Little Bit of a Massive Problem - Electronic Circuits Projects Diagrams Free

Alexsgarage · 31st October 2009, 03:58 PM

I am building a PWM IGBT driver and have encountered a little bit of a massive problem, when the transistor is off the voltage is still 5V or so above 0V. See the picture below, the straight line is 0V.

BrownOut · 31st October 2009, 04:07 PM

You're thread title made me chuckle. Can you post a schematic?

Alexsgarage · 31st October 2009, 04:11 PM

I had an original schematic but I have modified the circuit so much that I don`t think it will be much use but I will draw up a quick block diagram.

BrownOut · 31st October 2009, 04:16 PM

I would like to see details of the driver, and where the measurements are being made. My crystal ball doesn't work

Alexsgarage · 31st October 2009, 04:25 PM

Quote:

Originally Posted by BrownOut

My crystal ball doesn't work

LOL Funny

Here is the block diagram, the PWM is an SG3524 and it is transformer coupled to a comparator, the output which then drives the gate of a small transistor. The measurments are being made at the output and ground.
This is a high side driver.

BrownOut · 31st October 2009, 04:30 PM

I'd be concerned about the voltage at the base of the transistor; make sure it's really turned off. If it is, then the driven circuit might be holding a change. There is a potential problem with the "+" input to the comparator, the diode prevents the charge from flowing away from that node during turn off. But I suspect that there is a resistor in there somewhere that isn't shown on your diagram.

Alexsgarage · 31st October 2009, 04:35 PM

It can`t be the diode because the same thing happens on the low side driver and that one is just connected with a single wire. Could it possibly be the pull up resistors?

BrownOut · 31st October 2009, 04:37 PM

It could be. That's why I asked you to measure the base voltage. The pullup probably needs to stay in the circuit, but it might be too low valued.

kpatz · 31st October 2009, 04:46 PM

What's the value of the pullup? Try the highest value needed to turn the transistor on when the comparator is "off". For an IGBT something in the 1M+ range should work.

Is the comparator an open-collector output? Depending on the application you may be able to use an op-amp and eliminate the pullup.

Or use a PNP transistor off the comparator to drive the NPN, eliminating the need for the pullup.

Alexsgarage · 31st October 2009, 04:50 PM

Thanks kpatz the PNP transistor works nicely.

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31st October 2009, 03:58 PM	#1
Alexsgarage	Little Bit of a Massive Problem I am building a PWM IGBT driver and have encountered a little bit of a massive problem, when the transistor is off the voltage is still 5V or so above 0V. See the picture below, the straight line is 0V. Attached Thumbnails

31st October 2009, 04:07 PM	#2
BrownOut	You're thread title made me chuckle. Can you post a schematic? __________________ You don't need a quadraphonic Blaupunkt -- you need a curve ball.

31st October 2009, 04:11 PM	#3
Alexsgarage	I had an original schematic but I have modified the circuit so much that I don`t think it will be much use but I will draw up a quick block diagram.

31st October 2009, 04:16 PM	#4
BrownOut	I would like to see details of the driver, and where the measurements are being made. My crystal ball doesn't work __________________ You don't need a quadraphonic Blaupunkt -- you need a curve ball.

31st October 2009, 04:30 PM	#6
BrownOut	I'd be concerned about the voltage at the base of the transistor; make sure it's really turned off. If it is, then the driven circuit might be holding a change. There is a potential problem with the "+" input to the comparator, the diode prevents the charge from flowing away from that node during turn off. But I suspect that there is a resistor in there somewhere that isn't shown on your diagram. __________________ You don't need a quadraphonic Blaupunkt -- you need a curve ball.

31st October 2009, 04:35 PM	#7
Alexsgarage	It can`t be the diode because the same thing happens on the low side driver and that one is just connected with a single wire. Could it possibly be the pull up resistors?

31st October 2009, 04:37 PM	#8
BrownOut	It could be. That's why I asked you to measure the base voltage. The pullup probably needs to stay in the circuit, but it might be too low valued. __________________ You don't need a quadraphonic Blaupunkt -- you need a curve ball. Last edited by BrownOut; 31st October 2009 at 04:37 PM.

31st October 2009, 04:46 PM	#9
kpatz	What's the value of the pullup? Try the highest value needed to turn the transistor on when the comparator is "off". For an IGBT something in the 1M+ range should work. Is the comparator an open-collector output? Depending on the application you may be able to use an op-amp and eliminate the pullup. Or use a PNP transistor off the comparator to drive the NPN, eliminating the need for the pullup. Last edited by kpatz; 31st October 2009 at 04:47 PM.

31st October 2009, 04:50 PM	#10
Alexsgarage	Thanks kpatz the PNP transistor works nicely. Last edited by Alexsgarage; 31st October 2009 at 04:52 PM.