(LYT5228D Buck) Self-wound transformer causes high radiated emissions

Hello,

I wound a transformer for a LYT5228D circuit, according to the ACDC_LYTSwitch5_Buck_051424; Rev.2.0 design spreadsheet and its associated magnetics designer. I wound it exactly according to the spreadsheet (see attached a screenshot of the relevant sections of the spreadsheet). However, it causes strong radiated emissions as measured with our pre-compliance TEM cell. I have tried to correct this using a wire-wound shield, however this is not sufficient. I could confirm the transformer was the source of the issue by completely isolating it with copper tape that is connected to the PCB ground, which brings the emissions down. However, that is not something we can do for series production.

The magnetics designer seems inaccurate, as the gap I had to use to get the desired inductance was more than twice as wide as the one suggested in the spreadsheet, despite correctly entering the values of the cores used. A large gap like that is sure to cause radiated emissions, however using a smaller bobbin and core for the transformer is also not possible because most of them are too small for the inductance required, as per the magnetics designer.

My application has a VO of ~36V worst case (the LEDs used have a forward voltage of 2.6-3V) and 320mA. The LYT5228D should be suitable for that, or at least the design spreadsheet does not suggest any issues with this, however the suggested transformer is not usable like that as we have to conform to EMI standards.

Is there any way I can reduce the emissions of the transformer that is both practical and affordable for series production? Even the wire-wound shield is already increasing production cost of the transformer, nevermind grounding the core itself.