Multiple battery charging systems can be configured with various combinations of ac-dc and dc-dc converters. This paper reviews the reliability and economy of three representative configurations of multi-battery charging systems. Existing PSA (part stress analysis) can analyse the failure rate considering the environmental factors for the type of parts, the number of parts, the connection state of parts and the voltage and current stress of parts. However, since it does not reflect the system's operating characteristics, the Fault-tree analysis (FTA) method based on the fault tree is used. Still, it cannot be analysed considering the operational risk of the converter according to various load conditions. Therefore, in this study, by defining a failure according to the minimum and maximum load of the multiple battery charging systems and designing a separate fault tree, the converter operation characteristics were considered by reflecting the number of battery charges, the characteristics according to SOC (state of charge) and the redundancy effect that appears when cyclic charging is applied. Through the new enhanced FTA method, the multiple battery charging systems (Type 1) to which sequential charging is applied are quantitatively proven advantageous in terms of reliability and economy.

An input‐series quasi‐resonant (QR) flyback high‐voltage switching mode power supply (SMPS) based on an integrated transformer has been designed and analysed. It is capable of converting a high DC input voltage to lower DC voltage to power the control components of high‐power converters in a DC microgrid. An integrated transformer, combined with identical duty cycle control, is adopted to achieve automatic equalization of the series modules effectively even without employing any voltage or current equalization control strategy, of which effect is analysed theoretically. The design of the control system, based on QR flyback SMPS is given, which is implemented by peak current control, and a DCM small‐signal model for the designed SMPS is established, which indicates that the system has high stability and accuracy to ensure normal operation across a broad range of input voltages. Finally, the validity of the theoretical analysis and the proposed method is demonstrated by the development of a 48 W SMPS experimental prototype, which is composed of three input‐series modules. The overall efficiency of the prototype has been improved due to the adoption of QR. The proposed design and analysis methods can be extended to other input‐series isolated DC/DC topologies.

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