# Analysis and Implementation of a Hybrid DC Converter with Wide Voltage Variation
## 1 Introduction  
Bidirectional DC converters [1; 2; 3; 4; 5; 6] have been proposed to interface between the different DC voltage buses for renewable energy sources, battery charges/dischargers, uninterrupted power supplies, and electric vehicles. For electric vehicle applications [7; 8], bidirectional power factor correctors and bidirectional DC converters have been proposed to realize grid-to-vehicle and vehicle-to-grid operations. Phase-shift pulse-width modulation (PWM) converters have been proposed in [9; 10] to realize a forward power flow operation with a voltage step-down operation, and achieve reverse power flow with a voltage step-up operation. The main drawbacks of phase-shift PWM converters are a high freewheeling current and hard switching loss at light load conditions. Dual active bridge (DAB) converters have been studied and proposed in [11; 12; 13; 14] to realize bidirectional power flow using a phase-shift PWM technique and duty cycle control. However, two control variables, the phase-shift angle and duty cycle, are more difficult to be calculated and derived compared to the other bidirectional DC converters. Symmetric CLLC converters have been proposed in [15; 16] to accomplish forward and reverse power flow operations for DC distribution systems and battery charging/discharging systems. The primary-side and secondary-side have the same circuit structures in CLLC converters. Therefore, the forward and backward voltage transfer functions are identical and the circuit characteristics for both power flows are identical. The power switches can be turned on at soft switching operation due to the circuit characteristics of the resonant converter with frequency modulation. The main drawback of the CLLC converter is a narrow voltage operation range due to its low voltage gain. Therefore, the CLLC converter cannot work well under wide voltage variation conditions, such as solar power applications. To realize wide voltage variation capability, cascade DC converters have been proposed in [17; 18], with a buck or boostcircuit on the front-stage and a PWM converter on the rear stage. The main problem of cascade DC converters is high conduction loss. PWM converters or resonant converters with serial-parallel circuit structures have been studied in [19; 20; 21; 22] to realize wide input voltage operation (\(V_{in,max}/V_{in,min}<4\)). However, only forward power flow can be realized in these circuit topologies.  
According to the foregoing studies, a DC hybrid symmetric resonant converter for battery charging and discharging is proposed in this paper. One AC switch and one hybrid half/full-bridge circuit structure are used on both the primary-side and secondary-side to achieve wide voltage input operation and bidirectional power flow operation. The benefits of the proposed converter are as follows: (1) wide voltage operation from 60 V-480 V input; (2) symmetric circuit on both the primary and secondary sides to have the same voltage transfer function for backward and reverse power operations; and (3) soft switching operation on power semiconductors due to resonant circuit characteristics with frequency modulation. The proposed converter can be applied to photovoltaic power applications or battery charging/discharging systems. A 400 W prototype is provided to verify the proposed bidirectional power flow converter. The experimental results demonstrated that the maximum circuit efficiency of the prototype is approximately 93.6%. The circuit diagram of the proposed hybrid DC converter is discussed in Section 2. In Section 3, the principle operations of the converter are provided. The circuit characteristics and a design example are described in Section 4. Experimental results are provided and discussed in Section 5, to verify the feasibility of the theoretical discussions. The conclusions of the proposed circuit are described in Section 6.