The Investigation Of High Efficiency DC-DC Topology Structure And Control Strategy For High Voltage DC Server Power System

As the fast development of information technology, especially the mobile internet technology, the scale and power consumptions of datacenters are increasing rapidly and the datacenter with conventional 220V AC power supply system cannot meet the power and energy efficiency requirements anymore. One way to improve the power density and efficiency of the server system is to adopt the high voltage DC power supply concept and integrate DC/DC power modules into the mother board of the server. This integration requires high power density high efficiency bus converter module. Conventional PWM converters with output regulation ability cannot realize high power density and high efficiency at the same time. The unregulated DC transformer with fixed duty cycle and frequency achieves high efficiency and high power density, but it has no regulation ability. A second stage is required to regulate the output voltage but two stage structure decreases the conversion efficiency. This dissertation investigates the topology structures and control strategies for both PWM controlled phase-shift full-bridge converters and LLC resonant converters to achieve high efficiency, high power density as well as regulation requirements for DC/DC modules. It mainly contains the contests as below.Phase-shift full-bridge converter is popular in server power supply application for its simple structure and soft switching characteristics. A transformer winding series-parallel auto regulated rectifier unit is proposed in this dissertation, and a phase-shift full-bridge converter with transformer winding series-parallel auto regulated current doubler rectifier is derived. The connections of the secondary transformer windings are auto regulated according the duty cycle. This topology reduces the voltage stress of the rectifier components, makes the synchronous rectifier feasible in this application, reduces the conduction loss and improves the efficiency. What’s more, this topology decreases the current ripple in the output filter, reduces the loss of the output inductor and optimizes the system efficiency.Increasing switching frequency reduces the volume of passive components and improves the power density. Compared with conventional phase-shift full-bridge converter, resonant topology is more attractive in high frequency high power density applications for the inherent soft switching characteristics. Base on the efficiency problem of the existing two stage cascaded structure, this dissertation introduces a regulated DC transformer (RDCT) topology implemented by a controllable voltage source in series in the primary side. It regulates the output voltage by adjusting the voltage of the controllable voltage source and the driving schemes for both primary switches and secondary rectifiers are simplified. High efficiency is guaranteed because only partial energy is transmitted through the two stage conversion. Furthermore, the start-up strategy for resonant converter at high frequency is considered. A hybrid strategy based on limiting the current and varying the frequency is proposed to start-up a resonant converter under nominal input voltage, it also solves the current overshoot problem during the startup process.Interleaved control of DC-DC converters reduces the output current ripple, optimizes the power and thermal distribution and improves the system reliability. But for multi-phase resonant converters, the interleaved control requires the same driving frequencies, which means the mismatch of the resonant parameters cannot be compensated by regulating the frequency. This dissertation focus on the limitations of the available current sharing methods for interleaved resonant converters and proposes a novel current sharing control strategy for interleaved resonant converters based on the RDCT structure. Based on the power ratio of RDCT converter, the output current is shared by balancing the current of auxiliary PWM DC-DC modules. This method is suitable for high frequency high output current application. It samples only partial current, reduces the conduction loss of the sampling resistor and realizes the current sharing of resonant converters by PWM control.