Instabilities in current-mode controlled switching voltage regulators

Abstract

A survey of the instabilities of switching voltage regulators with different types of current-mode controllers (including LC3 or constant-hysteresis; MC2 or constant-frequency and constant off-time versions) is presented. Apart from the well-known open-loop oscillation of the constant frequency regulator above 50% duty ratio, other instabilities can also arise by increasing the ac loop-gain. At the constant-hysteresis controller the operating frequency changes, at the other types continuously or hysteretically arising subharmonic oscillation appears. These phenomena can be analysed by taking into account the effect of the amplified and fed-back ripple voltage to the operation of the current comparator. Calculations were accomplished in order to establish maximum usable ac gains for the three basic power converter topologies (buck, boost and buck-boost) and for the above mentioned controllers. Experiments were also carried out for several particular power circuit and controller combinations showing close correspondence to theoretical results.