Industrial Key Research and Development Projects of Shaanxi Province (2021GY-135)
三电平直流变换器因其主开关器件电压应力仅为Vin/2且具有拓扑结构简单和软开关特性好等优点，受到工业界和学术界的广泛关注。基于IGBT的零电压零电流开关ZVZCS（zero-voltage zero-current switching）三电平DC-DC变换器是大功率高压直流变换的主流方案，具有通流能力强、软开关负载范围宽和原边通态损耗低等优点。但目前ZVZCS三电平直流变换器仍存在原边电流复位困难和整流二极管电压应力高等问题，限制了该类变换器在大功率场合的应用。提出一种新型ZVZCS飞跨电容型不对称PWM半桥三电平DC-DC变换器，采用电压可变电流复位电路，在保证主开关器件宽负载范围实现软开关的同时不增加副边整流二极管的电压应力，且复位电路中的MOSETs全负载范围ZVS（zero-voltage switching）关断和ZCS（zero-current switching）开通。此外，该电路在飞跨电容两端串联一个双向开关，防止原边电流复位后反向，该双向开关在全负载范围可实现ZVS关断和ZCS开通，因此增加的损耗可以忽略。讨论了电路的结构、工作原理和特性，设计了一台6.5 kW实验样机验证其工作原理。
The three-level DC converter has attracted wide attention in both industry and academia because the voltage stress of primary switches is only Vin/2, and it also has advantages such as a simple structure and good soft-switching characteristics. In high-power HVDC conversion applications, the IGBT-based zero-voltage zero-current switching(ZVZCS) three-level DC-DC converter is the mainstream solution owing to its high current handling capacity, wide soft-switching load range, and low primary-side conduction loss. However, ZVZCS three-level DC converters still face some problems, such as the resetting difficulty in primary-side current and the increasing voltage stress of rectifier diode, which limit their applications in high-power scenarios. A novel ZVZCS flying capacitor asymmetric PWM half-bridge three-level DC-DC converter is proposed, which utilizes a current reset circuit with controllable voltage to ensure a wide soft-switching range of primary switches. Meanwhile, the voltage stress of rectifier diode one the secondary side is low. The MOSFETs in the reset circuit can achieve zero-voltage switching(ZVS) turning off and zero-current switching(ZCS) turning on. In addition, a bi-directional switch is series connected with the flying capacitor to prevent the reverse current of ip in the free-wheeling mode. This bi-directional switch can achieve ZVS turning off and ZCS turning on over the entire load range, so the added power loss is negligible. The circuit configuration, operation principle, and characteristics were discussed, and a 6.5 kW prototype was designed to verify the working principle.