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PMP21516: EMI Filter Design
Part Number: PMP21516
Can anyone explain why the two capacitors (C8, C9) in the CLC input filter are set up the way that they are? The screenshot is from this reference design:
I will provide some more context to my question:
I understand that there needs to be a smoothing capacitor on the output of that full-wave bridge rectifier to smooth rectified DC voltage. A rule of thumb of 2 - 3uF per watt of input power yields a value of 135uF - 405uF for Cin.
I also know that there needs to be an LC input filter to help mitigate differential-mode EMI.
According to TI's AN-2612 (linked below), the LC values are chosen based on equations outlined in section 4.2.
I cannot for the life of me figure out how we end up with those capacitor values (or inductor values for that matter) in that configuration. Can anyone offer their expertise?
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The bulk input caps on this design were sized for the steady state output power of 50W. If you want to size them for the peak loading of 100W, you are correct, it would require more capacitance. I typically use a factor of 1.5-2 uF/W for supplies that operate over universal AC input range.
There are some advantages to the PI filter approach of the differential choke and splitting the bulk cap into two capacitors, versus using a single larger capacitor and placing the differential choke before the bridge. With the PI filter approach, the current in the inductor will be mostly DC with a much lower peak current than if it were placed before the bridge. Additionally, the RMS current in the inductor will be lower. Plus you can get away with lower inductance to achieve the attenuation you need to pass EMI. Combine the lower current ratings with lower inductance, and you can improve efficiency and/or reduce the physical size of the inductor.
The trade off is that now you need two aluminum capacitors instead of one. This will likely be slightly more expensive and a slightly worse use of the available volume due to the cylindrical shape.
Either way of configuring your input filtering is OK, and you can easily find examples of either approach. It just depends on what your design criteria area on efficiency vs. size vs. cost.
Also, for more reference material on designing for EMI compliance, especially in flyback converters, check out the recent "Practical EMI Considerations for Low-Power AC/DC Supplies" topic from our Power Supply Design Seminar series.
thanks and regards,
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