Instabilities in current-mode controlled switching voltage regulators

Instabilities of switching voltage regulators with different power circuit and controller combinations are presented, and analytical methods and expressions for the determination of the maximum usable gain are introduced. The effect of the amplified and fed-back output ripple voltage on the operation of the current comparator is used for analysis. The constant-hysteresis controller can experience a shift of the frequency of free-running oscillation. The amount of change is a function of the ac loop gain, and a frequency-gain relation for a buck regulator is presented. In other types of convertors, such as constant frequency controlled convertors, subharmonic oscillation occurs, continuously or hysteretically. Equations are derived for determining stability criteria for buck and boost regulators, and constant hysteresis and constant frequency convertors, and calculated and measured values were in good correlation.