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Research on High Efficiency High Power Density Telecom Power Supply Modules

Author: RenXiaoYong
Tutor: RuanXinBo
School: Nanjing University of Aeronautics and Astronautics
Course: Power Electronics and Power Drives
Keywords: Distributed power system four-switch Buck-Boost converter three-mode two-edge modulation minimal power loss voltage ripple
CLC: TM461
Type: PhD thesis
Year: 2008
Downloads: 359
Quote: 9
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Abstract


The distributed power system (DPS) has been widely applied in telecom power system for easy maintainability, short design cycle, flexible system structure and high reliability. The intermediate bus architecture (IBA) is the dominant structure of DPS, and it consists of front-end converter, intermediate bus converter (IBC) and the point of load (POL). The front-end converter converts the AC line into 48VDC bus, while the IBC converts the 48VDC bus to the 12VDC bus, the intermediate bus, and the POL converts the 12VDC to the desired voltage the load needs. This dissertation focuses on the issues on the IBC and POL.To ensure uninterrupted power supply, battery is required to be paralleled with the 48V bus. When the power grid is lost, the battery will power the load. It is well known that the battery voltage has a wide range according to its state of charge, and as a result, the input voltage of the IBC is wide, e.g., 36V– 75V for 48V nominal voltage. However, as the grid is available at the most time, the IBC always operates at the nominal input voltage of 48V, so the efficient at the nominal input should be high enough. This dissertation introduces the concept of electrical stress factor, and based on which, the full-bridge converter is taken as the example to compare the efficiency under different input voltage range. The theoretical analysis and experimental results show that the wider the input voltage range is, the higher the electrical stress fact, and the lower the efficiency, and the more difficult the transformer optimum design. In order to achieve a high efficiency for the IBC at a wide input range, this dissertation proposes a two-stage structure, where the first stage is the four-switch Buck-Boost (FSBB) converter, which converts the wide input range into a narrow one or a constant voltage, and the second stage is a full-bridge converter, which realizes both voltage step-down and galvanic isolation. The FSBB features the function of both voltage step-up and step down, simple circuit, low voltage stress across the power switches, positive output, and direct power transfer path. Furthermore, two control freedom, i.e., two duty cycles, are available, providing the opportunity for optimum control strategies to achieve a high efficiency.There are two power transfer manner in switching converter, they are direct power transfer and indirect power transfer, and higher direct power ratio leads to higher efficiency. To increase the direct power ratio in FSBB, this dissertation proposes two kinds of three-mode control methods. One is with rough regulation, where the output voltage is in a narrow range, and the full-bridge converter converts it to a well regulated output voltage. Another is with precise regulation, where the full bridge converter acts as a DC transformer.In the three-mode rough regulation control strategy, the input range is divided into three regions, and the FSBB operates under Boost mode, filter mode and Buck mode respectively. When input voltage lies around the nominal input, FSBB will operate under filter mode, avoiding the frequently mode switching between Buck and Boost modes. In the filter mode, the controlling switch of Buck cell keeps on, while the one of Boost cell keeps off, and the output voltage equals to the input one, so it has a narrow variation range. In order to ensure the smooth mode switching, an improved output voltage setting is proposed, the output voltage range of FSBB keeps the same as the input range of the filter mode. The experimental results show that high efficiency in the full line range and the highest efficiency around nominal input is achieved.Similar to the rough regulation, in the three-mode precise regulation control strategy, the FSBB also operates under three modes, where the filter mode is replaced by the Buck-Boost mode with regulated output voltage. To increase the direct power ratio, the two-edge modulation (TEM) is proposed in this dissertation, where the Buck cell is in trailing-edge modulation, while the Boost cell is in leading-edge modulation. To further increase the direct power ratio in Buck-Boost mode, the Buck cell is allowed to operate with a pre-set maximum duty cycle, Dm, and the Boost cell regulates the output voltage. The inductor current ripple under Buck-Boost mode is relatively lower than that under Buck mode and Boost mode, however all the four switches are high-frequency switched, resulting in high switching loss. In order to reduce the switching loss, the switching frequency in Buck-Boost mode is decreased on the premise that the inductor current ripple is not higher than that of the other two modes. As there are two PWM methods, two switching frequencies and three operation modes, the control strategy is called three-mode dual-frequency two-edge modulation. The experimental results show the proposed control strategy achieves high efficiency in full line range and the highest efficiency around nominal line.Since the FSBB has multiple operation modes, and furthermore it operates in two switching frequencies in precise regulation control strategy, it is a challenge to achieve the stability for the two-stage IBC. This dissertation analyzes the characteristics of the output impedance of the FSBB under different modes and the input impedance of the full-bridge converter, and according to the impedance ratio criterion, the closed-loop design for the FSBB and full-bridge converter is discussed. Under the filter mode, the FSBB operates in open-loop, the output impedance has a bump at the resonant frequency. In order to damp the bump, an optimal damping circuit is designed without heavy penalty of efficiency. The experimental results indicate that the proposed design method is effective.The point of load (POL) converter is directly interconnected to the load. In low voltage high current output applications, high efficiency, high power density and low output voltage ripple are required. To achieve high efficiency, the minimal power loss (MPL) design for the filter inductor is proposed and verified by the experimental results. And in order to achieve low volume and satisfy the output voltage ripple requirement, the selection guideline for the output capacitors is proposed based on the analysis of the relationship between the voltage ripple and the parameters of the output capacitor. It is also well verified by the experimental results.

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CLC: > Industrial Technology > Electrotechnical > Transformers, converters and reactors > Converter > Rectifier
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