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Common mode choke off Power supply PCB is better

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Flyback

Well-Known Member
Hello,
We have failed Conducted mode emissions with our offline 150W LED driver. However, we added a Common Mode Choke in the mains cable (as in the attached) and then it passed conducted EMC.

We now are wondering about making room on the LED driver PCB so that we can fit the common mode choke on the PCB.

However, do you agree that if we place the very SAME common mode choke on the PCB, then we will have less chance of passing conducted emissions, because common mode noise is high frequency and can “radiate” around the common mode choke on the PCB and get into the earth return wire like that?

(The common mode noise is doubtless coming from the 1W Buck bias supply on this 150W Linear LED driver. The Earth wire in the mains cable does not actually get onto the PCB, but rather is screwed to the heatsink.)
 

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crutschow

Well-Known Member
Most Helpful Member
The line impedance is very low so I think that the amount of EMI that can radiate into the return is very small.
The EMI from the heatsink was likely conducted through the capacitance between the MOSFET and the heatsink.
 
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