PMP21462: Power supply design using UCC28742

Hi Jitender,

This transformer was designed to be magnetically resistant. The idea for this is to keep performance similar across the operating range even if an external magnet is placed close to the transformer.

To handle this a few design choices were made: first an iron powder core is used. While not the most efficient for isolated power conversion, its soft saturation curve prevents the primary inductance from dropping too much. The primary inductance for this design was 2.4 mH.

The turns ratio also had to be chosen to handle the wide input range. For the original transformer design the (pri : sec1) = (28.71 : 1), where sec1 is the 4 V output. The (sec1 : sec2) turns ratio is (1 : 2.57), where sec2 is the 10 V output. To increase sec2 to 12 V you would need to increase the turns on that output. I calculated that with diode drops considered a (sec1 : sec2) turns ratio of (1 : 2.67) will make the auxiliary output 12 V.

Hope this helps!

Thanks,

John