Input filter interaction with SMPS (9 of 13)

Filter Damping

Input Filter. Feedforward techniques have been proposed to prevent the peaking of the input filter from causing instability and performance degradation. See Kelkar and Lee for an example. This technique is controversial and not recommended by some investigators. However, the primary reason for damping the input filter is usually to control the amplification of input voltage modulation caused by the resonances of the input filter and feedforward should not be necessary.

Example. For example, MIL-STD-461, CS01, applies an 8 Vp-p signal (50 W maximum input) on a 28V input in the likely range of the input filter peak (20Hz to 9kHz). Other modulations invoked by specification or actual environment are similar. Very little gain from the input filter Q is tolerable, and a well-damped filter is called for. This necessarily well-damped filter usually aids in meeting the various criteria without the use of controversial feedforward techniques.

Output Filter. Explicit damping of the output filter is normally not required. However damping of the output filter may be desirably to control the minimum of the open-loop input impedance and the Rd degradation line.

Do not use this information for design without independent verification of the information.

Editor: Jerrold Foutz