Design of multifunctional charger system based on MCP1631HV

1 Overview
As portable rechargeable applications continue to grow, the demand for unique or customized battery charger designs is also increasing. In addition to the growth of portable rechargeable applications, battery chemistry is also constantly improving, and many new charging algorithms have emerged.
This article uses Microchip's high-speed PWM series device MCP1631HV to design a smart charger that has multiple charging algorithms coexisting, users can flexibly configure, and can meet the needs of rechargeable batteries with different characteristics. The entire hardware volume is 7 cm×6.7 cm×3 cm, which can meet the current social needs for small size, flexible configuration and high charging efficiency of smart chargers. The design of the entire smart charger system includes three parts: the principle of smart charger charging algorithm, the design of smart charger hardware system, and the design of charging algorithm system software.

2 Principles of multiple charging algorithms of smart chargers
Aiming at the common types of rechargeable batteries on the market, the charging curves of NiMH and NiCd batteries and lithium-ion batteries are specifically analyzed. Figure 1 is a characteristic diagram of the charging curve of NiMH and NiCd batteries.

As can be seen from Figure 1, the working process of the entire NiMH and NiCd charging curve is: once the MCU detects that there is a rechargeable battery, a controlled small current or a conditioned current will flow into the battery pack to start charging. If the voltage of each battery being charged is above 0.9 V, the battery pack will start fast charging or high current charging. For NiMH or NiCd batteries, the range of rechargeable batteries can reach (or even exceed) 50% to 100% of the battery capacity. When the battery reaches its capacity, the charging cycle is completed by gradually stopping the charging. When the battery charging
is completed, it is necessary to stop charging the battery pack. Generally, two methods are used to determine whether to stop charging:
① Based on the sudden increase in the battery pack temperature;
② Based on the slight drop in the battery pack voltage -dV/dt.
For NiMH and NiCd batteries, the slight drop in the battery pack voltage is not easy to detect because the rate of change is very small, but the -dT/dt rate of change is large and easy to detect. Therefore, in the following design, the first method is used to stop charging detection for the NiMH battery pack.
The characteristics of the lithium-ion battery charging curve are shown in Figure 2. Before charging lithium-ion batteries, the battery must be checked, and the voltage of each battery should be greater than 3 V before starting fast or high current charging. If it is less than 3 V, a low-value conditioning current is used to start the charging cycle. Once the MCU detects that the battery voltage is greater than the 3 V threshold, it will start fast or high current charging. As the battery voltage rises, the voltage reaches the maximum value before the battery is fully charged. The constant voltage of most lithium-ion batteries is 4.2 V. After reaching this voltage value, the battery charger becomes a constant voltage source (regulating current but not voltage). When in constant voltage mode, the charging cycle continues as the charging current decreases; when the charging current drops to about 7% of the fast charging current, charging stops. If the voltage drops below 4.0 V after charging is completed, a new charging cycle can be started.
There are generally two solutions to the power consumption problem of battery chargers: linear and switching mode. In order to improve the charging efficiency of smart chargers, this article adopts the design method of switching chargers, thereby increasing the charging efficiency of chargers to 85%.
There are many switching regulator power topologies: buck, boost, SEPIC, and flyback. Since the SEPIC power topology is superior to other topologies, this paper adopts the SEPIC power topology. The specific SEPIC power topology is shown in Figure 3.

3 Smart Charger Hardware System Design
The smart charger uses MCU to control the high-speed analog PWM device MCP1631HV to realize the function of the whole system. The programmability of MCU is used. Different charging algorithms are generated through software programming design. MCP1631HV is applied to the constant current SEPIC topology structure, and it provides a new high-speed analog PWM. Since pulse width modulation is realized, it has the advantages of high simulation speed and resolution when controlled by MCP1631HV.
The hardware design of the system mainly includes the following three parts: MCU core control and processing module, smart charger SEPIC module, and system configuration keyboard input and status display module.
3.1 MCU core control and processing module
The design of MCU core control and processing module mainly includes the peripheral minimum system design of Microchip's PIC16F883 microcontroller, the peripheral circuit of MCU and the program download and debugging interface design. The 10-bit ADC inside the MCU is used to collect the temperature of the battery pack during charging, and RA0 of PORTA port is allocated as the temperature input terminal, and RA4 is used as a common I/O port to control the SHDN enable terminal of MCP1631HV. The three input ports RA5 to RA7 are used as the input of the system configuration keyboard, where RA5 is used as the switch to start and stop charging, RA6 is used to select the type of rechargeable battery, and RA7 is used to select the number of batteries in the charger. The lower 4 bits RB0 to RB5 of the PORTB port are used as indicator lights when the system is working, and RB6 and RB7 are the program download and debugging interfaces of the MCU. The peripheral circuit of the MCU and its debugging interface circuit are shown in Figure 4.

3.2 Smart charger SEPIC module
The hardware circuit of the SEPIC power topology structure of the smart charger is shown in Figure 5.

The SEPIC power topology structure designed in Figure 5 is designed according to the principles discussed in Section 2. It mainly uses capacitor isolation, and there is no direct DC path between the input and the input, which reduces the use of power components and makes the charger safer; the SEPIC converter has an inductor L74487010 at the input, which can smooth the input current, reduce the necessary filtering, and reduce the source noise; the built-in low-side single switch of IRF7807VTRPBF reduces the complexity of MOSFET drive and current limiting protection; for applications where the input voltage may be higher or lower than the battery voltage, SEPIC can boost or buck the input voltage.
3.3 System Configuration Keyboard Input and Status Display Module
The realization of the multifunctionality of the intelligent charger requires user input configuration through the system configuration keyboard before the corresponding charging algorithm can be completed. The system configuration keyboard and status display circuit are shown in Figure 6.

The user selects the corresponding charging algorithm according to his needs, and makes corresponding settings through keyboard input and LED status indication. The keyboard part adopts an independent key design method.

4 Intelligent charger charging algorithm system software design
For a variety of charging algorithms, two charging algorithms are implemented through software programming inside the MCU: NiMH NiCd battery charging algorithm and lithium-ion battery charging algorithm. The overall software design process is shown in Figure 7. When the MCU is powered on and starts working, the system's I/O ports and the internal peripheral resources used are set first, and then the super cycle is detected, and the battery voltage, battery temperature, and whether the system button is pressed are continuously detected, and the corresponding events are processed according to the corresponding trigger conditions.

5 Conclusion
This paper designs an intelligent multifunctional charger based on the high-speed analog PWM device MCP1631HV, which can realize different charging algorithms and meet the market demand for multifunctionality, small size and high charging efficiency. It has certain advantages and high application value.