Stability issue and corresponding design considerations in a system of cascaded bidirectional DC-DC converters

In this paper, based on two cascaded bidirectional Buck/Boost Dc-Dc converters, stability issue in the bidirectional converter cascaded system is revealed. With Middlebrook impedance criterion, two minor loop gain, each for one power flow direction and which is defined as the ratio of the upstream converter output impedance to the downstream converter input impedance, are introduced for the system stability analysis. Time-domain simulation and mathematic calculation are done for the verification. Based on the analysis results, two conflicts in the bidirectional converter cascaded system are proposed. One is that with the shared common part in the power stage, it cannot easily optimize the system stability in both power flow directions. The other conflict is that in many cases with the shared common compensator in control stage, it cannot optimize the system stability in both power flow directions at the same time. By revealing these two conflicts, this paper proposes that the corresponding consideration and tradeoff should be done during the design stage.