Additional transformer question

[_stkx]

Greetings

I've seen many commercial smps having no additional regular transformer for powering ICs and stuff for example in monitors and tvs. However all of amateur design contains one, maybe 2x12V transformer. How to achieve such design that no additional transformer is needed? Or maybe you could tell me how this type of design is called so i can futher google it.

Cheers

 

[wally7856]

For no transformer a dropping resistor with a transistor is used to bring the voltage down to the controller level. The resistor only powers the controller until the smps starts up. Then a auxiliary winding on the transformer only 2 or 3 turns powers the control chip. a diode is used to block the voltage from the resistor once the aux winding powers up the circuit.

 

[_stkx]

I understand it now. Thank you for your reply.