CMC FILTER DESIGN

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Hi, I would like to learn and know more about how to design CMC filter for an AC to DC LED driver (75W to 100W). If there is any step by step guide with necessary formulas, I would be very grateful.

Thanks in advance

Hi,

In designing CMC filters, we usually iterate until we get to the needed inductance. You can start off with a small CMC and work your way up until you get a passing EMI scan. Usually for 75 to 100W designs, we start at around 10mH. Take note that this only filters common mode noise so you would need to optimize the other EMI filter components to get desired EMI filtering.

There are two ways of choosing a CMC: either you use an off-the-shelf CMC or you can create your own.

If you want to use an off-the-shelf CMC, here are some considerations:

  • Check current rating of chosen CMC - this should be higher than the highest input current of your LED driver (input current at lowest input voltage)
  • Check the DC resistance of the CMC - you would want this as low as possible to minimize losses
  • Check the CMC's impedance vs. frequency - this would dictate until what frequency it will be effective at filtering 

If you want to create your own custom CMC:

  • Pick the core - depending on your size restriction, pick the largest core you can use; also choose a core material with high AL (inductance per turn2)
  • Choosing wire gauge - choose wire gauge that can handle highest input current; choose the thickest wire gauge that fits the core chosen (take note that the core is divided into 2 equal sides) given the target inductance
  • Always check inductance of both sides after winding - make sure that both sides are balanced (same inductance) as any imbalance may lead to ineffectiveness of the filter, especially at high power applications

I've attached a sample custom CMC specification. These are used by DER-801 for input and output CMCs. 

TitleSize
Input CMC208.86 KB
Output CMC348.67 KB