Design of multifunctional charging system for various batteries based on MC9S12XS128

introduction

Due to the oil crisis and increasingly serious environmental pollution, the development of electric vehicles has become an inevitable trend. Batteries provide power for electric vehicles, and the charging performance of batteries directly affects the use and life of batteries. Batteries are generally divided into lead-acid batteries, nickel-cadmium batteries, nickel-metal hydride batteries and lithium-ion batteries. Due to the wide variety of batteries and different capacities, batteries of different types and capacities often require different chargers to match. If the battery chargers are not well matched, unsafe phenomena such as overcharging and overheating will occur, thereby affecting the normal use of the battery and shortening the battery life. Therefore, it is very necessary to design a multifunctional charging system based on single-chip microcomputer control that can charge various types of batteries. The multifunctional 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 the software, so that each battery can be charged under the best charging method. For batteries of different capacities, when choosing a good charging method, you only need to set the charging parameters to charge the battery quickly and stably.

1 Hardware Circuit Design

This system adopts a phase-shifted full-bridge soft switching circuit, that is, the Boost circuit and the full-bridge converter are combined to form a single-stage PFC circuit. The circuit has a simple structure and high efficiency. It can realize the adjustment of the input current and can work in higher power occasions, giving full play to the advantages of the full-bridge circuit.

The system is mainly composed of a charging main circuit and a charging control circuit. Figure 1 is a hardware schematic diagram of the multifunctional charging system.

1.1 System Working Principle

This design uses switching power supply technology with a maximum power of 3500W. First, the 220V single-phase industrial frequency AC power is rectified by a full-bridge circuit composed of 4 diodes, and then filtered by a large capacitor to obtain a DC power of about 300V. At this time, the ripple in the DC power is relatively large. The DC power passes through a full-bridge inverter composed of 4 insulated gate bipolar transistors (IGBTs) to obtain a high-frequency AC power with adjustable voltage, which is coupled to the secondary side through a high-frequency transformer, and then rectified by a full bridge. Finally, it is filtered by an inductor and capacitor to obtain a DC power with very small ripple to charge the battery. The multifunctional charging system can charge different types of batteries and batteries with different capacities. The charging voltage and current during the charging process are controlled in real time by a single-chip microcomputer. The entire charging system is a feedback control system. The single-chip microcomputer monitors the entire charging process by real-time detection of the current, voltage and temperature during the charging process, effectively avoiding overcurrent, overvoltage and overheating during the charging process, so that the charging process is safe and stable.

The air switch before the inverter bridge is to prevent short circuit or high current from damaging the battery or electronic devices. The microcontroller detects the charging current, voltage and temperature and compares them with the set values ​​before charging, and controls the output of 4 PWM waves to the gates of 4 IGBTs, thereby controlling the on-off time of the collector to emitter current to achieve the purpose of controlling the output voltage.

Since IGBT needs to be isolated and driven, this design uses Mitsubishi's IBGT dedicated driver chip M57962L. Figure 2 is its application circuit.

Since four IGBTs are selected to form a full-bridge inverter, each IGBT needs an M57962L chip to drive it, and each M57662L chip requires three voltage levels, namely 15V, l0V, and 5V to power it. The 5V voltage also powers the MC9S12XS128 microcontroller. This paper designs a 50W transformer to power the microcontroller and four M57962L chips. Its secondary winding outputs three groups of voltages. After rectification, filtering and voltage stabilization, the three required voltages are obtained.

1.2 Charging Control Circuit

Freescale MC9S12XS128 microcontroller is selected as the control core for data acquisition and control. It has an internal data memory of 8KB, a program memory of 128KB, 2 SCI, 1 SPI, 1 IIC, 1 CAN, 16-channel A/D, 8-channel PWM, and 8-channel ECT modules. Its operating frequency is 80MHz, and its operation speed is fast, and its processing capacity is greatly improved. The chip integrates 16-channel 12-bit high-precision A/D converters, 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 and off of the IGBT, simplifying the design of the microcontroller peripheral circuit.

1.2.1 Voltage Detection

This system uses a resistor voltage divider structure, which is connected in parallel in the charging circuit to monitor the voltage signal. The voltage signal is transmitted from the PAD0 port to the microcontroller through the microcontroller's built-in A/D converter for processing. This structure can automatically select the corresponding range detection voltage according to the actual voltage outside, so that the smaller the voltage, the higher the detected voltage accuracy, which helps to more accurately control the change of charging voltage during the charging process.

1.2.2 Current Detection

This system uses a Hall-type current sensor to detect the charging current signal, and after a certain conversion process, transmits the detected current signal from the PAD1 port to the microcontroller through the microcontroller's built-in A/D converter for processing. The sensor has high precision and can accurately detect changes in the charging current of 0.1A.

1.2.3 Temperature detection

This system uses a 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 in battery temperature during the charging process. The temperature signal is transmitted to the microcontroller through the PAD2 port 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 uses a 12864 LCD screen with a Chinese character library, and the LCD screen module is connected to the PA and PB ports of the microcontroller.

It can display the charging voltage, charging current, terminal voltage and temperature of the battery in real time during the charging process, and can display the calendar, duty cycle of 4-channel PWM waves, etc. when idle.

1.2.5 Key Input

A 4x4 matrix keyboard is used. By pressing the buttons, you can switch to the battery charging method selection, charging parameter setting, calendar adjustment, 4-way PWM wave duty cycle display, charging voltage, charging current, battery terminal voltage and temperature display and other interfaces.

1.2.6 PWM Output

The output frequency of PWM is determined by the high-frequency AC alternating cycle set by a timer/counter. The PWM waveform of this system uses the left-aligned method. The duty cycle of each PWM channel 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 multifunctional charging system is written in C language and written into the internal program memory of the microcontroller after assembly, simulation and debugging, so as to realize the hierarchical structure and modular functions of the system software, and the readability, maintainability and scalability of the software are strong.

The multifunctional charging system is designed with corresponding charging methods for different types of batteries. The software mainly consists of initialization, battery quality detection before charging, charging stage and charging protection.

This system mainly uses lithium iron phosphate for testing. Its charging stage consists of three parts: low current charging stage, constant current charging stage, and constant voltage charging stage. Its program flow chart is shown in Figure 3.

Charging stage: After the battery detection program is completed, the battery starts to be 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 stage, which is the fast charging stage of the battery, and 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 stage, and as the battery voltage gradually rises, the charging current gradually decreases; when the charging current decreases to the set reference value, the system determines that the battery is sufficient and stops charging.

Charging protection part: During the charging process, the battery voltage is constantly monitored to see if it exceeds the safety value, and the temperature or temperature change rate reaches the limit value. If the above situation occurs, charging will be terminated immediately. The battery voltage is detected to prevent lithium-ion batteries and lead-acid batteries from overcharging, and the temperature and temperature change rate are detected to see if they reach the limit value to prevent nickel-metal hydride and nickel-cadmium batteries from overcharging.

The above charging stages are designed for lithium-ion batteries. In practice, lithium iron phosphate battery packs are mainly used for experiments. For other types of batteries, corresponding charging methods are set in the software: the charging stage of lead-acid batteries is the same as that of lithium-ion batteries, that is, first pre-charge with a small current, then charge with a constant current, and finally charge with a constant voltage. When the constant-voltage charging current is small to a certain extent, the system determines that the battery is sufficient and stops charging; for nickel-cadmium batteries, first pre-charge with a small current, then charge with a fast constant current. When the battery voltage drops for the first time, the system determines that the battery is sufficient and stops charging; for nickel-metal hydride batteries, first pre-charge with a small current, then charge with a fast constant current. When the battery voltage shows zero growth, the battery is determined to be sufficient and stops charging.

Lead-acid batteries and lithium-ion batteries have low self-discharge rates, and charging can be stopped directly after the battery is fully charged. NiMH and NiCd batteries have high self-discharge rates. If they are unattended for charging at night, trickle charging can be used to replenish the battery after the battery is fully charged to keep the battery fully charged.

3 Conclusion

The experimental results show that the designed multifunctional charging system can work normally, the output DC voltage is stable and the ripple is small, the charging process control precision is high, it can charge various types of batteries quickly and stably, and stop charging in time after the battery is fully charged, which has practical application and promotion value.