Abstract：Unidirectional DC-DC converters are widely used in renewable energy system, electrical vehicles and electrical locomotives. The development of lightweight, high-voltage, and high-efficiency unidirectional DC-DC converters is of great significance. In the above applications, the DC-DC converter is usually required to achieve high voltage gain, and isolation is preferred to ensure the safety operation in some cases. On the other hand, diode is one of the commonly used rectifier components in the isolated unidirectional DC-DC converters. The parasitic capacitance of diode will resonant with the leakage inductance of the transformer, causing severe voltage oscillation. Reducing the voltage stress of the switching devices and improving the operation efficiency of the DC-DC converters are the research hotpot. Aunidirectional isolated resonant switched capacitor converter (UI-RSCC) is proposed in this paper. Simulation and experimental results verify the effectiveness of UI-RSCC. First, the topology and operation principle of UI-RSCC are analyzed, which consists of n DC capacitors, n-1 resonant switched capacitor units (RSCU) and K isolated rectifier unit (IRU), 1≤K≤n-2. The power transmission path and the voltage balancing principle of DC capacitors are studied. Then the output voltage control method is proposed, and threetypical operation modes of UI-RSCC are introduced, including current continuous mode with soft switching (CCMSS), current continuous mode with hard switching (CCMHS) and current discrete mode (CDM). Moreover, the soft switching condition and output performance of the three operation modesare also discussed. It shows that UI-RSCC has the advantage of automatic voltage equalization, and the switching voltage stress is suppressed. The required switching devices is reduced by multiplexing parts switching devices functions. Additionally, all the switching devices achieve soft switching under CCMSS with capacitor filtering, and additional clamping circuit is not required. The analysis of the three operation modes shows that CCMSS is preferred to achieve higher efficiency. The simulation modelis built to verify the correctness of theoretical analysis and the three operation modes are considered. Results showed that the voltage balancing of DC capacitors are achieved in all three operation modes. Moreover, zero voltage switching (ZVS) of all switching devices is achieved under CCMSS. While the ZVS of secondary side switching devices is not achieved under CCMHS. And under CDM, only the secondary side switching devices can achieve ZVS. Then 10 kW experimental platform is built, which consists of three RSCUs and one IRU. The peak efficiency of 97.37% is achieved and the operation efficiency under rated power is 96.33%. Experimental results show that the oscillation is alleviated with the proposed UI-RSCC. And the output voltage keeps stable when the input voltage swings from -5%~+10%. And the experimental result shows thatvoltage auto-balancing is also achieved in multilevel UI-RSCC, including five RSCUs and two IRUs. The following conclusions can be drawn: (1) UI-RSCC achieves voltage auto-balancing of input DC capacitors, and the output voltage is stable. The experimental results are basically consistent with theoretical analysis. (2) In continuous soft switching mode, ZVS of all switching devices is achieved without additional clamping circuits, and the voltage spike in diodes of secondary side is suppressed. (3) UI-RSCC suppresses the voltage stress of switching devices and reduces required switching devices, which has a promising application in the unidirectional energy flow, such as electric locomotive, urban rail transit auxiliary converter, etc.
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2024, Vol. 39 
