Input filter compensation for switching regulators

Abstract

An input filter is often required between a switching regulator and its power source due to the need of preventing the regulator switching current from being reflected back into the source. The presence of the input filter often results in various performance difficulties such as loop instability, degradation of transient response, audiosusceptibility and output impedance characteristics. These problems are caused mainly by the interaction between the peaking of the output impedance of the input filter and the regulator control loop. Conventional single-stage and two-stage input filters can be designed to minimize the peaking effect, however this often results in a penalty of weight or loss increase in the input filter.

A novel input filter compensation scheme for a buck regulator that eliminates the interaction between the input filter output impedance and the regulator control loop is presented. The scheme is implemented using a feedforward loop that senses the input filter state variables and uses this information to modulate the duty cycle signal. The feedforward design process presented is seen to be straightforward and the feedforward easy to implement. Extensive experimental data supported by analytical results show that significant performance improvement is achieved with the use of feedforward in the following performance categories: loop stability, audiosusceptibility, output impedance and transient response.

The use of feedforward results in isolating the switching regulator from its power source thus eliminating all interaction between the regulator and equipment upstream. In addition the use of feedforward removes some of the input filter design constraints and makes the input filter design process simpler thus making it possible to optimize the input filter. The concept of feedforward compensation can also be extended to other types of switching regulators.