Research on full-bridge converter with integrated and hybrid structure for on-board charger in electric vehicles

Abstract

Humankind has achieved the noticeable improvement of material prosperity and comfort with the use of fossil fuel since 18th century. In particular, the appearance of vehicle offers the convenience of movement, and vehicle industry has led the economic growth. These days, more than one billion vehicles exist over the world according to Ward’s research. Most of them have an internal combustion engine using fossil fuel. Thus, the increase in number of vehicles results in the mass consumption of fossil fuels such as gasoline and diesel as a fuel for the vehicles. Therefore, the vehicle is regarded as an emission source of carbon dioxide ($CO_2$) that is considered as the reason of the global warming. Since the damage of natural disasters due to the climate change caused by the global warming has been reported around the world, several countries have proposed $CO_2$ emission standard with a goal which is challenging compared to former regulations. However, the US Environmental Protection Agency (EPA) has anticipated the only 5% of vehicles using an internal combustion engine could meet $CO_2$ emission target in 2025. As a result, in order to depart from the vehicle technology based on fossil fuel, the Electric Vehicle Initiative (EVI) which is multi-government policy forum was established in 2010. The EVI has performed a role to accelerate the introduction and adoption of electric vehicles worldwide. Thus, the researches of eco-friendly electric vehicles (EVs) such as hybrid electric vehicle (HEV), plug-in HEV (PHEV), battery EV (BEV), and fuel cell vehicle (FCV) have gathered momentum. South Korea has become the member of EVI since May 2015. The EVs necessarily need rechargeable batteries as the power source of electric traction system. As a result, the interest for the EV battery charger has been increased as much as the annual growth of EV sales about 40%. The EV battery charger can be divided into three levels based on the power rating, and two types such as on-board and off-board with the location. In this research, the level 2 with the power rating from 3.3kW to 6.6kW and on-board type charger, which means that a charger is mounted on a vehicle, is focused. Therefore, the weight lightening, high power density, and high efficiency are being issued for EV on-board battery charger applications. The most of on-board EV charger system has the basic architecture of an AC-DC converter with power factor correction (PFC) operation followed by an isolated DC-DC converter. By the AC-DC converter, the AC input is converted to the constant 400V DC voltage, and the low distortion of input current and the high power factor can be achieved. The DC-DC converter modulates output voltage and current based on the status of the battery, and directly recharges it. The conventional phase-shift full-bridge (PSFB) topology with a full-bridge (FB) output rectifier is a very attractive topology for the isolated DC-DC converter in on-board battery charger because the PSFB topology has several advantages as follows

the voltage stress of all primary switches is clamped to the source voltage, the current stress of them is low compared to other topologies, the output voltage is easily controllable and stable with phase-shift manner despite of wide output voltage range, and zero-voltage-switching (ZVS) operation of all primary switches by utilizing the transformer leakage inductance and intrinsic capacitance of switches without any additional components. However, the PSFB topology also has several significant drawbacks to apply EV battery charger. Firstly, the large conduction power loss coming from the circulating current on the primary side during freewheeling phase. It is because the freewheeling period can be longer in constant current (CC) charging due to the wide output voltage range, and the large turns ratio to achieve high output voltage makes the circulating current to be larger. Secondly, it is difficult to achieve the ZVS operation of the lagging-leg switches at light load condition due to the insufficient ZVS energy at leakage inductance. Thus, the switching power loss can be enlarged during constant voltage (CV) charging which takes longer time than CC charging. At last, a large output filter inductor is demanded for low output current ripple. Due to these drawbacks, the PSFB converter has the limit to reduce its volume and weight, and to increase the conversion efficiency. In this dissertation, the research is mainly focused on the integrated structure and hybrid structure full-bridge topology development, and an optimal design and control method for high efficiency and high power density. The research is divided into three parts as follows:

Part 1. Integrated Dual Full-Bridge converter with Current-Doubler Rectifier for EV charger In the research, a novel wide-range soft-switching topology based on the integration of two PSFB circuit is proposed. The novel topology has three legs on the primary side, and the center-leg is shared. In the secondary side, the integrated form of two current doubler rectifier (CDR) circuits having FB structure is adopted, which shares output inductors. In order to extend the ZVS operation range of lagging-leg switches in the conventional PSFB converter, the external inductor is typically added to the primary side of converter. However, the core loss and conduction loss generated by the additional inductor reduce the conversion efficiency. Furthermore, the size of additional inductor decreases the power density. On the other hand, the proposed topology can achieve the wide ZVS range with the only one external inductor having small inductance. It is because the output current is reflected to primary side in light load condition due to the integration circuit of CDRs, which extends ZVS range. In addition, the size of output filter inductor can be reduced since the output current ripple is reduced by the output load-share operation. Therefore, the high efficiency and high power density can be achieved. The validation of the proposed converter is confirmed by the experiment with a prototype on-board charger of 5.7kW (13.5A). As a result, the efficiency is higher than 95% in whole output power range, and it achieves 97.3% peak efficiency.

Part 2. Hybrid Converter with Wide ZVS Range and Minimized Circulating Loss for EV chargers In this research, a hybrid structure converter, which can overcome the drawbacks of the conventional PSFB converter such as narrow ZVS range, conduction loss by circulating current, and large output filter inductor, is proposed. The hybrid converter discussed in this part consists of the conventional PSFB converter with a full-bridge rectifier and a half-bridge LLC series-resonant converter (SRC) with a voltage-doubler rectifier (VDR), sharing the lagging-leg switches and integrating the VDR of LLC SRC into the node of the secondary-side rectifier in the PSFB converter. This rectifier structure allows the proposed hybrid converter to have high efficiency due to ZVS operation in whole load condition for all the switches, zero-current-switching (ZCS) operation of the output rectifier diodes, and the reduction of circulating current during freewheeling phase by the output voltage of the LLC SRC. In addition, the size reduction of output filter inductor can be achieved. Its validity is confirmed by experimental results involving a 3.3kW (250-450V / 7.33A) prototype converter. As a result, the efficiency is higher than 95.7% in whole output power range, and it achieves 97.6% peak efficiency.

Part 3. Design and Control of Hybrid Converter for Optimal Conversion Efficiency in Electric Vehicle Chargers This research analyzes the power distribution of the targeted hybrid converter which is the same converter as the proposed converter in part 2. From the analysis, the reason that the power distribution is poor, which causes low efficiency and low power density, is clarified. Based on the analysis, the optimal design of transformer and the control strategy with pulse-frequency modulation and pulse-width modulation are presented. With the precise design and control, the output voltage on LLC SRC is modified, which can increase the conversion efficiency and power density. Its validity is confirmed by experimental results involving a 3.3kW (250-450V / 7.33A) prototype converter and three different types of transformer. As a result, the maximum efficiency is achieved as 97.7% as peak efficiency, and it is noted that the power density can be increased by using smaller transformer without additional loss.