Design of power supplies for the next generation of CPUs

Abstract

This project is a Teaching Company Scheme between The Hong Kong Polytechnic University and ON Semiconductor Company Limited on the practical design of power supplies for the CPUs of computers. In recent years, computer systems have become more complex and advanced. Not only their performance has been improved, but also higher speed logic has been developed. To reduce the overall power dissipation, low supply voltage is desired and this in turn requires power supplies to provide higher current capability. Moreover, notebook computers have become slimmer and lighter, so the performance demand placed on power electronics has become more stringent. The 1.1 V to 1.8 V supply rails appear in year 2002 notebook computers [1, 2, 3, 4], which supply power to RAMBUS, RDRAM and chipset rails etc. The new generation of CPUs requires power supplies to provide a large current at a low voltage (1.1 V to 1.8 V). These CPUs also require power supplies to have low output voltage ripple and exceptionally fast transient response. Conventional power supplies fail to meet such requirements. Using linear regulators is a solution to improve load regulation under transient conditions. However, switching power supplies provide better efficiency than linear regulators. A high efficiency is essential to most computers, especially portable ones in order to allow them to have a longer operation time. In this Teaching Company Scheme, a detailed study has been carried out on the design of power supplies for fast CPUs. The advantages and problems of existing circuits are identified. It is found that multiphase converters are used to increase the output current, reduce the output ripple and improve the transient characteristics. However, for each phase of the multiphase converter, a current sensor (together with the associated circuitry) is required. This makes the power supply complex and costly. In some circuits the resistive voltage drop across the output filtering inductor is detected and processed to form the current feedback signal (so as to eliminate the need for current sensors). But the production spread in the effective resistance of the inductor and its temperature-dependent characteristics introduce problems in the mass production. In order to solve the above mentioned problems, multiphase converters with coupled inductors are proposed. These converters (with coupled inductors) have the advantages of: Reduced number of magnetic components; Requiring no separate current sensors; Reduced output ripple. The characteristics and applications of multiphase converters with coupled inductors are studied in detail in this Teaching Company Scheme. The general guidelines on the design of PCBs for High speed CPUs are also discussed.