# Analysis of a Three-Level Bidirectional ZVS Resonant Converter
## 1 Introduction  
Renewable power to reduce the effect of global warming has been developed by using high efficiency power electronic based converters in local dc nanogrid or microgrid distribution [1; 2; 3; 4; 5; 6] between renewable energy power and local dc or ac loads. In order to maintain the voltage stability on dc distribution system, energy storage power units are usually demanded between battery banks and dc bus system to save (or restore) excess (or insufficient) energy on the dc bus. Therefore, the bidirectional pulse-width modulation (PWM) converters have been proposed for the battery-based systems [7; 8; 9; 10; 11; 12; 13; 14] such as electric vehicles, hybrid electric vehicles, and dc microgrids. In dc microgrids, the unipolar voltage (380 V) or bipolar voltage (\(\pm\)380 V or 760 V) distribution can be adopted on the dc bus voltage. High frequency link medium voltage converters have been used for dc traction power units, three phase industry power supplies and dc microgrids. Three-level dc converters with 600 V MOSFETs or conventional PWM converters with 1200 V IGBTs or SiCs have been presented in medium voltage input applications. The drawback of 1200 V IGBT is low switching frequency and the cost of 1200 V SiC is expensive. Bidirectional PWM converters with dual active bridge (DAB) structure have been studied to realize forward and reverse power transfer. Three-level bidirectional converters or cascaded converters with the high frequency MOSFETs have been developed for high voltage systems such as 760 V input. PWM scheme is widely adopted in bidirectional DAB systems to control power flow and realize soft switching turn-on characteristics. However, the control scheme for generating the PWM signals is complicated and the circulating current under low duty cycle is high. Resonant converters have the benefits of high circuit efficiency and low electromagnetic interference. A full-bridge resonant circuit topology was proposed in [15] to achieve bidirectional power transfer. However, the soft switching characteristics cannot be achieved in backward power flow. Bidirectional Full-bridge resonant converters presented in [16; 17; 18] have symmetric circuit structure to achieve forward and backward power flow so that power switches can realize zero-voltage switching. However, there is acirculating current on the parallel inductor in primary-side which will result in addition conduction loss during forward power flow.  
A soft switching three-level resonant converter is developed for high voltage to low voltage conversion. The profits of the developed converter are forward and backward power flow capability and zero-voltage turn-on characteristic. Three-level diode-clamp circuit topology is used on the primary-side and full-bridge circuit topology is adopted on the secondary-side. The _LLC_ circuit tank is employed to control load voltage and achieve zero-voltage switching on active devices. For forward power transfer, the three-level diode-clamp converter is controlled using the pulse-frequency modulation (PFM) to control low-side voltage and active devices of full-bridge converter on the secondary-side are operated as synchronous rectifiers. In order to implement the same resonant circuit structures for bidirectional power flow, an additional inductor is connected on the primary-side during the reverse power flow condition. In reverse power flow operation, the full-bridge converter on the low-voltage side is operated with PFM scheme to control high-side voltage. The proposed converter with bidirectional power flow capability can be applied in local dc nanogrid or microgrid distribution between renewable energy power and local dc or ac loads. The circuit schematic and circuit operation are provided and discussed in Sections 2 and 3. The circuit characteristic and experiments with a 1.44 kW laboratory circuit are demonstrated and discussed to show the feasibility of the studied bidirectional power converter in Section 4. Finally, a conclusion of the studied converter is given in Section 5.