Efficiency improvement at light load in bidirectional DC-DC converter by utilizing discontinuous current mode

This paper proposes a feedback current control for bidirectional DC-DC converter which is operated in Discontinuous Current Mode (DCM) at light load and Continuous Current Mode (CCM) at heavy load in order to improve light load efficiency. In the proposed method, the nonlinearity compensation for DCM operation is constructed by using the duty ratio at previous calculation period. Moreover, the introduction of DCM current feedback control into bidirectional power conversion is accomplished by detecting the operation mode at the output of the control system. This make the control becomes parameter-independent. The validity of the proposed control is confirmed by a 1-kW prototype. In the ramp response, the slope of the DCM current almost agrees to the design value with the error of 0.8%. Moreover, the smooth transition among 4 current modes: CCM-powering, DCM-powering, DCM-generation, CCM-generation, is also confirmed. On the other hand, in order to further improve the efficiency at light load, the synchronous switching for DCM is proposed. As a result, at load of 0.1 p.u. the efficiency of the DCM synchronous switching is improved by 1.5% from 97.2% to 98.7% compared with the CCM synchronous switching. Besides, it is confirmed that, the efficiency of the CCM/DCM synchronous switching is higher by 0.2% than that of the CCM/DCM asynchronous switching at all range of load. Furthermore, the efficiency at rated load is 98.8%, whereas the maximum efficiency is 99.0% at load of 0.45-0.65 p.u.