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Transformer design in SMPS


Swathika

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Is it really that low?

If you are doing a planar, or it's a wire wound non-isolated application, seems that that constant would be unusable. On the bench right now I have a interleaved flyback (current mode) 1/4 brick running at 250KHz with planar's (16 layer PCB with 3oz Cu) that uses RM6 N92 cores (8mil gap) and I'm getting a bit over 120W out if it with efficiencies in the low 90's. While I haven't calculated it out, I don't think it would be in the 40% range.

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Typical Ku factors for standard converters:

Forward                  0.4
Full and 1/2 bridge 0.4
Push Pull w CT        0.4 ~ 0.5 (0.5 for the single ended secondary output)

This sounds low but due to the shape of standard round copper wire, if you do the math only about 78% of the available window area can be filled. Take the insulation of the wire, any other layer insulation, mandatory creepage distances, RFI and safety screens. Add the bobbin and you can start to see the picture.

Construction with foil windings will also improve Ku

Planer designs may have slightly higher factors.

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