During the start of a Switching Mode Power Supply (SMPS), the control circuit (oscillator) can't get its power from the transformer because it's not powered up yet. At the same time, it can't get its power from the mains because won't handle that much voltage. So, what is the technique or the Supply circuit used to give power to the control circuit so that it oscillates and make the transformer work?

I understand that some SMPS use ICs that are able to handle the mains power but I'm asking about the ones that don't use ICs. They just use low power transistors that work as oscillator.

I would appreciate it if you provide a very basic schematic of SMPS. I don't care about the output voltage or the quality (ripples/harmonics). I just need a very basic schematic that answers my question and to understand the basics.

 

There is a switch which connects input power to the inductor in a buck converter. The switch is controlled by an oscilator with some initial duty cycle. The on/off be havior of the switch develops an output voltage across the load after the turn on transients have subsided. The output voltage will be in a stable steady state after those transients have subsided,

 

During the start of a Switching Mode Power Supply (SMPS), the control circuit (oscillator) can't get its power from the transformer because it's not powered up yet. At the same time, it can't get its power from the mains because won't handle that much voltage. So, what is the technique or the Supply circuit used to give power to the control circuit so that it oscillates and make the transformer work?

search: smps start up resistor

https://forum.allaboutcircuits.com/threads/how-smps-starts-up-auxiliary-winding.179881/

The above link is to a good explanation. I could add that:

a. It may take two or more series resistors as resistors have voltage limits and the 300+ DC voltage
exceedes the typical limits.

b. If one of these resistors fails open the power supply won't start -- easy to test & easy to fix..