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TPS40210 - radiated emissions

David Daggett
Intellectual 855 points
Other Parts Discussed in Thread: TPS40210

From our customer:

 

We have a SEPIC design based on a TPS40210, and I’m trying to reduce emissions from it.  Overall, it is not bad but we are getting some AM band (roughly 500 kHz to 2 MHz) emissions that we are trying to reduce.  We can get them under our limit line if we place a shield over the transformer, so the emissions are coming from the transformer itself.  A shield is not a great production solution, so I’m looking for alternatives.  The transformer is a bobbin/drum style, and I think it would help if we could use a toroidal transformer instead.  I’m going to probably order a couple toroidal transformers to try.

 

We originally started with a Coilcraft part number MSD1278T-183MLD, and now we are using Vishay part number IHCL-4040DZ-5A 33uH, but both have similar levels of emissions.  These both are shielded coupled inductors, but the shielding is done with ferrite, and I think it just isn’t very effective at low frequencies.  If the shielding is ineffective, then it basically becomes a loop antenna I think.

 

I was just wondering if TI has recommendations on transformers or coupled-inductors for this SEPIC architecture.  I’ll continue with our own experiments, but let me know if you or any of the power supply experts at TI have any advice for this.

  • 0
    TI__Mastermind 35115 points

    Hi David,

    In general when using a switching frequency below the AM band it's hard to keep the magnitude of the peaks at the switching frequency's harmonics low without adding a shield for the inductor. One thing to try is dithering the switching frequency to help to reduce these peaks. This would require an external circuit and I would need to think about how to do this with the TPS40210. It would look similar to what is done in the application note I've linked below.

    www.ti.com/lit/an/slua646/slua646.pdf

    One thing specific to a SEPIC to try is a loosely coupled inductor. The higher leakage inductance helps filter the circulating current in the primary winding and may also help reduce the emissons. However I have not tried to use them to reduce emissions before. The MSC1278 series is an example of loosely coupled inductors.

    Do you have some details on the limits the customer is trying to meet and how far away they are from hitting them?

    Best Regards,
    Anthony