Probing High side IGBT's VCE of an H bridge circuit

[rahul91]

Hi,
I have an H bridge IGBT circuit. I want to probe VCE of the two high side IGBTs. Obviously , the signals to be measured are differential. I have an oscilloscope which can measure differential signals. The issue is that it can measure a maximum of 25 V and I am applying 48V dc as the H bridge power. I have got an x10 single ended probe.Can I use this probe to apply the differential signal to the oscilloscope...?

 

[kabeer02]

Why cant you use voltage divider rule using resistor so that you can step the voltage down then you can use your oscilloscope probe to measured your signal?

 

[FvM]

I have got an x10 single ended probe.Can I use this probe to apply the differential signal to the oscilloscope...?

Yes, but you need two of it. Compensation adjustment of both probes must match well.

 

[rahul91]

Why cant you use voltage divider rule using resistor so that you can step the voltage down then you can use your oscilloscope probe to measured your signal?

Thank you...I tried that method...But had an issue with that probing....The high side collector emitter voltage was expected to swing between 48 V(OFF) and 0V(ON)...But the observation was from 48V(OFF) to -48V(ON)...Not sure it has turned ON ....But surely the gate emitter was sufficient to make it ON.... I am not sure whether Iam right or not, what I think is that when doing so , the measurement is not made with respect to ground..May be that is the reason...But calculations shows that the method should work...a little confused.....What made me think so is that, then I measured the two points, VCC and VEE with respect to ground..then using the math function available with my oscilloscope ,plotted their difference...got the result.... VCE=VCC-VEE.
Then I measured the load waveform in the above method...It also worked...ie .. load voltage was swinging from +48 V to -48 V...
So really the ground reference was the issue...???

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Yes, but you need two of it. Compensation adjustment of both probes must match well.

Thank you.. I tried as you suggested.. using two probes with respect to ground..and apllied the math function subtraction in my oscilloscope....it worked...

 

[schmitt trigger]

As FVM mentions, make sure that both probes are identical AND are properly compensated.
Otherwise, the resulting waveform will be distorted where it counts the most, the leading and trailing edges.