STRF6656f PCB

[mahmood]

Hi all

I am looking for flyback power transformer calculations to complte
my project with TV power supply STRF6656f . Its prototype PCB is ready
and work but when i connect a load to output the voltage that was about
32 V ( a zener & optocoupler ) drop to 6 V . i think it is because of transformer !! at the end any one know STR6656f working frq ?

[Cristi]

the STRF6656F is quasi-resonant switch. the operating frequency changes with the load. the transformer is the critical part in such design. must be made strictly according to the specifications. you must keep the leakage inductance within some certain values, and the feedback winding must be properly sized. also the extenal components on the feedback path must have the value indicated in the datasheet or app. notes, since the have influence on the frequency range on which the smps is working.
as an example, for FPS, for 50W the transformer should be made on EER 35 core, with 68 primary turns, split in two sections. the magnetizing inductance should be 780-800uH and the leakage inductance <15uH. the secondary for 12V should have 7 turns, and the feedback winding 20 turns.
take care at that pcb, you have the clearance and creepage too small, and there are some GND loops and overlaps between power traces and control traces. the optocoupler must be placed somewhere else, on the secondary section if the board cannot be wider.