TPSF12C1: Cx2 capacity

Hi Kobayashi-san,

Hirotsugu Kobayashi said:

Is it correct that the signal is injected on the N side via Cinj and on the L side via Cinj and Cx2?

Yes, this is correct. The signal can also be injected to the L-side as well.

Hirotsugu Kobayashi said:

Is there a guideline for how many times more Cx2 than Cinj is required?

There is no guideline. CX2 with leakage inductance of the chokes help with the DM noise as explained in the other thread. CINJ and the chokes will form an L-C filter to mitigate CM noise. 

Ben

Hi Ben-san,

If Cinj and Cx2 have the same capacitance, won't the signal to the L side be attenuated?

I think Cx2 requires a sufficiently larger capacity than Cinj, but am I wrong?

Best regards,

Hirotsugu Kobayashi said:

If Cinj and Cx2 have the same capacitance, won't the signal to the L side be attenuated?

It is not advise for both CINJ and Cx2 to have the same capacitance when designing an EMI filter. Because the X-capacitors are large compare to the Y-capacitors, the X-capacitors will appear to be a "short" across L and N. If the X-caps are lower in value, yes, the signal being injected from N to L will be attenuated.

Ben

Hi Ben-san,

Therefore, there is a relationship of Cx2>Cinj.

Hi Kobayashi-san,

Yes, that is correct.

Ben

Ben-san,

As an example, it might be a good idea to have a guideline of "Cinj is 1/10 or less of Cx2".