Design Considerations for Electric Vehicle Charger Circuit Topologies

0 Introduction

As early as the beginning of the 20th century, electric drive systems were used in the driving systems of cars in Europe and the United States. At that time, electric vehicles had replaced the old horse-drawn carriages and bicycles as the main means of transportation. The advantages of electric vehicles such as comfort, cleanliness, no noise, and little pollution once made people think that this would be a huge innovation in transportation. However, due to the problems of key technologies such as batteries at that time and the development of fuel vehicles, the development of electric vehicles has been restricted for 100 years.

With the development of modern high-tech and the increasing prominence of the two major problems of the world today, environmental and energy, electric vehicles have become a hot topic in the research, development and use of the automotive industry. Since the 1980s, countries around the world have launched a large-scale development of electric vehicles. However, the marketization of electric vehicles has been plagued by some key technologies. Among them, a more prominent problem is to ensure that the charging technology of electric vehicle battery packs is safe, efficient, user-friendly, strong and cost-effective [1][2].

1 Charging Technology

Electric vehicle battery charging is one of the key technologies that must be solved before electric vehicles are put on the market. There are generally two basic methods for charging electric vehicle batteries: contact charging and inductive coupling charging.

1.1 Contact Charging

The contact charging method uses a traditional contactor, and the user connects the charging source connector to the car. A typical example is shown in Figure 1. The disadvantages of this method are: the conductor is exposed outside, which is unsafe. In addition, due to repeated plugging and unplugging operations, mechanical wear will occur, resulting in loose contacts and ineffective transmission of electrical energy.

Figure 1 Schematic diagram of contact charging

1.2 Inductively coupled charging

Inductive coupling charging is a method whereby the charging source and the vehicle receiving device do not use direct electrical contact, but instead use a combination of separate high-frequency transformers to transmit energy contactlessly through inductive coupling. The use of inductive coupling charging can solve the defects of contact charging methods [3][4].

Figure 2 shows a simplified power flow diagram of the inductively coupled charging system for electric vehicles. In the figure, after the input grid AC is rectified, it passes through the high-frequency inverter link, is transmitted through the cable through the inductive coupler, and is transmitted to the input end of the electric vehicle, and then passes through the rectification and filtering link to charge the on-board battery of the electric vehicle.

Figure 2 Simplified power flow diagram of EV inductively coupled charging system

The inductive coupling charging method can also be further designed into a fully automatic charging method without human intervention. That is, the primary and secondary sides of the magnetic coupling device of the inductive coupler are separated by a larger distance, and the charging source is installed in a fixed location. Once the car is parked in this fixed area, it can receive the energy of the charging source without contact, realizing inductive charging, thus realizing fully automatic charging without the intervention of the car user or the charging station staff.

2 Inductively coupled charging standard—SAE J-1773

In order to realize the marketization of electric vehicles, the American Society of Automotive Engineers has formulated corresponding standards based on system requirements. Among them, two charging standards, SAE J-1772 and SAE J-1773, have been formulated for electric vehicle chargers, corresponding to contact charging and inductive coupling charging respectively. Electric vehicle charging system manufacturers must comply with these standards when designing, developing and producing electric vehicle chargers.

The SAE J-1773 standard specifies the minimum practical size and electrical performance requirements for inductive charging couplers for electric vehicles in the United States.

The charging coupler consists of two parts: the coupler and the car socket. The combination is equivalent to a transformer with primary and secondary separation working at a frequency between 80 and 300 kHz.

For inductively coupled electric vehicle charging, SAEJ-1773 recommends three charging methods, as shown in Table 1. For different charging methods, the design of the charger will also be different accordingly. Among them, the most commonly used method is home charging, with a charger power of 6.6kW. Chargers with higher power levels are generally used in charging stations and other occasions.

Table 1 Three charging modes recommended by SAEJ-1773

According to the SAE J-1773 standard, the inductive coupler can be represented by the equivalent circuit model shown in Figure 3. The corresponding component values ​​are listed in Table 2.