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UCC28180EVM-573: UCC28180 | Power Loss in Output Capacitor

Part Number: UCC28180EVM-573
Other Parts Discussed in Thread: UCC28070

Hi,

 I have the following doubt;

1. While designing the PFC (Boost, CCM), the losses in the output bulk capacitor is Irms2*ESR.

Let us say, i am designing a 1200W PFC over the universal input with 400V as my DC bus and 100% efficiency, 

my capacitor losses are coming out to be 9.91 W.

Even if I use MLCC on the output side, only the high frequency ripple is what is bypassed and low frequency ripple goes through only the bulk capacitor.

I have seen some reference designs on boost PFC. There the loss percentage of capacitors is also mentioned in the overall loss split-up.

How is this issue addressed in practical designs without extra forced cooling? The thermal resistance of the capacitor is also so high that it is not that easy to transfer heat!

Thanks,

Siri

  • Hello,

    If you can't use external cooling and the capacitors are running too hot based on your ambient teampature.  You could select capacitors with lower ESR to dissipate heat.  You could add ceramic capacitors in parallel to reduce the total ESR on the boost stage to reduce heating.  The other option is to run more parallel boost capacitors to get the total ESR down.

    You could tryin interleaved PFC as well for your 1.2 kW design.  (ucc28070).

    1. It uses two stages in parallel that operate 180 degrees out of phase.

    2.The capacitor RMS current in interleaved PFC is half of what it is in a single stage.

    3.Running stages in paralell cuts all conduction losses in half. 

    4. You can also achieve a 30% reduction in total magnetic volume.

    The following link will bring you to a seminar paper on the benefits of interleaved PFC.

    Regards,