Today I started taking a look at the 8.4V converter. I found some surprises in the results and challenges in trying to scope them. The attachments are a set of signals I captured while scoping Q1's drain and D8's anode. Note that these captures are from the new Owon PSU that is currently installed in my scope.
The main challenge was dealing with the signal references, Q1 is in the primary side and is referenced to GND-A, D8 is in the secondary side and is referenced to GND-B. Note that connecting a ground lead to GND-A would cause a short circuit and as a minimum will blow the fuse, so that is out of the question. Connecting a ground lead to GND-B is not a good idea either, it probably wouldn't be catastrophic, but it will short out the CC current sense resistors (R14/R14A/R14B). So the only option is using GND-C which is also Chassis gnd. GND-C is separated from GND-B by no more than 125mV, so it's a fair reference for D8. However, it's a different story for Q1, because GND-A is nearly the same as GND-C half the time, but separated from it by 100s of volts the other half, it depends where on the mains AC cycle you are at. However, this knowledge is helpful, because you can focus on just the waveforms during the time that GND-A is nearly the same as GND-C and ignore the rest. Just use, for example, single trigger, until you capture something with the correct baseline.
Some of the captures were done while I powered the scope with my Variac. I wanted to see the effect of different mains voltages, and using the Variac I was able to vary the voltage between 100VAC and 140VAC. Unfortunately my Variac is almost as old as me and doesn't have a ground lug on it's power cord or on its output outlet. So I had to operate the scope without a ground and that changes the way GND-A is referenced. In this case it is floating in respect to GND-C and the signals referenced to it act as if the scope channel was AC coupled. So for those captures I manually set the signal's ground base line to the third graticule below the x axis. Of course in these cases the channel's ground marker should be ignored.
The surprises include the frequency, which varies depending on the mains voltage, and the converter's operation mode, which is CCM at low mains voltages, is in transition at around 120VAC, and is DCM at higher voltages.
#1 - Shows the effect of GND-A on Q1's signal. For this capture the scope was directly connected to the earthed mains (120VAC) and the persistence was set to 1s.
#2 & 3 - Shows that at 120VAC the operating mode is in transition, sometimes CCM, some times DCM. Also note the frequency, at 120VAC it's around 58KHz, even though the R7731A is programmed to operate at 65KHz.
#4 thru #9 - These were obtained using the Variac, and show the effect of the mains voltage on the operating mode. Also note the change in frequency. At 100VAC it's 65KHz and as the mains voltage increase the frequency decreases, the last image (140VAC) has a frequency of 56KHz.