance?)

Can it be that my drive circuit is not performing as it should ? Or the problem may be with the load ?

Any ideas or sugestions on how to improve the drive circuit?

Thank you all.

Best Rergards
Bdrt
 

The gate of the IGBT appears as a Capacitor and so trying to drive this at 15kHz with that amount of impedance is not going to work. I strongly suggest you use a proper gate drive opto instead from say Avago. Looking at the datasheet for the IGBT you will see a suggested gate resistor value and this will dictate your current pulses required to switch the IGBT 'smartly'.
 

Hello,

This is a Back-up Solution that i need to get to work while i cant purchase the semikron drive.

I've researched this design on a application note from vishay ( if im not mistaken ) on IBGT gate drivers with Opotocopoulers.

The IGBT datasheet states that the IGBT already has a gate resistor, thats why i didnt place one. Or did i understand it incorrectly ?

As for the opotocoupolers, the HCPL3180 can deliver up to 2,5Amp. Can you recommend any better optocoupoler to driver the IGBTs?

Thank you.

Best Regards
Bdrt
 

Hi,

I doubt that with a 15V drive voltage you get 30V output voltage.
I expect less than 15V output voltage.

What about the connection of the low side IGBT's gate? --> don't leave it unconnected.

Klaus
 

Hi,

I doubt that with a 15V drive voltage you get 30V output voltage.
I expect less than 15V output voltage.

Hellow Klaus,

Why do you doubt ? I thought that +15V/-5V was a suitable voltage to drive IGBTs. Am i Wrong ?

In the Load (as you can see in the osciloscope picture) the voltage when the top IGBT is conducting is around 30V...

What about the connection of the low side IGBT's gate? --> don't leave it unconnected.

Should i just conect it to the ground instead of leave it unconected?

Thank you .
 
Last edited:

Hi,

I thought that +15V/-5V was a suitable voltage to drive IGBTs.
Yes, 15V is correct....but above emitter voltage!

So when the high side is ON, then emitter voltage is about 30V and thus the gate voltage needs to be 42V or more.

Should i just conect it to the ground instead of leave it unconected?
Sure!

Klaus
 

OK,

So i've corrected my schematics.

CircuitoAcc.png

Ive aplied GND to the lower IGBT and I'm Aplying a signal of +15V referenced to a Isolated GND in the upper IGBT.

The problem is that the waveforms of voltage and current in the load are even worst.

ResultNewConnections.jpg

Can someone please help me understand what am i doing wrong here?

Thanks,

Bdrt
 

Hi,

With your now given schematic it is impossible to drive the gate at all.
The gate is driven low by the pulldown or it is drven low with the optocoupler, but never driven high.
It´s totally impossible to get the shown waveforms. Or am I wrong?

Are you fooling us?

Klaus
 

Dear Klaus,

It was my fault. The schematic is Wrong. Not fooling anyone ... just trying to understand.

I'll Correct it and post the correct waveforms.

Thank you for the help.
 

Hi,
Yes some IGBTs have an internal resistance but you need an additional resistor which is quoted in the turn on and off times. You also need a high current pulse in order to smartly turn on and off the IGBT. You will need to do this is you want it to run at 15kHz. The use of a proper IGBT gate drive opto is essential. Using an ordinary opto and a pull up resistor will not do what you want, and probably explains the poor waveform too probably.

Klaus is also absolutely right about the clamping the gate to the emitter of lower device too to ensure it is held off.

Rob.
 

Hi,
The use of a proper IGBT gate drive opto is essential. Using an ordinary opto and a pull up resistor will not do what you want, and probably explains the poor waveform too probably.

Rob,

What Opto would you recommend ? FOD3184?

Thanks.

- - - Updated - - -

- - - Updated - - -

I've Try this circuit:
http://circuits.datasheetdir.com/142/ACPL-H312-circuits.jpg
But with the lower IGBT gate conected to the ground.

The wave forms were the same:

P60809-165012.jpg

The Current rises after a delay and fall only a couple of uS after the IGBT is turned off.
 

In case you are using a current probe to measure current, it may have lagging effect due to its conversion time.
 

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