Abstract

Performance of a current-fed bidirectional LLC resonant converter for energy storage system in DC grid applications is analyzed and tested. The well match of the voltages on two sides of the LLC circuit is ensured by varying the duty cycle of buck-boost circuit under wide battery voltage range. Hence LLC circuit is operated like a DC transformer to improve efficiency. In the battery discharging mode, the pulse-width modulation (PWM) control is employed. While in the battery charging mode, the phase-shift modulation (PSM) and PWM control is adopted. To achieve the zero-voltage-switching (ZVS) of all the MOSFETs, a frequency feed-forward loop is introduced in the battery charging mode. Compared with conventional bidirectional LLC converter, the frequency regulation range of this converter is narrowed which is beneficial for the design of magnetic components. The operation principles are introduced, the main features such as current ripple of battery, and the zero-voltage-switching (ZVS) of the two modes are analyzed and compared with conventional solutions. A 1.6-kW prototype with a battery voltage of 160V–240V and a DC-bus voltage of 400V has been built and tested to verify the operational principle analysis.

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