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measuring power of load in a PWM low side mosfet circuit.

Posted: May 4th, 2019, 1:07 pm
by Mark07

I'm looking for help in making a proper power measurement of a non-inductive resistor (1Ω) in a low side mosfet PWM circuit.

I am using all brand new tools from Tektronix and they were all auto-deskewed based on recommended settings.

- MDO3024
- TCP0030A X 2 (current probe)
- TMDP0200 (diff probe)
- TPP0250 (stock S.E. probe). ... 3MTft-zLWq ... BQ98iZtW72

Power IN ... Ls9iAfLQ88

Power OUT ... RiLx7NQr4F

The problem I'm having is that when I have the scope do a math function (CH1 X CH2) and I "measure" Power RMS on the input and compare it to Power RMS on the output (CH3 X CH4) I get typical C.O.P ≥ 1.67.

I am very careful with making sure the current probes are degaussed, and I've been trying all types of "Acquire" settings and even Gates Cycles etc. and different "record lengths" and I keep getting similar results.

Is there some measurement technique that I'm missing? Or is there some pdf to read or video to watch?

Thank you Kindly.


Re: measuring power of load in a PWM low side mosfet circuit.

Posted: May 6th, 2019, 3:35 pm
by Mark07
Just today I learned that when I change trigger to a different channel, the recommended deskew numbers will adjust to the trigger channel as primary... Not sure how much that would affect power results if I've been switching triggers during testing.

I do know that the difference between deskewed and skewed power measurements is significant enough to warrant real calibration.

The RMS power measurement is consistently showing me over unity type results... The Mean power seems to be as expected.

Is the RMS power measurement not intended for loads with low-side mosfet switch and PWM signal?