In the mean time I designed the test circuit/board for the power supply of the low voltage parts of the circuit. The MCU, the measuring amplifiers, the display will run from 3.3V (I planed to use 5V supply originally, but the display I'm using is not able to run from it). This supply finally not as cheap I wanted to have. It was clear that I need a switching mode supply, and a chip with high voltage capability. I know that a few hundred milliamps is not to much, and the whole circuit is powered from the mains and not batteries. This is a good candidate for some three leg standard regulator or LDO, but...

Always, that but.

the whole circuit will run from a single 48V supply. Generating just 100mA with a standard regulator means you have to dissipate 4.47W on the regulator. In addition the standard three leg regulators are not designed for 48V input voltage.

So the decision is to use HV buck regulator. Especially the LM2594HV-3.3 model from TI.

I designed a circuit around it:

And a PCB for testing: