Three-phase power factor correction (PFC) converters capable of step-down voltage are attractive in lower components voltage stress, and optimal design of following dcdc stage, which is an alternative for next-generation datacenter power conversion. An improved three-phase step-down PFCconverter (swiss rectifier) based on harmonic-current-injection(HCI) concept is proposed. It improves input current quality by eliminating switching dead zone of HCI network and avoids short circuit fault from hardware level. According to one-cycle control(OCC), a novel nonlinear control strategy (termed as closed-loop OCC) is presented, which reduces the impact of de inductor current ripple on input current. The principles of the improved swiss rectifier and closed-loop OCC are analyzed in detail, verified by simulation and on an 80 kHz, 300 V/2 kW prototype with digital controller. At rated condition, input current THD <2%.