Good day
I'm having a seemingly simple problem, but its one that I can't wrap my head around, tried google searches with no avail, so thought this is the best place to come.
My goal is to have a tiny, portable USB-C powered PSU (using the XYS3580, step up, step down DC-DC) a USB-C decoy board and the constant current dummy load Dave showed in EP102.
Here are the links to what I'm using
XYS3580:
https://www.aliexpress.com/item/4001078475001.html?spm=a2g0o.new_account_index.0.0.6d7d25b9Am7johUSB-C Decoy:
https://www.aliexpress.com/item/1005004406266189.html?spm=a2g0o.detail.0.0.6ad789Xb89XbQ4&mp=1Constant current dummy load:
Case:
https://www.printables.com/model/271405-usb-type-c-adjustable-power-supply (same width and height as PSU module, ie tiny)
The project is dead simple, use the USB-C decoy board to power both the PSU and the constant current load, and then have three terminals. One for the PSU, one for a common ground (part of the problem) and one for the positive side of the constant current load. The plan was also to use a switch to allow the PSU output to connect to the dummy load input, which would allow the dummy load current to be set, without an additional screen and would allow the voltage to be read of the dummy load in action (as long as the PSU output is turned off, as it measures the voltage at the output). I know that it will get real hot, real quick, but I am aiming for as small a unit as possible as I travel alot and do alot of electronics repairs (its actually my PhD topic), so I want a portable PSU and dummy load. (still waiting for µSupply...

)
The problem is that this type of PSU has two negative connections (I think due to low side current measurement), Vin- and OUT-, I assumed that these two could be connected together but the website of the product says
"'VIN-' and 'OUT-' can not be connected together, otherwise module can not support constant current output". Now the problem is that the current sink would have a different ground to the PSU, thus they could not share a common ground terminal, and even if they had separate terminals, I would not be able to connect the PSU outputs to the current sink inputs, as the grounds would then be tied together. I was thinking one solution would be to give the current sink its own separate isolated supply (in the form of a battery) but id prefer not to as the whole goal is to make the project as small as possible.
Any ideas? The best outcome would allow both the PSU and current sink to use the same USB-C supply, but also allow them to be connected together to allow the PSU to feed into the current sink.
I have attached the super simple schematic diagram for clarity
Any suggestions would be appreciated