# Hybrid Resonant Converter with Three Half-Bridge Legs for Wide Voltage Operation
## 1 Introduction  
Power electronic converters with high efficiency are developed for consumer electronics, battery chargers, telecommunication power units, direct current (DC) nano-grid power conversions, internet of things (IOT) power units, and renewable energy conversions. Soft switching techniques for flyback circuits, full bridge converters, and half bridge converters [1; 2; 3; 4; 5; 6; 7] have been developed to improve the converter efficiency and eliminate the switching losses. Phase-shift pulse-width modulation (PSPWM) full bridge converter [1; 2] uses the phase angle control between the lagging-leg switches and leading-leg switches to control the load voltage and also achieve soft switching operation. However, the lagging-leg switches are hard switching operation at the light load. The high circulating current loss at low duty cycle case is the other drawback. Soft switching flybacks have been discussed in [3; 4] for low power applications. Active clamp converter was discussed in [5] to reduce voltage spike on power switches by using active clamp and switch. However, the rectifier diodes have unbalanced voltage and current stresses on the output side. Resonant converters discussed in [6; 7; 8] can achieve low switching losses on power semiconductors. However, the output or input voltage range of the resonant converter is limited due to its low voltage gain of the series resonant circuit characteristics. For solar _photovoltaic or fuel cell_ systems [9; 10; 11; 12; 13], the output voltage of a solar panel depends on the geographical location and solar intensity. Therefore, the output voltage range of solar panel is wide variation. Power converters with wide input voltage operation have been discussed and developed in [9; 10; 11; 12] for fuel cell and solar power converters with the series or parallel connected structure to extend the input voltage range. Full bridge resonant converter with one alternating current (AC) switch and two isolated transformers was developed in [13] to have a wide voltage operation range (\(V_{in,max}=2V_{in,max}\))\(4V_{in,min}\)). However, the control algorithm is much more complicated due to four equivalent sub-circuit topologies. The power converters with wide voltage operation are also demanded for the power units in the outdoor LED lighting systems, battery chargers in electric vehicles, and railway vehicles. Due to the variable parallel or series connection of several LED strings for output lighting system, the power converters with variable output voltage are normally implemented. For railway low power supplies, the input voltages of DC converters are between 24 and 110 V for the braking system, electric door system, lighting system, and motor drive controller. For electric vehicle (EV) systems, the output voltage of battery charger is variable from 210 to 450 V. Therefore, DC converters with wide voltage range output is required for EV applications. To achieve wide voltage range output, the two-stage DC converters [14; 15; 16] based on the non-isolated DC/DC converter in the first-stage and the isolated DC converter in the second-stage are the simplest way to accomplish wide voltage operation. The main disadvantage of the two-stage converters is low circuit efficiency. Parallel- or series-connected converters [9; 17] were developed to have wide voltage operations. However, a large number of circuit components are needed in these circuit topologies. Series resonant converters published in [18; 19; 20; 21] have a wide voltage range operation such as \(V_{in,max}=4V_{in,min}\) or \(V_{o,max}=4V_{o,min}\). By the proper control of power switches, the resonant converter can be operated at the half or full bridge circuit topology. Therefore, the resonant converters with 4:1, that is, \(V_{in,max}=4V_{in,min}\), wide voltage range operation are achieved. However, the voltage operation range of these circuit topologies is less than 4, that is, \(V_{in,max}\leq 4V_{in,min}\).