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UCC28C40-Q1: Transformer choice and turns ratio description

Part Number: UCC28C40-Q1

Hi all,

I've been trying to spec a suitable transformer based on the Power Designer generated design I've got for a 12-24V to 5V 3A isolated power supply.

The ideal transformer is described as;

Ns1toNp= 0.376
Ns2toNp= 0.728

Two things I find confusing about this; these two metrics seem to describe a ratio between a dual secondary to single primary transformer, but the generated design report clearly shows a dual primary to single secondary, is this a misinterpretation?

Secondly, I've spoken to Wurth about this and they've suggested the 750311589 and 750311597 transformers (datasheets attached), are these suitable for the given spec?

I'm not a power engineer, so I'm in the dark as to how these are wired also, it looks like the Ns1toNp could be satisfied by a single 3:1 ratio coil arrangement but the Ns2toNp would have to be two 3:1 ratio coils in series?

Thanks in advance for any help you can provide.

  • Hello Louis,

    1. Normally S1 is the secondary winding that delivered the output load power. S2 is the winding at the lower end on the left side of the transformer and it acts as an aux winding to supply voltage and current to power the controller driving the power FETs.

    2. 750311597 is a better match from the two transformers because it has a tighter margin of output current, hence more suitable for your design.

    3. Turns ratio is determined by the number of turns in the winding. For example your NS1: Np is 3:1 turns ratio which means for 10 turns on the primary side there would be 30 turns on the secondary S1 side of the transformer. Similarly for NS2: NP would be 10 turns in primary to 15 turns in sec S2.

    I hope this answers your questions.


    Regards,

    Sonal

  • Hi Sonal,

    Thanks for your reply. Can you confirm I've wired the 750311597 correctly then?

    I'm using 1-3 : 10-7 for my Ns1toNp and 5-6 : 10-7 for my Ns2toNp, does this satisfy the requirements?

    Regards,

    Louis