Study on half-bridge converter and control method for high power density laptop adapter and high efficiency server power supply

Abstract

Nowadays, a high power efficiency has become more important because many electric devices have been developed and their power consumption has been increased continuously. Because a lot of power is supplied in worldwide, the environmental problems such as the energy resource depletion and global warming have been issued. Therefore, a high power efficiency in many electric devices is strongly required, and more guidelines are recommended. Moreover, since the consumers tend to prefer compact electric devices because of easy portability, most manufacturers make an effort to minimize their size. Meanwhile, the power consumption has been more increased with the development of the information technology (IT) industry. After the IT market has been dramatically grown all over the world, many data processing devices such as laptop computers have been developed, and the processing data has been increased continuously. As a result, more server systems have been required, and the number of data centers, where many server systems are gathered, has been surged accordingly. The laptop computer is one of the early models in the portable computers. Since it has outstanding performance compared with recent models like the smartphones and tablet PCs, it has been widely utilized until now. That is, the laptop computer has been developed with high performance and wide screen, which is differentiated from the other equipment. Therefore, the adapter for the laptop computer is required to have a high power rating, and small size for easy portability. However, when the adapter for the laptop computer operates at a high power level, it provides high power loss and heat. Since there is no cooling fan in the adapter, its low operating temperature is one of its most important issues, which indicates a high efficiency. Generally, to achieve a high efficiency in the adapter with a high power rating, the zero-voltage-switching (ZVS) topologies should be adopted, which consists of many components. Therefore, small-sized adapter with a high power rating is strongly required, obtaining a high efficiency. Meanwhile, the server computer has also been developed highly with the increase of data centers, and its power rating has been increased because of a large amount of data. Therefore, the server power supply, which supplies the power to the server computer, is required to achieve a high power efficiency. Moreover, in the server power system, a high reliability is demanded due to the characteristic of the server system, thus the redundant structure is widely used in the server power applications, which indicates that many server power supplies are paralleled to prevent the system fault. That is, if something goes wrong with one or two server power supplies, other supplies make up for them. Accordingly, each server power supply is mainly operated under light load conditions, rather than heavy load conditions. Therefore, the efficiency in light load conditions has also become more important. In this dissertation, the research is mainly focused on the power supplies with a high efficiency and power density, which is closely related to data processing devices widely utilized in IT industry. The research is divided into two parts as follows.

*Part 1. A Novel Accurate Primary Side Control Method for Half-Bridge LLC Converter

The conventional half-bridge (HB) LLC converter can be a good solution for high-efficiency adapter because of its simple structure and low voltage stresses on the primary switches. Additionally, since the zero-voltage-switching (ZVS) of the primary switches and zero-current-switching (ZCS) of the secondary switches can be achieved at the entire load conditions, the switching loss can be decreased. The high-efficiency HB LLC converter is generally implemented by the secondary side control (SSC), using a HB LLC controller in the primary side, operational amplifier in the secondary side, and photocoupler in between the primary and secondary side to regulate the output voltage. However, since this SSC method requires at least three ICs with different grounds, it is difficult to integrate them into a single-chip, which increases the size and cost of the control stage. In this part, a new PSC method is proposed to have low component count in the control stage and accurately regulate the output voltage in high-efficiency HB LLC converter for a high power adapter. In the proposed PSC method, the output voltage is regulated by obtaining the voltage across the primary side of the transformer when the external resonant inductor voltage becomes 0V. At this time, since the voltage across the transformer secondary leakage inductor is small, the proposed method can accurately regulate the output voltage. Moreover, the proposed PSC method can decrease the size and cost of the control stage by eliminating the photocoupler and secondary feedback circuitry. The circuit configuration, operation principle, and relevant analysis for the proposed PSC method are presented and verified by 85W prototype realized with a high power laptop adapter (400V input, 20V/4.25A output).

*Part 2. A High Efficiency PFM Half-Bridge Converter Utilizing a Half-Bridge LLC Converter under Light Load Conditions

The conventional half-bridge (HB) converters with the output inductor have common problems such as the primary and secondary switch turn-off losses and snubber loss in the secondary side caused by the output inductor, which degrades light load efficiency. To relieve these limitations of the conventional HB converters, a new HB converter, which employs one additional switch and capacitor in the secondary side, is proposed for a high efficiency under light load conditions in this part. Since the proposed converter operates like the HB LLC converter with below operation by turning on additional switch under light load conditions, the switch turn-off losses and snubber loss can be minimized, and the zero-voltage-switching (ZVS) capability can be improved. Consequently, the proposed converter can achieve a high efficiency under light load conditions. Moreover, since additional components can be implemented with compact size, the power density of the proposed converter can be maintained compared with that of the conventional HB converters. The circuit configuration, operation principle, and relevant analysis for the proposed converter are described and verified by 300W prototype for a server power supply (330~400V input, 12V/25A output).