1 Introduction
The battery is the key power output unit of electric vehicles. In the common batteries such as lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, lithium batteries and fuel cells, it has low energy ratio, light weight, good temperature characteristics and low pollution. The characteristics of memory are not obvious, and lead-acid batteries and nickel-hydrogen batteries are widely used in electric vehicles. However, the charging method is incorrect. The life of the rechargeable battery is much lower than the specified life. That is to say, many batteries are not used up, but are burnt. It can be seen that the quality of the charger has a great impact on battery life. Based on. This paper proposes a smart charger design using C805lF040 microcontroller intelligent charging control scheme, which can effectively improve the charging efficiency and extend the service life of the battery.
2 hardware design
2.1 System Block Diagram
Figure 1 Electric vehicle smart charger system block diagram
The electric vehicle intelligent charger adopts c8051040F single chip as the control core, mainly including AC/DC converter, IGBT power module, high frequency transformer, rectifier filter circuit, single chip control circuit, pulse width adjustment circuit and status display circuit. Figure 1 is a block diagram of its system.
In this scheme, the maximum output power of the switching power supply is 2.6KW, and the AC input range is l70V-270V. The charger circuit mainly includes two parts: the main charging circuit and the single-chip control circuit. The whole circuit works as follows: 220v single-phase AC power passes through the full bridge. The rectification is filtered by a capacitor. A direct current of about 300v is obtained. After passing through an inverter bridge composed of four IGBTs, high-frequency alternating current is obtained, which is coupled to the secondary side via a high-frequency transformer. Then through the rectifier D6, D7 rectification. Finally, a stable DC output is obtained after filtering through the inductor L3 and the capacitor C7. Due to the use of smart charging, the charging voltage and charging current are different for each stage of the battery. Therefore, the cygnal C8051040F MCU is used as the charging process control device. When charging, the MCU detects the charging current, charging voltage and battery temperature of the rechargeable battery to prevent overvoltage and overcurrent of the circuit. If the battery temperature is too high, it is also possible to determine whether it is switching to the next charging phase by detecting the battery voltage and current values. At the same time, the voltage value or current value of each stage of charging is given by the single chip microcomputer, and compared with the corresponding voltage and current value obtained by sampling. The on-time of the power tube is changed by changing the PWM value by the phase shift control chip uCC3895. Achieve different stable output values ​​at different stages of different batteries.
2.2 MCU control power introduction
The charging control circuit uses C8051F040 microcontroller for data acquisition and control. The chip is a fully integrated mixed-signal system-on-chip (soc). It has a CIP-51 core that is fully compatible with the 805l instruction set. It integrates almost all analog and digital peripherals and other functional components needed to form a single-chip data acquisition or control system in a single chip. These peripherals or features include: ADC, programmable gain amplifier, DAC, temperature sensor, 12C bus, UART, SPI, timer, programmable counter, timer array, and more. The C805lF040 MCU adopts a pipeline structure. The machine cycle is reduced from the standard 12 system clock cycles to one system clock cycle. The processing capacity is greatly improved and the peak performance can reach 25 MIPS. Built-in 64K bytes of Flash program memory and 256B of internal RAM and 4KB of XRAM located in the external data memory space. The C805lF040 has an on-chip JTAG debug circuit. Non-intrusive, full-speed in-system debugging is possible through the 4-pin JTAG interface and using devices installed in the final application. Because it has up to eight 12-bit ADc and eight 8-bit ADCs, it can sample single-ended input voltages and currents from port PORTC. 6-channel PWM, on-chip programmable watchdog timer. It can greatly simplify the peripheral design of the MCU control circuit and ensure the safe operation of the program. The ADC is responsible for the voltage during charging, the current J2C is responsible for the temperature data acquisition, the reference value of the voltage and current when the PWM output is charged to the comparison circuit, and the single-chip microcomputer controls the switching power supply control module UCC3895.
Voltage detection circuit: The voltage sampling circuit is composed of a precision resistor and an adjustable resistor. The maximum setting range of the AD measurement of the single chip microcomputer is 5V. Therefore, the battery pack voltage should be scaled down to within 5V. Then use the internal AD conversion function of C805lF040 to convert. The one-chip computer calculates the battery voltage internally, and the circuit adopts the built-in l2 bit AD conversion inside the single chip microcomputer. Reduce the complexity of the design circuit. And improve reliability and accuracy. In order to resist electrical interference and high voltage electric shock. The circuit is isolated by a high-speed isolation diaphragm PC8l7.
Current detection circuit: Generally, when the current is collected, a sampling resistor with a small resistance value is connected in series in the circuit. The voltage on the sampling resistor is input into the conversion channel of the single chip microcomputer to perform A towel conversion. Then calculate the voltage value into a current value. However, due to the large charging current in this scheme. Sampling with resistors consumes more power, so. This solution uses current transformers for current sampling.
Temperature detection circuit: Temperature sampling uses temperature sensor LM92. The LM92 is a monolithic high precision digital temperature sensor from National Semiconductor Corporation. At room temperature, the temperature measurement accuracy can reach plus or minus 0.33 degrees. It can be compared with the temperature points set by the user. The internal register of the sensor can be read and written by the 12C bus interface. Its programming is easy. It is easy to use and widely used in high-precision temperature measurement and temperature control.
In the pre-treatment stage before charging begins. Depending on the battery, the software selects the appropriate charging algorithm. Write the channel selection control word to the mode register PCAOCPMn of the C805lF040 microcontroller. And initialize the counter, the timer register PCA0 and the module capture/compare register PCAOCPn. The frequency of the PWM output signal depends on the time base of the PCA0 counter/timer. Changing the value of the module capture/compare register PCA0CPn changes the duty cycle of the PWM output pulse.
After charging starts. The software periodically collects the voltage value on the voltage dividing resistor of the sampling battery, at the same time. The current transformer circuit detects the charging current in real time. Has been calculated. Set the output parameters of the PCAOCPn microcontroller PwM. Achieve optimal intelligent charging control.
2.3 state liquid crystal display module circuit
LCD1286A dot matrix liquid crystal display is selected as the status display. The liquid crystal display module circuit can be directly connected to P5 and P3 of the I/O port of the single chip C8051F040, P5 is used as the data port (D0.D7); P3.0, P3.1, P3.2, P3.3, P3.4 and P3.5 connects the 6 signal lines LCDD/l, LCDR / W.LCDE, LCDCSl.LCDCS2 and LCDRST of the LCD module to control the reading and writing of the LCD. There are status displays at each stage of charging, such as: the battery is in the charging state, the battery enters the temperature control state due to the temperature is too high, and the battery is fully charged and the charging state is completed.
3 software design
The software is mainly composed of system initialization, pre-processing, selecting pulse fast charging module and algorithm or constant current, constant voltage, floating charging module and algorithm according to different battery types and states. The process is shown in Figure 2.
Figure 2 main block diagram
3.1 initialization
In the initial stage of the program, the C805lFU40 microcontroller should be initialized first. Set the input/output status of the I/O port by setting the I/O port code crossbar. Determine chip pin functions, set interrupts, TIM timer parameters, and more.
3.2 pretreatment
The pre-processing stage is the preparation before entering the fast charging.
After the program is initialized, first use the internal temperature sensor of the C805lF040 microcontroller to detect the ambient temperature. When the ambient temperature is too low or too high. Neither can charge the battery, otherwise it will damage the battery.
Then, set the A/D conversion parameters and channels to detect the terminal voltage of the battery. Compare the test data with the theoretical experience value to determine the type of battery and whether it is connected correctly. A battery with a low voltage at the opposite end is charged with a short-time pulsating current, which is beneficial to activate the chemical reaction substance in the battery. Partially restored damaged battery unit. The battery with the opposite terminal voltage within the nominal range selects the corresponding charging control module and algorithm, and the opposite terminal voltage is not within the nominal range of the battery. The software automatically removes it.
3.3 fast charging
According to the predetermined charging control module and algorithm, the C805lF040 microcontroller PWM control register PCAOCN, the mode register PCAOMD and the 16-bit capture, compare register PCAOCPn. open the interrupt enable bit. Start charging quickly.
During fast charging, the C8051F04J0 MCU must continuously detect the following key technical indicators: whether the circuit is open circuited, whether the battery is unbalanced, whether the battery reaches the specified safe voltage, whether the battery is overheated, and whether the battery meets -△v or △ T / Δt condition.
The disconnection of the battery is mainly determined by detecting the magnitude of the current on the sampling resistor. And in order to avoid misjudgment, it should be tested repeatedly. When there is an open circuit, you should return to the pre-processing stage. The timing of the disconnection should be made when the battery terminal voltage has reached a predetermined value, otherwise the charging current is relatively small when the battery terminal voltage does not reach the predetermined value. Misjudgment may occur.
The terminal voltage detection of the battery uses the on-chip 12-bit high-precision A/D module of the C8051F040 microcontroller. Use interrupt control. This can save the waiting time of the C805lF040 microcontroller during the conversion period. The data of the terminal voltage detection is calculated by the charging algorithm to calculate whether the voltage negative growth of the battery - ΔV satisfies the fast charging termination condition, and the output parameter of the C805lF040 single-chip PwM is modified to control the charging current.
The temperature detection of the battery is performed after the terminal voltage detection. The C805lF MCU accesses the corresponding digital temperature sensor LM92 by setting different address codes to read the temperature data. Through the charging algorithm, calculate whether the temperature change rate ΔT/△t of the battery satisfies the fast charge termination condition, and modify the output parameter of the C805lF040 single-chip PWM to control the charging current.
In order to prevent the battery from being damaged, the battery should be stopped immediately when the battery voltage reaches the highest terminal voltage Vmax or the highest temperature Tmax, otherwise the battery will be damaged.
4 Conclusion
The experimental results show that the smart fast charger with C805lF040 microcontroller as the control core can work normally. Because C805lF040 has good performance and price ratio, its unique analog circuit module, high-precision A/D conversion, 12C bus interface and high-speed PwM are applied to the charging control. The on-chip and off-chip functions of the C8051FD40 are effectively used. Increase the intelligence and practicality of the product. It saves the development time and cost of the product, reduces the production cost, and also improves the consistency and reliability of the product, and has a good promotion value.
The author of this article is innovative: This design takes the SoC microcontroller C805lF040 as the main body, and builds the hardware design platform and software design method of the electric vehicle battery charging system. The μC/OS II real-time operating system is embedded in the C805lF040, which can greatly improve the stability and real-time response of the system, and enhance the reliability, scalability and portability of the system.
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