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Little Bit of a Massive Problem

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Alexsgarage

New Member
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.
 

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Alexsgarage

New Member
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

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

Alexsgarage

New Member
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.
 

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BrownOut

Banned
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

New Member
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

Banned
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.
 
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kpatz

New Member
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.
 
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