Hi Mitch,
Thank you for posting the results of your PFC converter. I would like to help you get better results. But I will apologize in advance since it's possible some of my answers may not necessarily be applicable to your converter. I don't know much about PFC converters.
1) Low noise mode is not likely to be very useful for SMPS measurements. I assume you tried that that to go below the 119 Hz limitation. The issue with low noise mode is that it assumes there is not significant noise such as the ripple and switching transients common to SMPS. Specifically, what it does is sample some modest amount above the stimulus frequency. At some stimulus frequencies, that will be well below the Nyquist frequency of the main noise source. Depending on the relationship between the noise source frequency and the sampling frequency, the noise might show up as an alias. In the case where the noise is much higher amplitude than the stimulus (as is common with SMPS measurements), the aliased noise would be very destructive.
2)There is an enhancement I plan to make to relax the lower frequency limitation. It will give an ability to make tradeoffs between the lower limit and measurement quality. But I can't predict how it will impact the quality at lower frequencies. Here's what I plan to do, which may help explain the trade off. In the current design, the lower stimulus frequency limit is determined by the combination of high-noise sampling frequency, scope buffer size, and an imposed limit of 16 stimulus cycles. (See the end of this post:
topic14311-90.html#p67481) What I plan to do is remove the min (16) and max (100) stimulus cycles (min. would probably still be 1), and replace with a fixed or maximum filter bandwidth. At those lower frequencies, the filter (DFT bin) width will be relatively larger compared to the stimulus. In the case of 1 cycle, the width will be Fstim +/- Fstim/2. That may or may not be OK for some applications. But in any case, it will be good to let the user have the flexibility to choose. For your scope that value would be adjustable to around 8 Hz.
3) I notice your input amplitude is set to the maximum, 4.0 V. I don't know how that relates to the overall voltages your dealing with, perhaps it's relatively small? In the DC SMPS I am dealing with, I am using stimulus in the 10's or millivolts. You might get better results with smaller stimulus amplitude.
4) One other limitation related to input amplitude is that there is an important enhancement (adaptive stimulus signal amplitude) not yet available in the application. As mentioned in several sources about this SMPS measurement technique, the stimulus needs to be "right sized", which varies by frequency. By right sized it means not so large that it affects the converter operation (e.g. drive it into non-linear operation), but large enough that the SNR provides for good measurement quality. For the TPS5420 I have, I've found that I can get away with fixed stimulus, but might get better results with this enhancement. Briefly, the way adaptive stimulus would work is to find the lowest stimulus capable of achieving some SNR metric, while also limiting the maximum output.
Again, I apologize if some of this doesn't really apply to your PFC. I'll probably learn something in this process
The enhancements I mention above are the two highest priority enhancements on my list.
Thanks,
Aaron.
