Comparison of buck converter small signal dynamics for pulse-width modulated and quasi-resonant switches

The generalized canonical model obtained from extended state-space averaging is employed as a design tool for evaluation of the buck converter dynamics in different switching schemes. Designs are given at a specified constant conversion ratio and load for the pulse-width modulated, zero-current, zero-voltage, and nonlinear resonant switch full and half-wave converters. The small-signal equivalent-circuit model of each is discussed, and the feedback effects introduced by resonant switching on line and control transfer functions are evaluated. The small-signal transfer functions of half-wave converters are heavily load-current dependent, resulting in much greater damping at light loads than for full-wave converters, even to the point of causing two real poles in the converter response instead of a complex conjugate pair. This damping effect is evaluated over the entire normalized load current range for the linear and nonlinear zero-current switching converters, and simple approximate expressions are given for the real poles. Experimental verification of the half-wave analysis is presented, and good agreement between experiment and theory is obtained.<>