Stability analysis of PWM converters connected to general load subsystems

This paper investigates stability of dc-to-dc converters deriving another dc-to-dc converter through a filter stage. The impacts of the downstream filter/converter stage on the absolute and relative stabilities of the upstream converter are analyzed. The absolute stability is examined using the minor loop gain, defined as the ratio of the impedances seen at the output port of the upstream converter. The relative stability is assessed using the loop gain expression of the upstream converter. This paper reveals that the downstream filter/converter stage rarely destabilizes the upstream converter, but usually dictates the loop gain characteristics of the upstream converter and frequently determines the phase margin and 0 dB crossover frequency of the loop gain. This paper proposes a simple method to determine the stability margins of the converter from the minor loop gain. The theoretical predictions are supported by both small-signal simulations and experimental measurements.