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

A small-signal model for the dynamic analysis of the Class-D power 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 the amplitude modulation (AM) from communication theory. The key to the derivation 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. The required dynamic responses are obtained by adding these two frequency components for the appropriate variables to obtain the desired transfer function. All the calculated results of the dynamic responses are verified based on the experimental measurements from a prototype Class-D power converter operating at 350 kHz