# Hybrid LLC Converter with Wide Range of Zero-Voltage Switching and Wide Input Voltage Operation
## 1 Introduction
When input voltage is in low voltage range, the full bridge LLC resonant circuit (S\({}_{1}\)-S\({}_{4}\) are active,) is operated. On the other hand, the half bridge LLC resonant circuit (S\({}_{1}\) and S\({}_{2}\) are active, S\({}_{3}\) OFF and S\({}_{4}\) ON) is used under high voltage input case. Therefore, wide input voltage operation is achieved in this circuit topology. However, there is a dc voltage level on the resonant capacitor when LLC circuit is operated at half bridge circuit structure. A hybrid LLC converter [18] has been studied to achieve 8:1 wide output voltage operation (V\({}_{\text{o,max}}\) = 8 V\({}_{\text{o,min}}\)). This circuit topology is based on a full bridge resonant circuit and an ac switch on the primary-side and a hybrid diode rectifier on the output-side. Compares to Reference [17], the half bridge LLC circuit with split input capacitors is used in Reference [18] to avoid a dc voltage level on the resonant capacitor under low voltage output operation. In order to extend output voltage range, a hybrid output rectifier with a full-wave rectification or a half-wave rectification structure is adopted on the secondary-side in Reference [18]. The main disadvantage of this circuit topology in Reference [18] is four diode conduction losses on the secondary-side when full-wave rectification structure is operated. A new LLC converter is presented to realize wide range of input voltage operation. The LLC resonant circuit is used in the studied converter to achieve soft switching operation for all power devices due to inductive input impedance operation by pulse frequency modulation. To realize wide voltage input operation, the half bridge with split capacitors or full bridge equivalent LLC circuit can be operated on high-voltage side and the voltage doubler rectifier with different secondary turns is adopted on low-voltage side. Due to the different input voltage range, three circuit structures with different voltage gains are controlled to overcome wide range of input voltage variation. The input voltage range of the studied circuit topology is 50 V-400 V. For high voltage input (200 V-400 V), half bridge LLC circuit with fewer winding turns is used to obtain the least voltage gain and regulate load voltage. For medium voltage input (100 V-200 V), full bridge LLC circuit with fewer secondary turns is controlled to attain high voltage gain. For low voltage input (50 V-100 V), full bridge LLC circuit with more winding turns is adopted to obtain the largest voltage gain. Thus, the wide voltage operation (50 V-400 V) is realized in the studied LLC converter. Compared to the multistage converters or series/parallel converters, the proposed converter has simple circuit structure and control scheme by using the general analog integrated circuit. Compared to the 4:1 wide voltage LLC converter in Reference [17] with 4 switches (3 switches) conduction loss at low (high) voltage range, the proposed converter does not a dc voltage level on the resonant capacitor when half bridge LLC equivalent circuit structure with 4 switches conduction loss is operated and the proposed converter has 8:1 wide voltage operation capability. Compared to the 8:1 wide voltage LLC converter in Reference [18], the proposed converter has only two instead of four diode conduction losses on the secondary-side for high current output applications. The laboratory prototype has been developed and the experiments are given to investigate the presentation of the presented converter.