Asymmetrical PWM Scheme to Widen the Operating Range of the Three-Phase Series-Capacitor Buck Converter

The three-phase series-capacitor (3P-SC) buck converter is analyzed extensively in this study for high-voltage step-down applications with a wide input voltage range. This structure is attractive due to its high step-down conversion ratio and inherent current balancing. However, the inductor currents of the 3P-SC converter become unbalanced when the duty cycle exceeds 1/3. This results in significant stress and reduces the utilization of switching devices. This article proposes an asymmetrical pulsewidth modulation (PWM) scheme for the 3P-SC converter to overcome the limitations of the conventional PWM scheme. By using the proposed switching scheme, the duty cycle can be increased to one while retaining the high conversion ratio and current balancing functionality. Thereafter, the proposed asymmetrical PWM strategy is generalized for multiphase series-capacitor (nP-SC) converters. Finally, to validate the performance of the proposed PWM strategy, a 1.2-kW prototype 3P-SC converter is fabricated and tested.