A small signal model for V2 control with composite output capacitors based on describing function approach

A small signal model based on describing function method is proposed for V² control with composite output capacitors (combination of various types of output capacitors). V² control structure is widely used in DC/DC converters to provide fast dynamic performance. By adding ceramic capacitors in parallel with OSCON or SP output capacitors, the output impedance characteristic can be improved. However the original stable system may run into sub-harmonic oscillation with large amount of ceramic capacitance. The proposed model can indicate how much ceramic capacitance we can add and still keep sufficient stability margin. The effect coming from the parallel ceramic capacitors is also explained. The modeling result is verified using Simplis simulation and measurement result.