The paper develops computer aided procedures for the analysis and design of input filters of PWM DC-DC converters operating in voltage or current mode. The proposed methodology is based on the Switched Inductor Model (SIM) which is an equivalent circuit approach for modelling and simulation of switch mode systems. The method is compatible with all versions of SPICE derived electronic circuit simulators. Following a brief discussion on the modelling aspect, the paper examines the question of instability due to the input filters. It is shown that this issue can be easily studied by examining the small signal loop-gain function. Furthermore, the small signal analysis which includes the effect of the filter, can then be used to optimize the feedback network to meet desired closed loop dynamic responses. Namely, to obtain a wide closed-loop bandwidth, to minimize the audiosusceptibility and to reduce the output impedance of the filter-equipped converter in closed loop.