How Ycap reduce radiation EMI

Zhihong

I recommend you review this TI/Unitrode PSDS paper on EMI https://www.ti.com/seclit/ml/slup202/slup202.pdf.  As it describes in the beginning of Section 8 on radiated EMI (page 12), it highlights that conducted and radiated EMI are related and impact one another.  Another good paper I recommend you review is https://www.ti.com/seclit/ml/slup338/slup338.pdf, specifically section "EMI shielding and cancellation techniques" that starts on page 21 since it focuses specifically on the EMI noise you are working with in your design.

The flyback topology generates common mode EMI due do its single ended PWM operation.  Since there is parasitic capacitance across the transformer due to the interleaved winding (which for a flyback transformer can be high since a high amount of interleaving is commonly used to reduce leakage inductance), this noise couples to the output and causes conducted and radiated EMI measurements to fail.  By adding a y-capacitor across the transformer, a low impedance path shunts the energy back to the source rather than have it radiate outwards.

Another way to think about it is with Kirchhoff's current law (KCL) and impedance divider.  By adding a y-capacitor, you are adding a capacitor in series with the winding capacitance back to the voltage source.  This creates a capacitance impedance divider on the output, which reduces the magnitude of the common mode voltage source that generates the radiated EMI you are measuring.

Best Regards,

Eric

Hi Eric,

Thanks for your sharing and reply.

Here is my analysis about how the Y capacitor between the primary side and secondary side improve radiation EMI.

Could you help me double check my analysis to see if my analysis make sense and give your advice?

1. The area of common mode current loop is reduced, so the radiated EMI is reduced

• Before adding Cy2, the flow direction of common mode current is as shown in the blue circle below, and the area of current loop is large.
• Flow direction of ICM before adding Cy2:
• Primary side , Transformer parasitic capacitance , Secondary side , CS1 , Cy1 ,Primary side.
• After adding Cy2, part of the common mode current is shunted, and the flow direction is shown in the red circle in the figure below. The area of this part of the common mode current loop is small.
• Flow direction of ICM after adding cy2：Primary side , Transformer parasitic capacitance , Secondary side , Cy2 ,Primary side.

2. Reduce the common mode current flowing to the long wire that may become the antenna, thus reducing the radiated EMI

• Before Cy2 is added, the high-frequency ICM will flow to the rear circuit through the long wire connected at the front and rear stages, so the long wire is likely to become an antenna and radiate outward under this condition.
• After adding Cy2, the high-frequency ICM of part of the secondary side will flow back to the original side through Cy2, so as to reduce the ICM flowing to the rear long wire, and thus reduce the radiation generated when the long wire is used as the antenna。

Best Regards

Wesley Huang

wesley-huang@ti.com