# Hybrid LLC Converter with Wide Range of Zero-Voltage Switching and Wide Input Voltage Operation
## 1 Introduction  
The renewable energy systems have been presented and established to reduce fossil fuel demand and prevent the influence of global warming. The output of renewable energy sources may be unstable voltage or current with dc or ac waveform. Power electronics play the principle of energy transfer between renewable energy sources and energy storage elements or loads. The output of photovoltaic (PV) panel or small-scale wind turbine generator is an unstable and non-constant dc voltage. Therefore, dc converters with wide voltage input capability are normally demanded to achieve the principle of energy transfer between PV panel (or small-scale wind generator) and energy storage unit or dc load. The power switches of dc-dc converters can be controlled by pulse-width modulation (PWM) [1; 2; 3] or frequency modulation (FM) [4; 5; 6]. In PWM control, the switching frequency is fixed but duty cycle is variable in order to control load voltage. In FM control, the duty cycle is fixed and the switching frequency is variable to control voltage transfer function of dc converters and regulate load voltage. Gallium Nitride (GaN) FET, Insulated Gate Bipolar Transistors (IGBTs), Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and Silicon Carbide (SiC) devices are widely used in power electronics. The main drawback of IGBT devices is low frequency operation. SiC and GaN devices have advantages of high frequency operation and low switching loss for high power density applications. However, SiC and GaN devices have high cost problem compared to MOSFET and IGBT devices. In medium power applications, MOSFET devices have advantages of low cost and medium frequency operation capability. High frequency and high efficiency power converters are demanded to achieve energy transfer from renewable energy sources to the energy storage units. Due to wide voltage variation of the solar panel output or low power wind generator, the soft switching converters with wide voltage input or output operation have studied. The series-connected dc-dc converter has studied in Reference [7] to accomplish wide input voltage operation for power conversion and battery charger applications. The disadvantage of the series-connected dc-dc converter is poor efficiency. The series/parallel-connected converters with wide voltage operation for vehicles were presented in References [8; 9; 10; 11]. The circuit topology needs more active and passive components so that the efficiency and reliability are decreased. Phase-shift PWM scheme with a wide voltage input/output operation were studied in References [12; 13; 14]. The main problems of these topologies are the complicated control algorithm, high freewheeling current and hard switching operation at low load condition. Interleaved converters with frequency control have been presented in Reference [15] to have low input ripple current and achieve wide voltage input. This circuit topology is still a multistage circuit. The inductor-inductor-capacitor (LLC) converter with different winding turns was reported in Reference [16] to increase hold-up time on power units of personal computer applications. Four secondary windings are needed so that the conduction loss is increased on the transformer windings. In conventional LLC converter, the circuit can operate well at the limit voltage range to achieve low freewheeling current loss and high circuit efficiency. If the wide voltage range operation is demanded, the low inductor ratio between the magnetizing inductor and resonant inductor and low quality factor are needed in conventional LLC converter to achieve high voltage gain. However, the magnetizing inductance is decreased and the circulating current loss is increased. Therefore, the LLC converter efficiency is decreased. To solve this problem, a hybrid LLC circuit operation [17] shown in Figure 1a has been proposed to realize 4:1 wide voltage operation (V\({}_{\text{max}}\) = 4 V\({}_{\text{min}}\)).