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On-Board Charging(OBC) System Design


1. Overview of on-board charging system

The on-board charging system is installed on an electric vehicle to adjust the AC or DC input power to a calibrated voltage/electricity.

It is a system that provides electricity for the power battery of electric vehicles, and can also additionally supply power for on-board electrical equipment. Different from traditional industrial-grade products, as an automotive-grade product, the on-board charging system has higher requirements and is more difficult to manufacture.

The AC vehicle-mounted charging system consists of the vehicle-mounted charging system and the control and guidance device. Among them, the on-board charging system consists of AC Input interface, power unit, control unit, DC output interface and other parts are composed, as shown the following. In the process of allowing electricity, the on-board charging system provides low-voltage power supplies such as the power battery management system (BMS), charging contactor, instrument panel, and cooling system. The structure of the digging guide device will be introduced.

The AC input interface contains 7 pins, divided into three types of connections, including high-voltage source connection (L/NCI / NCL)

Pressure neutral line (N); vehicle chassis ground (PE); low voltage signal confirmation and control confirmation (CC/CP). The standard input interface adopts power frequency single-phase input voltage of 220v. If power is needed, two spare pins can be enabled to achieve three-phase 380V input.

In the control unit, the sampled output current and voltage are processed and passed the real-time value to the control loop. The controller compares the difference between the measured value and the expected value, and then transfers the adjustment requirements to the PWM loop (pulse width modulation technology) or PFM loop (pulse frequency modulation technology) uses pulse changes to control the switching duty or frequency of the power devices in the high-voltage loop, and finally realizes that the output current and voltage are as close as possible to the values required by the main control system.

The low-voltage auxiliary unit is a standard low-voltage power supply with an output voltage of 12V or 24V, which is used to supply power to the electrical appliances on the electric vehicle during charging, such as the power battery management system, thermal management system, and automobile instrumentation.

The power unit circuit structure is shown, which generally includes four parts: input rectification, PFC (power factor correction), inverter and output rectification circuits. It converts the input power frequency AC power into a DC power suitable for the proper voltage that the power battery system can accept for charging.

The DC output interface includes two positive and negative pins of the low-voltage auxiliary power supply, two positive and negative pins of the high-voltage charging circuit, chassis ground, communication lines CAN — H and CAN — L two pins, and charging request signal pins. Among them, the two pins of the high-voltage charging circuit are connected to the power storage system; the charging request signal line is used to confirm the charging connection between the input port of the charging system and the external power supply, and then send it to the vehicle controller through the "charging request signal" pin Send a charge request signal.

2 Electrical characteristics of on-board charging system

The electrical parameters of the on-board charging system mainly include electrical parameters such as rated input/output voltage and current, rated frequency, efficiency, power factor, and electromagnetic compatibility.

2.1 Rated input/output voltage, current and rated frequency of the car charging system

Generally, the allowable voltage fluctuation range of the rated input voltage of the on-board charging system is the nominal voltage (1 ± 15%). The rated frequency is 50 Hz ± I Hz.

2.2. Charging efficiency and power factor of on-board charging system

①The efficiency of the on-board charging system should not be less than 90% under the condition of rated input voltage and rated load

②The power factor of the on-board charging system should not be lower than 0.92 under the condition of rated input voltage and rated load.

2.3. EMC requirements for on-board charging systems

(1) Anti-electromagnetic interference performance

(2) Electromagnetic interference generated

3 Communication protocol requirements between the on-board charging system and the power battery management system (BMS)

1. Charging method of on-board charging system

Common charging methods for on-board charging systems include constant voltage charging, constant current charging, pulse charging, and phased charging. ①Constant voltage charging: The charging voltage remains unchanged during the entire charging process, and the charging current gradually decreases with the increase of the charging time. When the charging current is less than a certain value, the charging stops.

②Constant current charging: At the beginning, the power battery pack is charged with a constant current. When it is about to be fully charged, a constant low current is used for floating charge to fully charge the remaining power and compensate for battery self-discharge. When the charging voltage reaches the rated voltage Stop charging.

③Pulse charging: usually used in fast charging, it is conducive to the full reaction of the active substances inside the battery, effectively reducing and eliminating the occurrence of polarization, and can be charged with a larger current, which can effectively improve the charging efficiency and shorten the charging time , Delay battery life.

④Constant current and constant voltage two-stage charging method: It combines the advantages of both the constant voltage charging method and the constant current charging method. In the initial stage of charging, the constant current and voltage-limiting charging method is used to avoid the shortcomings of excessive current during simple constant voltage charging; when the voltage reaches a certain value, it is changed to the constant voltage and current-limiting charging method, which solves the problem of simple constant current charging in the later stage of the battery. The problem of insufficient ability to accept input energy. At present, constant current and constant voltage two-stage charging is a commonly used charging method for vehicle-mounted charging systems. The voltage and current curves of the charging process are shown.

Control and guidance of on-board charging system

The input control and guidance circuit of the on-board charging system is shown. When the charging plug is plugged into the socket, the input control guidance circuit has the function of making the vehicle in a non-driving state. The vehicle control device can determine whether the vehicle charging interface plug is completely connected to the socket by detecting the voltage value of point 3, and the vehicle control device passes The PWM signal of detection point 2 can determine whether the vehicle charging connection device is fully connected.

The output control and guidance circuit of the on-board charging system is shown. After the power supply equipment and the electric vehicle establish a connection and connection, the on-board charging system sends a charging request signal to the vehicle control device and supplies power to the vehicle control device. According to the signals of detection point 2 and detection point 3, the rated input current of the power supply equipment, vehicle connection device and on-board charging system is compared, and the minimum value is determined as the maximum allowable input current of the on-board charging system. When the charging connection device is fully connected and the maximum allowable input current setting of the charging system is completed, the vehicle control device controls the contactor in Figure 3-147 to close, and the on-board charging system starts to charge the electric vehicle. During the charging process, the vehicle control device can monitor the PWM duty cycle of the signal at detection point 3 in the picture. When the charging is completed or other charging conditions are not met, the vehicle control device sends a charging stop signal to the vehicle. Charging system, on-board charging system stop DC output, CAN communication and low-voltage auxiliary power output.

Development trend of on-board charging system

1. Function integration

The power components of electric vehicles mainly include motor controllers, on-board power allowing systems, DC/DC converters and other parts. Have advantages. Therefore, effectively integrating these power components into a whole, sharing some of the circuits and cooling substrates, is a relatively obvious development trend at present.

Of course, there is also a degree of control in the process of achieving integration. If the integration is not sufficient, the cost reduction is not significant; if there are too many integrated components, the product flexibility is insufficient, and it is not easy to become a standardized product. In addition, this kind of integration can't just be physical integration, it must be system integration to be able to bring out the advantages of volume and cost. At present, integrating the motor controller, DC/DC converter and on-board charging system is one of the more feasible solutions

2. High power and high efficiency

3. Efficient thermal management

4. Two-way charging and discharging function

5. Wireless charging system

The basic composition of the wireless charging system is shown. It transfers energy through the power transmitting coil and the receiving coil, instead of the conductive charging plug, so it can improve the convenience and safety of charging. In addition, the combination of wireless charging technology and future driverless technology can further enhance the convenience of charging.

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