5 Passive Filter Limitations

C_F connects to the main power rail and must be rated for the maximum DC voltage. Designs requiring a large C_F and a higher DC voltage will necessitate the use of a larger and more expensive capacitor. Similarly, L_IN must be rated for the maximum input current (I_IN) at minimum input voltages. In higher-power systems, the inductor size and cost will further increase given the larger inductance and DC current requirements. Transients such as load dump and cold crank exacerbate the V_IN and I_IN requirements.

A second-order effect is that the self-resonant frequency (SRF) decreases as the size of C_F increases. At the SRF, the impedance of the equivalent series inductance (ESL) equals that of the capacitance and marks the lowest impedance point over the frequency range. A higher ESL deteriorates the filter’s performance and its ability to attenuate noise at high frequencies.

Figure 5-1 shows two capacitors where the larger capacitor (in blue) is less effective at higher frequencies compared to the smaller capacitor (in black). The figure shows both the impedance and effective capacitance. At frequencies above the SRF, the capacitance rolls off and the parasitic inductance dominates the capacitor impedance. A similar SRF effect can limit L_IN in filter performance, where parasitic capacitance dominates the inductor and deteriorates performance at high frequencies.

Figure 5-1 Impedance of two different package and value capacitors: the larger capacitor (blue) is in a larger package and has a lower SRF, which results in a larger impedance at high frequencies compared to the smaller-package capacitor (black).