High active mode efficiency switching mode power supply

Power supply standby mode efficiency has attracted much of the focus in the past years. However, larger total energy use and energy savings opportunities were in power supplies´ active mode [1]. Recently, higher active mode efficiency was required and the 80 PLUS program has become one of the specification of ENERGY STAR. In redundant application, efficiency even required to be higher than 80% from full load to 10% load. Here, based on the analysis of power losses changing with load, this paper gives the general concept of how to design high active mode efficiency switching mode power supply. Then, the system structure aimed to high active mode efficiency was proposed for 1 kW workstation power supply. In PFC stage, instead of powder core, ferrite core was selected as the PFC choke to decrease core loss, which is especially helpful to increase PFC light load efficiency. In D2D stage, two-choke approach ITTF (interleaved two-transistor forward) was selected, which operates just like two independent two-transistor forward converter phases paralleled. Through shielding one phase at light load, light load efficiency can be further increased. Dynamic issue in phase shielding control was analyzed in detail. Finally, a prototype based on the proposed solution was designed, which can meet the requirement of 80 PLUS even at 10% load.