Technical principle and application of electric vehicle multi-function charging system

introduction

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Due to the oil crisis and increasingly serious environmental pollution, the development of electric vehicles has become the trend of the times. The battery provides power for the electric vehicle, and the battery charging performance directly affects the use and life of the battery. The battery is generally divided into a lead battery, a nickel cadmium battery, a nickel hydrogen battery and a lithium ion battery. Due to the wide variety of batteries and different capacities, different types and capacities of batteries often require different chargers to match. If the battery chargers are not well matched, unsafe phenomena such as overcharging and overheating may occur, thereby affecting the normal use of the battery and shortening the battery. life. Therefore, it is necessary to design a multi-function charging system based on single-chip control that can charge all kinds of batteries. The multi-function charging system can quickly and stably charge batteries of different types and capacities. We have designed corresponding charging methods for different types of batteries in software so that each battery can be charged under the optimal charging method. For batteries of different capacities, the battery can be charged quickly and stably by setting the charging parameters when selecting the charging method.

1 hardware circuit design

The system adopts a phase-shifted full-bridge soft-switching circuit, which combines a Boost circuit and a full-bridge converter to form a single-stage PFC circuit. The circuit has a simple structure and high efficiency, and can realize setting of an input current and can work at a large power. Occasionally, the advantages of the full bridge circuit are exerted.

The system is mainly composed of charging main circuit and charging control circuit. Figure 1 is the hardware schematic diagram of multi-function charging system.

1.1 How the system works

This design adopts switching power supply technology, the maximum power is 3500W, first 220V single-phase power frequency AC, through 4 diodes to form a full-bridge circuit for rectification, and then through large-capacitance filter to obtain about 300V DC, at this time in DC ripple Larger. The DC power is passed through a full-bridge inverter consisting of four insulated gate bipolar transistors (IGBTs) to obtain a high-frequency alternating current with adjustable voltage. It is coupled to the secondary side via a high-frequency transformer, then rectified by a full bridge, and finally filtered by an inductor. A DC with a small ripple is charged to charge the battery. The multi-function charging system can charge different types of batteries and batteries with different capacities. The charging voltage and current in the charging process are controlled by the single-chip microcomputer in real time. The whole charging system is a feedback control system, and the single-chip microcomputer detects the current and voltage in the charging process in real time. And temperature monitoring the entire charging process, effectively avoiding overcurrent, overvoltage and overheating during charging, so that the charging process is carried out safely and steadily.

The air switch in front of the inverter bridge is to prevent short circuits or large currents in the circuit from damaging the battery or electronic device. The MCU compares the charging current, voltage and temperature with the set value before charging, and controls the output of 4 PWM waves to the gates of 4 IGBTs, thereby controlling the collector-to-emitter current on-off time to reach the control output voltage. the goal of.

Since the IGBT needs to be driven by isolation, this design selects Mitsubishi IBGT dedicated driver chip M57962L, and Figure 2 is its application circuit.

Since 4 IGBTs are selected to form a full-bridge inverter, each IGBT needs one M57962L chip driver, and each M57662L chip needs three voltage levels, namely 15V, l0V, 5v, for power supply, and 5v voltage simultaneously supplies power to MC9S12XS128 microcontroller. In this paper, a transformer with a power of 50W is designed to supply power to the MCU and four M57962L chips. The secondary winding outputs three sets of voltages. After rectification and filtering, the three voltages required are obtained.

1.2 Charging control loop

Select Freescale MC9S12XS128 microcontroller as the control core for data acquisition and control, its internal data memory 8KB, program memory 128KB, 2 SCI, 1 SPI, 1 IIC, 1 CAN, 16 A/D, 8 PWM The 8-channel ECT module has a working frequency of 80MHz, which is fast in operation and greatly improved in processing capability. The chip integrates l6 channel 12-bit high-precision A/D converter, which can directly detect the charging voltage, current and temperature of the battery. The 8-channel PWM can be directly output to the M57962L chip to control the on-off of the IGBT, simplifying the peripheral of the microcontroller. Circuit design.

1.2.1 Voltage detection

The system adopts a resistor-divided structure, and monitors the voltage signal in parallel in the charging circuit. The voltage signal is transmitted from the PAD0 port to the single-chip microcomputer through the self-contained A/D converter of the single-chip microcomputer for processing. This structure can automatically select according to the actual voltage outside. The range detection voltage is such that the smaller the voltage, the higher the accuracy of the detected voltage, which helps to more accurately control the change of the charging voltage during charging.

1.2.2 Current detection

The system selects the Hall-type current sensor to detect the charging current signal, and the detected current signal is transmitted from the PAD1 port to the single-chip microcomputer through the A/D converter provided by the MCU through a certain conversion process. The sensor has high precision and can Accurate detection of a change in charging current of 0.1A.

1.2.3 Temperature detection

The system selects the thermistor to detect the battery temperature signal during the charging process. In actual application, the thermistor is attached to the battery to detect the battery temperature. The thermistor can accurately detect the change of the battery temperature during the charging process, and the temperature signal passes through the PAD2. The mouth is transmitted to the MCU for processing to prevent the battery from overheating during the charging process, so that the charging process can be carried out smoothly and safely.

1.2.4 LCD module

This system selects 12864 LCD screen with Chinese character library, LCD module is connected with PA and PB port of single chip microcomputer.

The charging voltage, the charging current, and the terminal voltage and temperature of the battery can be displayed in real time, and the calendar, the duty ratio of the four PWM waves, and the like can be displayed when idle.

1.2.5 Key input

Use a 4x4 matrix keyboard. By pressing the button, you can switch to battery charging method selection, charging parameter setting, calendar adjustment, duty cycle display of 4 PWM waves, charging voltage, charging current, battery terminal voltage and temperature display.

1.2.6 PWM output

The output frequency of PWM is determined by the high-frequency AC alternating current period set by a timer/counter. The PWM waveform of this system is left-aligned, and the duty ratio of each PWM is: [(PWMPERx-PWMDTYx)/PWMPERx]×100 %, where PWMPERx represents the PWM channel register and PWMDTYx represents the PWM channel duty cycle register.

2 software design

The system software of the multi-function charging system is written in C language. It is written into the internal program memory of the MCU through assembly and simulation debugging. The system software is structured hierarchically and functionally modularized. The software is readable, maintainable and expandable. Strong.

The multi-function charging system is designed for different types of batteries, and the corresponding charging method is mainly composed of initialization, pre-charging battery detection, charging phase and charging protection.

The system mainly uses lithium iron phosphate for testing. The charging phase consists of a small current charging phase, a constant current charging phase, and a constant voltage charging phase. The program flow chart is shown in Figure 3.

Charging phase: After the battery testing procedure is completed, the battery is charged with a small current, and the charging rate is about 1/5C. When the small current is charged until the battery voltage reaches the reference value, the system enters the constant current charging phase, which is the battery. In the fast charging phase, the charging rate is 1-2C; when the charging voltage reaches the set maximum charging voltage of the battery, the system enters the constant voltage charging phase, and as the battery voltage gradually rises, the charging current gradually decreases; when the charging current decreases d When N sets the reference value, the system determines that the battery is sufficient to stop charging.

Charging protection part: During the charging process, continuously monitor whether the battery voltage exceeds the safe value, whether the temperature or temperature change rate reaches the limit value, and terminate the charging immediately if any of the above conditions occur. The battery voltage is detected to prevent overcharging of the lithium-ion battery and the lead-acid battery, and whether the temperature and temperature change rate are detected to a limit value is to prevent over-charging of the nickel-hydrogen and nickel-cadmium batteries.

The above charging stage is designed for lithium ion batteries. In practice, the experiment is mainly carried out with a lithium iron phosphate battery pack. For other types of batteries, a corresponding charging method is set in the software: the lead battery charging stage is the same as the lithium ion battery, that is, the first Small current pre-charge, constant current charging, and finally constant voltage charging. When the constant voltage charging current is small to a certain extent, the system judges that the battery is sufficient and stops charging; nickel-cadmium battery, first small current pre-charge, then fast constant current charging, When it is detected that the battery voltage drops for the first time, the system judges that the battery is sufficient and stops charging; the nickel-hydrogen battery is pre-charged with a small current, and then fast-current constant charging. When the battery voltage shows zero growth, it is judged that the battery is sufficient and the charging is stopped.

Lead-acid batteries and lithium-ion batteries have a low self-discharge rate. When the battery is fully charged, the battery can be directly stopped. The self-discharge rate of nickel-metal hydride and nickel-cadmium is high. If the battery is unattended at night, the battery can be trickle-charged after the battery is sufficient. Replenish the charge to keep the battery fully charged.

3 Conclusion

The experimental results show that the designed multi-function charging system can work normally, the output DC voltage is stable, the ripple is small, the charging process control precision is high, the battery can be charged quickly and stably, and the charging is stopped in time after the battery is fully charged. , there is practical application and promotion value.