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LC Noise Filter Effect Using Common Mode Choke Coils (Normal Mode)

Make the following connections to check the attenuation effect of normal mode noise in a filter circuit in a comparison of actual measurements and simulation for a noise filter using a common mode choke coil.

In normal mode, due to the cancellation effect of the magnetic field in common mode choke coils, they do not function as common mode choke coils but leakage inductance is formed by magnetic flux leakage.
The equivalent circuit in normal mode is shown below.

However, if the capacitance of Cy is not negligible compared to the capacitance of Cx, the total capacitance of capacitor X is (Cx+(Cy/2)), and the cutoff frequency will decline when in normal mode.

With capacitor Y, in normal mode, the phase relationship is reversed between the Hi side and Lo side corresponding to the signal line.
The Capacitor Y ground corresponds to earth and can be omitted in normal mode as no current flows through capacitor Y in this case.
Also, since the characteristic impedance Z0 of a filter consisting of leakage inductance and capacitor X is Z0 = √(2Lleak/Cx), then it is necessary to align the load impedance.

With the LDFL002302LS-V0E, the load resistance is the value obtained by subtracting the DCR component (4 terminals) from the value obtained by the above formula. The simulation is performed with the following circuit constants.

【Circuit constants for LDFL002302LS-V0E】
Each component Circuit Numerical value of the equivalent circuit (input values)
stray capacitance C 2.1pF
iron loss(Loss of magnetic material) R 11.6kΩ
Self-inductance L 15.0mH
coupling coefficient k 0.9996333
copper loss(Loss of wire) DCR 30mΩ
Dummy resistance Rd 1GΩ
Load resistance R-Load 0.18Ω

The total leakage inductance is the sum of the leakage inductance on the primary and secondary sides, which is double the value (5.5μH × 2).
Based on the above, the equivalent circuit in normal mode forms a low-pass filter with leakage inductance and Capacitor X, and the cutoff frequency Fc can be expressed by the following formula.

The comparison results of the measured values ​​and SPICE simulation attenuation characteristics whenLDFL002302LS-V0E and our conductive polymer hybrid aluminum electrolytic capacitor HHSE630ELL101MJC5S are combined are shown below.
The attenuation characteristic is approximately -40dB/Dec., and we see that the actual measurement and the simulation match well.

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