Current-Programmed Criteria

Y-Parameters. Using y parameters, both the analytical and graphical criteria for determining stability and degradation with the addition of an input filter have been worked out by Erich-and-Polivka, and by Kohut. Erich-and-Polivka use impedance for their graphical criteria and Kohut uses admittance.

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.

Practical Placement of Input Filter

Placement. P5 is the most practical placement of the input filter.

Input L. In this location, added input inductance from a system EMI filter or long leads to the power source will not cause the power supply to go unstable since Ls will migrate to the left, which is safe, and will not increase the Q.

Placement of Input Filter

No Problem in the Old Days. When 20kHz switching-mode power supplies began to be used in the mid 1960's usually the only EMI specification invoked was for conducted emissions above 150kHz. This placed the input filter out to the right of location P1, beyond the negative input resistance frequency of the power supply, and adding a filter never caused instability.