Modeling instabilities in mating switchmode power supplies to input LC filters

Switchmode power supplies are the dominant type of power supplies used in electronic equipment. Their high efficiencies and low cost overcome the disadvantage of noisy electrical performance. One type of noise produced at the input of these converters is referred to as conducted EMI (Electro-Magnetic Interference). This emission is regulated so that it does not exceed limits set by the FCC. Circuit designers as well as system designers place low-pass filters comprised of inductors and capacitors in front ofthe power supply to meet this conducted EMI limit. This can cause a problem. This paper proposes a circuit that demonstrates the essentials of this problem. Then the circuit is put into the form of a Spice model and run to determine what the instability looks like in graphical form