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High switching noise when connecting injection transformer for loop response measurements


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Junior Member level 1
Jan 16, 2023
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I am attempting to take some loop measurements so I can tune a digital compensator for DAB converter (12V to 300V, 600W, 150KHz). The output voltage is sensed using an isolated amplifier (AMC1202).

The method I am using is similar to the following application note:
But for safety reasons, I am injecting the test voltage after the isolated amplifier. The entire signal chain is shown below:

Since the impedance looking into the output of U2 is low, and the impedance looking into the input of U3 is high, this injection point should be acceptable. I have also used this method in the past with success.

The problem I am having is I see high amplitude switching noise on both channels AFTER connecting the injection transformer. Here is a plot of the Channels 1 and 2 (AC coupled) BEFORE connecting the injection transformer:

and here is AFTER connecting the injection transformer (note the difference in the voltage scales, +/-50mV vs +/-500mV):

The function generator connected to the injection transformer is Off for the plot above. The AFTER was actually much worse, but I but a ferrite on the input and output of the injection transformer and saw the switching noise reduce by ~50% (I can't seem to get it any better than that though).

I am using a Picoscope 2205A, a homemade injection transformer, and the FRA for Picoscope utility. I am concerned the switching noise is affecting the FRA for Picoscope's ability to calculate the gain and phase.

Has anyone experienced something similar to this or have any suggestions for reducing the noise (or identifying the true source)? I'm happy to provide more info if necessary.

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