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UCC28C54: EMI troubles

Marek Šmíd
Prodigy 20 points
Part Number: UCC28C54

Tool/software:

Hi,

I would like to ask about EMI tests. I have cca 100W flyback without pfc, with UCC28C54 and Synchronious rectifier. Switching frequency cca 95 kHz. I tried to handle peaks caused by switching frequency by dumping the filter, but without change. So it looks like I have to make better differential mode filter.

Do you think that better differential mode filter will effect the peaks up to 30 MHz ? Or it will effect peaks only up to certain frequency, for ex. 10 MHz ? I want to know if only the improvement of differential filter can handle the requirements, or even common mode filter needs to be improved.

Or any another ideas to meet EMC requirements ?

Thanks a lot.

  • 0
    TI__Mastermind 26955 points

    Marek,

    Your first harmonic is happening at 190kHz (2fs) and is about 25dB over the peak limit. Since the scan you are showing includes the total CM+DM emissions, it's not clear which is the dominant offender. Ideally you should measure the CM and DM contributions independently but if you are unable to do this then lets assume the total scan is 100% DM and 100% CM and design a filter to target both. Make sure all your EMI components are properly rated current and voltage and consider DC bias effect for inductors, capacitors, als, consider appropriate inductor, capacitor SRF for the frequencies you are targeting.

    A typical EMI filter would be something similar to the arrangement below. Prefer to use LDM as 1/2LDM+1/2LDM like shown below but designers often combine as a a single LDM. Also, common practice to include a Y-cap directly across the flyback transformer primary-to-secondary isolation barrier:

    Helpful EMI Filter Design Tools for plotting insertion loss:

    1. Wurth Red Expert
    2. Murata Simsurfing 

    Regards,

    Steve