Design of energy harvesting charger based on EasyARM1138

With the development of computer technology and power electronics technology, mobile phones, digital products, laptops, portable instruments and other devices are becoming indispensable tools in people's lives. The design of the chargers corresponding to these products is also receiving more and more attention, and the quality of the charger will directly affect the performance and service life of the product. The number of charging devices in modern life is increasing rapidly, and the charging interfaces of various manufacturers are different. At the same time, there are many types of chargers on the market, and the more typical implementation method of the charger is to use a special function integrated circuit IC to control the range of charging current/voltage. This ordinary constant current or constant voltage charger has the defects of low charging efficiency, long charging time, and reduced battery life. This article focuses on the charging and discharging characteristics of lithium-ion rechargeable batteries and the needs in actual use. It uses the new embedded chip LM3S1138 as the main controller to perform intelligent control during the charging process of lithium-ion batteries, strictly control the physical parameters such as charging current, voltage, and temperature, so as to achieve the characteristics of digitalization, intelligence, and energy saving.

1. Hardware Design of Energy Harvesting Charger

The hardware design of the energy harvesting charger mainly includes the design and integration of DC power supply, power converter, EasyARM1138, PWM generator, sampling circuit, rechargeable battery, etc., forming a circulation system. Its circuit module is shown in Figure 1.

1.1 EasyARM1138 Embedded Microprocessor

The EasyARM1138 embedded microprocessor uses the LM3S1138 chip based on the Cortex-M3 core of the Stellaris series of Luminary Micro. The chip includes a low-dropout voltage regulator, integrated power-down reset and power-on reset functions, an emulated comparator, a 10-bit ADC, SSI, GPIO, watchdog and general timer, UART, I2C ^and motion control PWM and other rich peripheral functions, which can be directly connected to the GPIO pins without the need for feature multiplexing [1]. It is very suitable for use as a control unit for smart chargers.

The task of EasyARM1138 is to collect the battery charging status in real time from the sampling circuit, determine the charging current of the next stage by calculation, and generate a suitable PWM signal to control the charging current; transmit and display the sampling data in real time through UART and LCD, and generate an alarm signal when the collected battery parameters are abnormal.

1.2 Power conversion and control circuit

1.2.1 BUCK power conversion circuit

In the process of energy collection, the charger implements different charging strategies by controlling voltage or current. The design adopts the easy-to-control and high-efficiency BUCK converter. The BUCK converter is controlled by the PWM signal generated by EasyARM1138. By controlling the duty cycle of PWM, the output voltage or current of the switch tube Q2 is controlled. The BUCK conversion circuit is shown in Figure 2.

Vi and Vo are the input and output voltages respectively, and D1 is a freewheeling diode. The working principle of the BUCK converter: When the PWM output is high, the switch tube is turned on, and the current passes through the transistor and the inductor to the battery. In this stage, the inductor absorbs energy and the capacitor is charged. When the PWM output is low, the switch tube is turned off, the current flows through the diode D1, the voltage across the inductor is reversed, and the current is provided by the diode. The inductor and capacitor are used as filters to output voltage and current.

1.2.2 PWM Generator

The PWM generator is integrated in the EasyARM1138 system, and uses the 16-bit PWM function of the timer module to generate PWM signals. In PWM mode, TimerA or TimerB is configured as a 16-bit down counter, and automatically generates a PWM square wave signal by setting the appropriate load value (determines the PWM period) and match value (determines the PWM duty cycle), and outputs it from the corresponding CCP pin.

The basic idea of ​​this method is to use the PWM (CCP) port of EasyARM1138 to adjust the PWM duty cycle by software without changing the PWM square wave period. Before adjusting the charging current, the processor quickly reads the charging current, and then compares the set charging current with the actual charging current. If the actual current is too small, the PWM duty cycle is adjusted to increase the charging current; if the actual current is too large, the PWM duty cycle is adjusted to reduce the charging current. In the process of adjusting the software PWM, attention should be paid to the ripple interference introduced by the ADC reading deviation and the power supply operating voltage, and digital filtering techniques such as the arithmetic average method should be reasonably used.

1.3 Sampling Circuit

Sampling includes sampling of charging current and charging battery terminal voltage. The sampled voltage and current are sent to the LM3S1138 control chip through an integrated 10-bit ADC module in EasyARM1138, and LM3S1138 processes and saves the data. The ADC module supports 8 input channels, with a maximum output error of ±3 mV, a ±3.3 V power supply, and contains 4 programmable sequencers that can sample multiple analog input sources without controller intervention. The current and voltage sampling schematic is shown in Figure 3.

(1) Current and voltage sampling

In order to reduce costs, the design does not use an external sensor for current sampling. Instead, the current flowing through the battery is converted into voltage through a sensing resistor R6, and then the ADC is converted and sampled. The current flowing through the battery may be very large (more than 1 A). If the sensing resistor is large, a large voltage drop will be generated. According to the power calculation formula: P=I2R, if the power consumption is too large, more heat will be generated. Obviously, this is not advisable. In this design, R6=0.1 Ω, and the LM358 operational amplifier is used to amplify the voltage to about 3 V, and then transmitted to the ADC1 pin of the ADC converter [2]. Voltage sampling is directly achieved by changing the size of the sliding resistor R4 to make the output voltage within the rated range of 0 to 3 V, and then transmitted to the ADC0 pin of the ADC converter for data conversion.

(2) Protection circuit and reference voltage regulator

If the voltage entering the ADC pin is too large, it may cause damage to the chip. The correct approach is to use voltage limiting protection measures, and the typical usage is to use clamping protection diodes. In order to suppress the interference on the ADC input signal, RC low-pass filtering is generally required, as shown in Figure 4.

The ADC converter needs a reference voltage as a reference to complete the quantization of analog voltage signals to digital signals. The reference voltage directly affects the results of voltage and current sampling. EasyARM1138 has an internally integrated programmable 3.3 V reference voltage regulator to ensure the accuracy of the ADC reference voltage. No external voltage regulator is required, which can save design costs.

2 Software Programming

The charging current and voltage of the energy harvesting charger are limited, and all the data of the "battery characteristics" are calculated based on the scaling factor. These data are defined in the include file, calculated at compile time, and processed as constants when the program is running. All data output from the ADC can be directly compared with these constants. In other words, during the program running, no real-time calculation is required, thus saving calculation time and program space. Lithium-ion rechargeable batteries use a constant current-constant voltage charging method, and the charging main control program flow is shown in Figure 5 [3-6].

This electric energy collection charger uses the microcontroller LM3S1138 as the CPU, has intelligent and energy-saving charging performance, and the electric energy collection rate is very ideal. Therefore, it has good promotion and use value.