Study on isolated DC/DC half-bridge type converter for improving light-load efficiency and performance

Abstract

The modern society can be said to be the sea of information and the world of information. Various IT products, such as laptop, tablet PC, smart phones, which are commonly found around us, are becoming indispensable to modern society. IT industries are the fastest growing industries in recent years. In addition, they store and share all the information in a data. Recently, due to the development of Internet of things (IOT), it is also possible to exchange the information in various IT products. Moreover, the demand of server computers and data centers are rapidly increasing worldwide to cover the increased data throughput due to online, Internet, and IOT. Electrical energy is essential for IT products, and IT products have continuously required the power conversion devices, which convert electrical energy to each application level. In other words, since these IT products require the battery charger and power supplies, the growth of the IT product market represents the growth of the power supply market. Therefore, the studies on the high efficient power converters are also actively under way for using IT products and energy saving. Among these power converters, as the data throughput rapidly increases around the world, the demand of server power supply, which supplies the power to data centers and server computers, are increased to cover these huge data throughput. The server power supply is the collection of the best existing technologies to satisfy the various regulations and requirements. Looking at recent trends in power supply area based on this server power supply, they require high conversion efficiency and high power density. Especially, the efficiency and performance improvements under light-load condition are the most studied area in the power supply market. Indeed, due to the requirement of 10% efficiency in 80PLUS titanium level, no-load operation, and standby mode, there are also many requirements for light-load condition. In this study, according to the trend, the light-load characteristics of power supplies are improved by DC/DC converter with half-bridge structure. DC/DC converters with half-bridge structure have disadvantages that the control is limited and the light-load efficiency is lower than other topologies because of small number of active devices. In this dissertation, among the half-bridge converters, a new asymmetric half-bridge converter with fixed switching frequency, which regulates output voltage using asymmetric duty of primary switches and additional switch, is proposed to improve the conversion efficiency over the entire load condition, especially light-load condition. In addition, the problem of light-load regulation in the half-bridge LLC resonant converter is analyzed by Bode plot and impedance asymptote, and the circuit for solving this problem is proposed. Part 1. Study on the efficiency improvement of asymmetric half-bridge converter The asymmetric half-bridge converter is a topology that can be used in various applications because it can carry a simple structure, control, and double-ended type rectifier which can use the transformer optimally. The asymmetric half-bridge converter applied to the server power supply has to have a wide input range to satisfy the hold-up requirement, which is the time to regulate the output voltage for 16~20 (ms) after input AC line is disconnected, to protect the data. As a result, it should be designed to have small duty cycle at nominal input condition and the overall efficiency is reduced, especially under light-load condition. In order to improve the conversion efficiency, the circuit of adding an external inductor to achieve zero voltage switching and a converter which can eliminate DC-offset current in transformer have been proposed. However, since the conversion efficiency drops are caused by asymmetric operation, this study proposes a new circuit that can provide an additional gain in order to use the duty cycle close to 0.5 at the nominal input condition. The proposed converter is based on an asymmetric half-bridge converter with an external inductor and two clamp diodes. The clamp diode in lower side is switched to a switch and auxiliary winding of transformer is located in series to the external inductor. Through this result, it is possible to reduce the current of external inductor by using the auxiliary winding in the nominal input condition and to achieve the optimal design of asymmetric half-bridge converter. In the hold-up condition, in order to compensate for the insufficient gain, the additional switch operates and makes the external inductor current build up to change the current path and turn-ratio of the transformer. This operation achieves the additional gain so that the proposed converter has less condition loss, a wider zero voltage switching range, and higher conversion efficiency than the conventional asymmetric half-bridge converters in the nominal input condition. The proposed converter was verified by a 500 (W) proto-type converter and it satisfies the hold-up requirement, while it achieves the high conversion efficiency. Part 2. Study on the light-load regulation problem of half-bridge LLC resonant converter Among the resonant converters which regulate the output voltage by changing the switching frequency, the half-bridge LLC resonant converter has many advantages such as, high efficiency, small number of elements, and capability of high frequency operation so that it is the most used topology in recent years. However, LLC resonant converter has a problem that the output voltage increases under the light-load condition, so that it is limited to use the applications, which frequently operate under no-load condition or standby mode. The main reason of the light-load regulation problem in LLC resonant converter is the junction capacitance of rectifier diode in secondary side. For the analysis, since the frequency characteristic with AC equivalent circuit of LLC resonant converter is plotted by Bode plot and impedance asymptotes, there are two high frequency poles located at the frequencies, where the impedance magnitude of $R_{ac}$ and $1/sC_{j,eq}$ are equal and the impedance magnitude of $R_{ac}$ and $sL_r$ are equal, move closer to each other under the light-load condition. Through this result, they generate a double poles and it can make the undesired boosting region due to the second order slope. Therefore, even though the switching frequency increases, the gain curve and output voltage increases continuously under the light-load condition. In this study, the light-load regulation is achieved by changing the resonance tank to eliminate the influence of $C_{j,eq}$ which commonly observed under light-load condition. By adding an additional capacitor in parallel to the resonant inductor, the impedance is changed to achieve the reduced gain in the high frequency region. The proposed circuit can be controlled by the pulse frequency modulation, which is the conventional method and does not require the additional control method. In addition, the proposed converter has the same gain curve and performance as the conventional LLC resonant converter in normal frequency region, and has reduced gain curve in high frequency region to compensate the effect of $C_{j,eq}$. Because the additional elements of proposed converter are minimized and the additional losses are very small, the proposed converter achieves the light-load and no-load regulation while it maintains the high conversion efficiency. To verify the characteristics of proposed converter, 200 (W) prototype converter was designed and it can achieve no-load regulation which is less than 3% and higher efficiency under the light-load condition due to smaller switching losses. Up to date, the usage of LLC resonant converter has been limited due to the light-load regulation problem. However, with the proposed method, LLC resonant converter achieves the no-load regulation without any side effect. Therefore, LLC resonant converter with the proposed technique can be extended more to overall power electronics industry. In this dissertation, a new topology and its design have been studied to improve the light-load efficiency and the performance of half-bridge typed DC/DC converter, which has simple structure and control. Since the proposed circuits can achieve the optimal design of each topology under nominal state, they are expected to be used widely in various applications that especially require the high reliability and conversion efficiency.