# 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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• PA310404.JPG
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#### Alexsgarage

##### New Member
I had an original schematic but I have modified the circuit so much that I dont 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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• block diagram.jpg
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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 cant 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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#### Alexsgarage

##### New Member
Thanks kpatz the PNP transistor works nicely.

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