Hi,
If the results from 6500uH and 68mH common mode chokes are compared:
There is very little difference between the two results. This is consistent with the result predicted by modelling.
In the high frequencies there is no difference. This the important area. This is where the phase margin is measured.
At the lower frequencies there is more noise with the 6500uH magnetizing inductance.
Time DomainTo determine what is happening here we can look at the signals in the time domain with an oscilloscope. The signals from the injection transformer were connected directly to the scope inputs (no 10x probes).
At 50kHz the signal both signals are a reasonable size. This is in the 'noise free' part of the Bode plot.
At 220kHz, close to the 0dB point for this power supply, the signals are equal in amplitude.
At 10kHz the loop gain is higher, so the signal from TP5, is decreasing in amplitude.
At 1kHz the loop gain is even higher and the signal on TP is very small.
The loop gain is around 46dB so the signal on TP5 is 200x smaller than the signal on TP7.
If the vertical setting on the scope is changed to 1mV /div and reduce the bandwidth:
the waveform can be seen.
The signal on TP7 is 40mV p-p (15mV RMS)
The signal at TP5 is 15mV / 200 = 75uV
Go down to 100Hz and the signal on TP5 is around 7.5uV RMS, if you assume a single pole slope.
ObservationThe LF -3dB of the injection transformer does not directly impact the loop gain measurement.
The LF -3dB reduces the signal size, and makes a difficult measurement harder because of signal to noise ratio, SNR issues.
The low frequency performance of the power supply being measured is of little interest. The interesting part is where the gain is 0dB and the phase margin is measured.
If the power supply has a low bandwidth control loop, the gain is lower and the signals are bigger at low frequencies. SNR is less of an issue.
Regards,
Jay_Diddy_B