Power Design Tip 4: Damping Input Filter (Part 2)

Filter 18.432M

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In "Power Supply Tip 3", we discussed how the source impedance of the input filter becomes resistive and how it interacts with the negative input impedance of the switching regulator. In extreme cases, these impedance amplitudes can be equal, but their signs are opposite to form an oscillator. A common standard in the industry is that the source impedance of the input filter should be at least 6 dB lower than the input impedance of the switching regulator as a safety margin to minimize the probability of oscillation. The input filter design typically begins with the selection of the input capacitor (CO shown in Figure 1) based on the ripple current rating or hold requirements. The second step usually involves selecting the inductor (LO) based on the EMI requirements of the system. As we discussed last month, near the resonance, the source impedance of these two components can be very high, resulting in system instability. Figure 1 depicts a method of controlling this impedance by placing a series resistor (RD) and a capacitor (CD) in parallel with the input filter. The filter can be damped with a resistor across the CO. However, in most cases, this will result in excessive power loss. Another method is to add a series connected inductor and resistor across the filter inductor.