Buck converter small-signal models and dynamics: comparison of quasi-resonant and pulsewidth modulated switches

The generalized canonical model obtained from extended state-space averaging is used as a design tool for the 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 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, and exhibit significant damping at light loads, which can result in 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. Simple approximate expressions are given for the real poles. Experimental verification of the half-wave analysis is presented, and the effects of converter efficiency on model accuracy are discussed