Stability Analysis for Cascaded Converter Using Equivalent Multi-PWL Current Ripple

In a cascaded converter, the input current ripple of the load converter is fed forward to the source converter. The feedforward current ripple can induce instability in the source converter. The cascaded converter is usually a high-order system, which makes it hard to investigate the stability effect of this feedforward current ripple. To solve this issue, a multi-piecewise linear current load (PWLCL) is proposed to equivalently represent the load converter with input inductor for stability analysis. Taking the peak-current-mode (PCM) controlled buck converter with two-PWLCL as an example, a nonlinear switched model is established to demonstrate the effectiveness of multi-PWLCL. Based on the model, the stability analysis of the PCM controlled buck converter with two-PWLCL is carried out by Filippov's method and Floquet theory. Thereafter, the stability boundaries predicted by theoretical analyses are compared with those obtained by circuit simulations using two-PWLCL and load converter. The results manifest that the stability effect of feedforward current ripple can be studied effectively by using equivalent multi-PWLCL. The equivalent method can also be extended to the other three cases. Finally, the correctness of the theoretical and simulation results is confirmed by experiments.