Other Parts Discussed in Thread: PMP21098
Hello
Out of WEBENCH I got a design for an AC/DC 240VAC --> 12VDC 7A converter.
It uses the UCC28C54.
It also uses a custom transformer with a "turn/ratio" of about 10 turns Np:Ns, 146:14
According to the datasheet for the UCC28C54, Vreflected must not exceed 130V in the datasheet's application example, in this case it is the same in my WEBENCH design as in the datasheet's example.
This is because the MOSFET used for the switching has Vds max = 650 volts, and the bulk voltage that comes from the maximum AC supply voltage, as well as some margins.
It is thus the input voltage and the MOSFET's Vds that determine the maximum number of winding turns on the transformer, and in this case it will therefore be a maximum of 10 turns.
If you then look at the PMP21098 reference design, where a transformer is used that has a turn ratio of 35, but which still leaves 12V out and has 240VAC in, i.e. the same spec as in my case above on these points. So no consideration is given to Vreflected in this case. Here the MOSFET used for switching "only" has 550Vds even though the supply voltage (V bulk max) is the same.
Why doesn't Vreflected need to be taken into account in design according to PMP21098, there must be a reflected voltage there in the same way as in WEBENCH design, can someone explain this to me?
The WEBENCH design I received uses a custom made transformer, and the PMP21098 uses an off-the-shelf variant, which is why I would like to use the latter, but in the slightly less space-consuming design from WEBENCH.
Texas Reference Design PMP21098:
www.ti.com/.../PMP21098
WEBENCH design according to my wishes
https://file.io/7PR7Wi26FaI7
Data sheet on UCC28C54
https://www.ti.com/lit/gpn/ucc28c54
Thanks, Thomas Nilsson
