An Improved modulation method to Series Capacitor Buck Converter in Discontinuous Conduction Mode

Series capacitor buck converter (SCBC) in DCM is a unique solution for the 48 V power supply system. The converter has the merits of high step-down ratio, low current ripple and voltage stress, self-sharing current. However, two problems were revealed in this paper through the analysis of multi-phase SCBC in light load condition: the self-current sharing can only realize in a limited region of 1 < D ≤ 1/M, the negative current conduction will be generated in a very low current situation. Through discussing the causes of the unbalanced current and the negative current conduction, an edge-connected phase shift modulation method is introduced to solve above defects. Without any addition devices, the self-sharing current operating region is extended to 0 < D ≤ 1/2, and the negative current is eliminated. By applying the proposed method, the phase current can be balanced which has the same slope and duration, the same peak-to-peak current. And the negative current is eliminated, which conforms to the operating conditions of 48 V power supply system. Experimental results from a 3-phase SCBC prototype validate the correctness of proposed method with an improved efficiency of 98.6%.