###### Abstract  
The isolated three-level DC/DC converter (ITLDC) can be used to charge electric vehicles. During the constant current charging stage, the ITLDC can be designed to realize nature zero voltage switching (ZVS). However, during the constant voltage charging stage, the charging current is small; thus, nature ZVS cannot be realized. This paper presents an active commutation auxiliary circuit (ACAC) for the ITLDC to realize the full load range ZVS. With the proposed ACACs, all the main switches achieve zero-voltage turn-on and quasi zero-voltage turn-off, and the auxiliary switches realize zero current turn-on and zero-voltage turn-off; thus, the efficiency will be high. The auxiliary currents generated by the ACACs are controllable. During the constant current charging stage, the ITLDC realizes nature ZVS and the auxiliary currents are controlled to zero; thus, the ACACs do not result in high current stress or bring in additional losses, and the efficiency will be high. During the constant voltage charging stage, the charging current decreases with charging time and the charging current is too small to realize nature ZVS. Thus, the ITLDC can work with the proposed ACACs and the auxiliary currents can be controlled within a suitable value to realize ZVS. With the proposed ACACs, the ITLDC can realize ZVS during the whole charging process; thus, the efficiency will be high. The structure and operating principle of the ITLDC with ACACs are introduced and the performance of the proposed TLDC is experimentally verified on a 1.5 kW prototype converter. s +
Footnote †: journal: Applied  
currents. To achieve high efficiency in an ITLDC, soft switching should be realized. This paper will focus on the zero-voltage switching (ZVS) of the four-switch ITLDC. There are many research works focusing on the ZVS of the diode clamping ITLDC. For example, in [11; 12], the ZVS is realized with a large load range and reduced circulating loss; in [13; 14], the ZVZCS methods are proposed, and some switches realize zero-current turn-on and other switches realize zero-voltage turn-on. However, the research works focusing on the ZVS of the four-switch ITLDC are relatively few and these research works are as follows. The ITLDC will achieve natural ZVS with a high output current and the natural ZVS range can be extended by increasing the leakage inductance of the transformer or adding an external inductor in series with the primary winding of the transformer [11]. However, a too large leakage inductance will result in high duty cycle loss and high circulating loss [11; 15]. In [16], a combined DC/DC converter is proposed to realize wide load range ZVS, two ITLDCs' parallel operation and share two switches of the down bridge; thus, the current stress of the down bridge switches will be high. A wide load range ZVZCS ITLDC with two main switches realized ZVS and another two main switches realized zero-current switching (ZCS) in [17]. However, when the ITLDC is implemented with MOSFETs, the switching losses will not be substantially reduced with ZCS, due to the large output capacitance of MOSFETs. In [6], a zero-voltage switching PWM strategy with capacitor current-balancing control is proposed; the ZVS achievement conditions of the upper bridge switches and the down bridge switches are shifted towards each other in every two switching periods; thus, the switching losses caused by the hard switching at the light load situation can be distributed evenly among the four power switches. However, the soft switching range is not extended with this ZVS method. In [18], a ZVS non-isolated bidirectional DC/DC converter with transient current build-up technique is proposed with a small average auxiliary current, which can be used in ITLDC to realize full load range ZVS. However, this ZVS method needs two auxiliary switches for each bridge and the auxiliary switches are turned off under hard switching conditions. To extend the ZVS range of the ITLDC, a commutation auxiliary circuit (CAC) can be added.