A small-signal model utilizing amplitude modulation for the class-D converter at fixed frequency

A small-signal model for dynamic analysis of the Class-D converter with a variable capacitance switch (VCS) is presented. Conventional small-signal analysis is not suitable because of the time-varying capacitor in the VCS. The new model Is derived based on amplitude modulation (AM). The key is to define two frequency components, lower and upper sidebands, which are related to the switching and injected small-signal perturbation frequencies, as the resonant state variables such as tank current. Required dynamic responses are obtained by adding these two frequency components for the desired variables to obtain the desired transfer function. All calculated results of the dynamic responses are verified by experimental measurements on a prototype class-D converter operating at 350 kHz.