8.2.2.5.1 EMI Filter Design

EMI Filter Design Steps:

• Calculate the required attenuation
• Capacitor C_IN represents the existing capacitor at the input of the switching converter (10 µF was used for this application)
• Inductor LF is usually selected between 1 μH and 10 μH (1.8 µH was used for this application), but can be smaller to reduce losses in a high current design
• Calculate capacitor C_F

Figure 29. Input EMI Filter

By calculating the first harmonic current from the Fourier series of the input current waveform and multiplying it by the input impedance (the impedance is defined by the existing input capacitor C_IN), a formula can be derived to obtain the required attenuation:

Equation 37.

Equation 38.

V_MAX is the allowed dBμV noise level for the particular EMI standard. C_IN is the existing input capacitors of the Buck converter, for this application 10 µF was selected. D_MAX is the maximum duty cycle, Ipk is the inductor current, the current at the input can be modeled as a square wave, F_SW is the switching frequency.

Equation 39.

Equation 40.

For this application, C_F was chosen to be 1 μF. Adding an input filter to a switching regulator modifies the control-to output transfer function. The output impedance of the filter must be sufficiently small such that the input filter does not significantly affect the loop gain of the buck converter. The impedance of the filter peaks at the filter resonant frequency.

Equation 41.

Equation 42.

Referring to Figure 29, the purpose of R_D is to reduce the peak output impedance of the filter at the cutoff frequency. The capacitor C_D blocks the dc component of the input voltage, and avoids excessive power dissipation on R_D. The capacitor C_D should have lower impedance than R_D at the resonant frequency, with a capacitance value greater than 5 times the filter capacitor C_IN. This will prevent it from interfering with the cutoff frequency of the main filter. Added damping is needed when the output impedance is high at the resonant frequency (Q) of filter formed by C_IN and L_F is too high):

An electrolytic cap C_D can be used as damping device, with value:

Equation 43.

For this design C_D = 47 µF was selected

Equation 44.