# DC Converter with Wide Soft Switching Operation, Wide Input Voltage and Low Current Ripple
## 1 Introduction  
A fuel cell or solar cell is the clean renewable power energy to change chemical or photovoltaic energy to utility power by using a power electronic circuit, such as dc-dc or dc-ac power converters [1]. The output voltage of single solar cell stack is low and related to solar illuminance (or intensity). Therefore, the high-frequency-link power electronic converters with wide voltage operation are essentially demanded to produce an sTable 400 V on a dc bus between PV (photovoltaic)panels and ac (or dc) grids. However, the input voltage range of the conventional dc-dc converters or dc-ac converters for solar power conversion applications is limited. For dc-dc converters, the allowed maximum input voltage is less than three times of the minimum input voltage, i.e., \(V_{in,\ max}<3V_{in,\ min}\), due to the available duty cycle control. The high-frequency-link dc-dc converters have two types: current source converters [2; 3] and voltage source converters [4; 5]. Current source converters have less input current ripple compared to the voltage source converters. To increase converter efficiency, the zero-voltage switching dc-dc converters have developed to decrease the turn-on switching losses on power switches. Soft switching flyback with active clamped pulse-width modulation (PWM) [6] have been implemented for adaptor applications with low power rating. Asymmetric PWM converters have been studied in [7; 8] to accomplish high-efficiency power converters. However, the problems of an asymmetric half-bridge converter are unbalance current rating on power switches and rectifier diodes. Full-bridge PWM converters have been presented and discussed in [9; 10] for high-power applications and a wide range of zero-voltage switching operationa. However, the drawbacks of this circuit topology are the large circulating current on the primary side under freewheeling state and the serious switching losses at low load condition. To solve the above problems, _LLC_ (inductor-inductor-capacitor) resonant converters have been implemented in [11; 12; 13; 14; 15] to accomplish soft switching operation over a wide load range. Half-bridge and full-bridge _LLC_ converters are adopted for low and medium power application due to the fundamental ac input voltage of a half-bridge _LLC_ converter is only one-half of a full-bridge_LLC_ converter. However, the input voltage operation of an _LLC_ converter is limited. It is not easy for an _LLC_ converter to be applied in fuel cells or solar power applications with wide input voltage variation.  
A hybrid _LLC_ circuit is proposed in this paper to implement wide voltage operation from \(V_{in,\ min}\) to \(4V_{in,\ min}\), zero-voltage switching for all power semiconductors and input current ripple-free. The interleaved PWM converter is used on the input side to realize current ripple reduction. The second-stage is a full-bridge (low voltage mode) or half-ridge (high voltage mode) _LLC_ resonant circuit to realize wide input voltage capability and accomplish wide zero-voltage or zero-current switching operation on active devices or rectifier diodes. Due to the fundamental ac, the input voltage of the half-bridge _LLC_ converter is only one-half of full-bridge _LLC_ converter, half-bridge circuit topology is operated at a high input voltage range and full-bridge circuit topology is operated at a low input voltage range. The selection of a half-bridge or full-bridge converter is implemented with an additional switch to realize wide input voltage operation. The interleaved PWM circuits and full-bridge-type _LLC_ converter share same power switches. Thus, the switch counts are reduced and the single-stage current source dc converter is implemented in the presented circuit. The voltage double rectification topology is adopted on the output side to decrease diode counts and voltage rating on diodes. Finally, experimental verifications based on a 500 W circuit are demonstrated to validate the performance of the studied single-stage dc-dc converter.