Rsearch on new AC/DC converter for high efficiency and low cost server power system

Abstract

As the global warming has been focused on the world-wide problem, many projects for reducing CO2 emissions are in progress. For the server power system, climate savers computing initiative (CSCI) have regulated CO2 emissions from the operation of computers. In this case, the server power system with single output must be satisfied with the gold level efficiency by 2010, which is 88%-92%-88% at 20%-50%-100% load conditions, respectively. This high efficiency becomes a big obstacle to the server power system with the power factor correction (PFC) circuit, the DC/DC converter, the stand-by circuit, hold up time regulation, and hot-swap function. Furthermore, low cost is required as the cost to the server power system is rapidly increased. To meet these market needs, this dissertation suggests three approaches which can achieve high efficiency and low cost server power system. First, a new modified flying capacitor passive lossess snubber with wide snubber reset ranges is proposed for the PFC stage. The proposed snubber has a new branch that consists of one coupled inductor and one diode compared to the conventional flying-capacitor passive lossless snubber. When the main switch is turned on, the voltage across flying-capacitor is charged to $2V_{in}$/n using a new branch without the help of the reverse recovery current of output diode. Therefore, it can effectively reduce the reverse recovery current maintaining wide snubber reset ranges. Furthermore, the output diode can achieve low voltage stress since the clamping diode is turned off before the voltage across flying capacitor reaches its peak value. Secondly, a new transformer integrated with additional resonant inductor is proposed for the DC/DC stage. For the DC/DC stage of server power system, phase-shift full-bridge (PSFB) converter with additional resonant inductor and clamping diodes is widely used due to the wide zero voltage switching (ZVS) ranges and the low voltage stress of secondary rectifiers. ...