Against the backdrop of high data center loads, improving the power density of server power supplies has become an industry consensus. At present, server power supplies mainly adopt a two-stage architecture: a front-end PFC and a rear-end DCDC. For the front-end PFC, multilevel converters can significantly improve power density while ensuring high efficiency.

However, one of the application pain points of flying-capacitor multilevel converters is the voltage balancing problem of the flying capacitors, especially the dynamic voltage balancing problem. Based on a four-level flying-capacitor totem-pole PFC topology, the Zhejiang University PMIC team proposed a modulation strategy that dynamically maintains flying-capacitor voltage balancing, supporting next-generation server power supply systems.

Voltage Balancing Challenges of Flying-Capacitor Multilevel Topologies

Although multilevel topologies can effectively reduce the voltage stress of switching devices and increase the equivalent switching frequency, the voltage balancing problem of flying capacitors has always been key to constraining their performance and reliability. The voltage balancing problem of flying capacitors faces many challenges:

• Traditional voltage loop control has limited bandwidth, making it difficult to control high-frequency oscillations on the flying-capacitor voltage during transient instants
• The flying-capacitor voltage needs to be sampled, bringing additional hardware costs, complicating the control loop, and affecting system efficiency and power density

Hybrid Dual Phase-Sequence Modulation Strategy

The Zhejiang University PMIC team proposed a dynamic voltage balancing modulation strategy that requires no flying-capacitor voltage sampling—hybrid dual phase-sequence modulation. On the basis of phase-dependent carrier modulation, only the phase-dependent sequence needs to be changed to dynamically maintain flying-capacitor voltage balancing.

Impressive Measured Verification Results

Peak efficiency exceeds 99.4%, and the overall power density reaches 320W/in³

Voltage balancing effect achieved during the restart process from an initial state of unbalanced flying-capacitor voltages after an AC Drop transient

Summary: Dynamic Voltage Balancing Modulation Promotes the Practical Application of Multilevel Topologies

The dynamic voltage balancing modulation strategy proposed in this study effectively solves the flying-capacitor voltage balancing challenge of four-level flying-capacitor topologies in practical applications, providing new technical support for realizing high-efficiency, high-power-density AC/DC converters. By changing the carrier modulation sequence, it not only breaks through the bandwidth limitation of traditional voltage balancing loops, but also eliminates the need for voltage sampling of flying-capacitor voltages, reducing software and hardware complexity and providing an important reference for multilevel topologies moving toward server power supplies.