Design of digital controlled current source based on single chip microcomputer

1 Introduction

Power supply technology, especially digital control power supply technology, is a very practical engineering technology, involving many disciplines such as electrical, electronic, system integration, control theory, and materials. The development of computer and communication technology has provided a broad development prospect for power electronics technology, and at the same time has put forward higher requirements for power supply. Ordinary power supplies can no longer meet the actual needs due to their shortcomings such as low accuracy. It was not until the emergence of single-chip microcomputer technology and voltage conversion modules that the development of precise digital control power supplies became possible. The digital control current source designed in this paper uses PIC16F877A single-chip microcomputer as the core component, and keyboard, display, D/A, switching power supply and other modules as peripheral circuits.

2 Design requirements and overall design ideas

2.1 Design requirements

This design requires: input 220V, output up to 12V; output current is controlled by keyboard, with a step size of 0.01A; LED is used to display output current, with an accuracy of 0.02A; the current source steady current range is (0.2-1)A.

2.2 Overall design ideas

This design uses a switching power supply to meet the requirements of output range, accuracy and ripple. According to the system requirements, the D/A conversion is followed by a power amplifier connected to an operational amplifier to control the D/A input, thereby controlling the current value. This system is mainly composed of a numerical control part, a power supply part, and a keyboard display circuit. The system principle block diagram is shown in Figure 1.

Design of digital current source based on single chip microcomputer

3 Hardware circuit design and software selection

According to the design requirements of digital current source, the system is mainly composed of control module, power module, D/A module and keyboard display module.

3.1 Selection of control module

This design uses PIC16F877A single-chip microcomputer control. Compared with AT89C51 single-chip microcomputer, PIC16F877A adopts Harvard structure, can realize single-finger instruction, has reduced instruction set technology, simple addressing mode, strong I/O port driving ability, I2C and SPI serial bus ports, simple peripheral circuit, not only easy to develop, but also can save users' circuit board space and manufacturing costs. The program has strong confidentiality, low power consumption, wide voltage design, can combine a considerable number of peripheral devices together, easy to use, and improved anti-interference performance.

3.4 Software Selection

Protel 99 SE software design system is an EAD circuit integrated design software platform built on IBM PC compatible environment. It has functions such as circuit schematic design, PCB (printed circuit board) design, circuit hierarchical design, report making, circuit simulation and logic device design.

MICROCHIP has equipped the PIC series of microcontrollers with a powerful software integrated development system, Mp lab, which is a full-featured software package that integrates a variety of microcontroller application development tool software.

This paper uses Protel 99 SE software design system to design the PCB board and Mp lab to perform system simulation

4 MCU Program Implementation of Digital Control Current Source

The PIC16F877A microcontroller used in this article is a new product developed by MICROCHIP. It has the function of FLASH programming . It can directly perform operations such as pausing CPU execution and observing register contents on the microcontroller. It is currently the most widely used PIC microcontroller.

The functions to be realized by the microcontroller program are: the independent keyboard inputs data to the PIC16F877A microcontroller, the PIC16F877A microcontroller processes the obtained data and sends it to the 10-bit digital-to-analog converter TLC5615 to realize the control of current.

C language programming is used here, which has the advantages of high code writing efficiency, intuitive software debugging, convenient maintenance and upgrading, high code reuse rate, and convenient cross-platform code transplantation. The main program flow chart is shown in Figure 2.

5 System Testing

This design requires the output current range to be 0.2A-1A, and the voltage across the sampling resistor of the constant current source module is 200mV-2000mV. From the voltage value, it can be inferred that the reference voltage |Vref| of the digital-to-analog conversion module is at least 2V (Vref<0). The Vref of this design is -2.15V, and the analog voltage range of the output end is (0-12)V, so the output current is 0.2A-1A. The design requires any preset value within 0.2A-1A. This design inputs the current value through the keyboard and sends it to the microcontroller. The microcontroller converts the analog quantity into a digital quantity and sends it to the digital-to-analog conversion circuit according to the input key value, and then outputs the analog quantity. Some current values ​​and their corresponding theoretical and actual digital quantities are listed in Table 1. The corresponding theoretical code value is the code value required for the digital-to-analog conversion corresponding to the input current, and the actual code value is the code value sent to the digital-to-analog converter by the microcontroller after processing the input current value.

The test results listed in Table 1 show that the maximum error of the output of this design is 33mA when the input current is 32mA, and the error is 1mA. The exercise part of the question requires that the absolute value of the output current change is ≤ 0.1% of the output current + 1mA, that is, 1.032, so the error value measured by this design reaches the error value specified in the design requirements.

6 Conclusion

The digital current source designed in this paper uses PID algorithm to achieve the functions of selectable range, adjustable output, precise stepping, and extremely small ripple current, and the preset value and measured value of the output current can be displayed on the LED at the same time. The human-machine interface uses an independent keyboard and LED display, and the control interface is intuitive and concise, with good human-machine interaction performance. It has the advantages of flexible control, convenient system upgrade, improved reliability of the control system, easy standardization, convenient system maintenance, good consistency, low cost, and convenient production and manufacturing.