sm transformer design frequency?

Hi all hopefully just a quick basic question When designing switch mode transformer for a full bridge converter do you use the fundamental or the pwm frequency as the design frequency? i.e for a 100hz output transformer would you use 100hz as the frequency or the 50khz pwm frequency? obviously it effects turns numbers rather a lot

thanks in advance
Harry

If the full bridge output is 100 Hz, the transformer must be designed for 100 Hz. But it doesn't sound like a good design idea to connect a transformer in this place.