A General Control Strategy for Input-Series–Output-Series Modular DC–DC Converters

Input-series-output-series (ISOS) connected dc-dc converters enable the utilization of low-voltage rating switches in high-voltage-input and high-voltage-output applications that require galvanic isolation. To achieve input or output power balance among all the constituent modules, a general control strategy eliminating the use of input-voltage sharing (IVS) loops is proposed. The instability mechanism of independent output-voltage sharing control has been investigated by using the Routh-Hurwitz criterion. From the same criterion, a stability design method for the proposed control strategy has been developed. Detailed comparison of the proposed control technique, voltage-mode IVS, and current-mode IVS control methods is made. In comparison, the proposed control strategy has the same performance compared with current-mode IVS control. The control system design is simplified by avoiding mandatory IVS loops which are needed for IVS control. Both the steady-state and dynamic performance are improved with the proposed control strategy in comparison with voltage-mode IVS control. Using the proposed control strategy, excellent input and output power balance can be achieved for the ISOS connection during transient and steady-state conditions. The proposed control strategy performance is validated by experimental results of a 960-W ISOS system composed of three two-transistor forward dc-dc converters.