Intelligent voltage-stabilized power supply based on single-chip microcomputer

1 Introduction

DC regulated power supply is a common electronic instrument, widely used in electronic circuits, teaching experiments and scientific research. Most of the DC regulated power supplies currently used are linear power supplies, which are composed of discrete devices. They are large in size, low in efficiency, poor in reliability, inconvenient to operate and use, and have insufficient self-protection functions, resulting in a high failure rate. With the rapid development of electronic technology, various electronic and electrical equipment have increasingly higher performance requirements for regulated power supplies, and regulated power supplies are constantly developing in the direction of miniaturization, high efficiency, low cost, high reliability, low electromagnetic interference, modularization and intelligence. The new generation of intelligent regulated power supplies designed and manufactured with the single-chip microcomputer system as the core not only has a simple circuit, compact structure, low price and excellent performance, but also because the single-chip microcomputer has computing and control capabilities, it can be used to perform various calculations on the sampled data, thereby eliminating and reducing the errors caused by interference signals and analog circuits, greatly improving the output voltage and control current accuracy of the regulated power supply, and reducing the requirements for analog circuits. The intelligent regulated power supply can use the single-chip microcomputer to set up a thorough protection monitoring system to ensure reliable power supply operation. The output voltage and limited current are displayed digitally, and the input is via keyboard. The power supply has beautiful appearance, convenient operation and use, and has high use value.

2 Working Principle

This intelligent voltage-stabilized power supply is based on the switching power supply circuit, with a high-performance single-chip microcomputer as the control core, and a data processing circuit. Under the support of detection and control software, the output current and voltage of the switching power supply are sampled and compared with the given data, so as to adjust and control the working state of the switching power supply, and monitor the working temperature and output current of the switching circuit at the same time. The working principle block diagram is shown in Figure 1. The city power is rectified and filtered to become direct current and sent to the switch adjustment circuit. The switch adjustment circuit outputs stable direct current under the control of the single-chip microcomputer. Users can set the output voltage value and maximum output current value of the voltage-stabilized power supply through the keyboard as needed. The single-chip microcomputer system automatically samples the output voltage and current of the power supply and compares them with the user-given data. Then, the switch adjustment circuit is controlled according to the set adjustment algorithm to make the output voltage of the power supply meet the given value. While adjusting the output voltage of the power supply, the single-chip microcomputer also detects the working temperature and output current of the circuit. If it exceeds the given value, the protection circuit is activated.

Figure 1 Block diagram of intelligent voltage-stabilized power supply

3 Hardware Design

3.1 MCU system

The single-chip computer system of the intelligent voltage-stabilized power supply uses 8031 ​​as the CPU, including 8kRAM (chip 6264 data memory) and 16kROM (chip 27128 program memory), as well as 1kEEROM. EEROM is used to save the voltage, current data, temperature, pulse width adjustment data, etc. input from the keyboard for the last time. Each time the power is turned on, the single-chip computer reads the data from the EEROM to control the power output. In addition, an integrated circuit 8155 is expanded to supplement the I/O port of 8031, of which port A of 8155 is used as an output to provide an LED display data port, port B is used as a keyboard input port, and port C is used as an output to provide an excitation pulse signal for the switch adjustment circuit. See Figure 2 for the specific block diagram.

Figure 2 MCU system block diagram

3.2 Sensor input channel and A/D conversion

The current sensor is made of a section of constantan sheet connected in series in the power output circuit, and the voltage sensor uses a resistor voltage divider. The single-chip system detects current and voltage through current and voltage sensors, and measures two analog signals. They are first amplified by their respective amplifiers into signals that match the A/D converter, and then sent to the A/D converter through the multi-way selection switch CD4051. The single-chip CPU controls the selection of relevant channels for time-sharing switching to achieve a two-choice, and sends the two analog signals to the AD1674 converter in turn. After A/D conversion, they become digital signals and are then sent to the 8031 ​​single-chip computer through the photoelectric coupler.

The temperature sensor uses the solid-state temperature adjustment switch AD22105 launched by ADI, and its application circuit principle is shown in Figure 3. It only needs an external resistor to accurately set the temperature at any point in the wide operating temperature range of -40℃ to +150℃. When installing, the temperature sensor is in contact with the switch tube to detect the temperature rise of the switch tube during operation. When the temperature rise of the switch tube exceeds a given value (such as 80℃), the output of AD22105 will switch, and its conversion change is directly input into 8031.

Figure 3 Temperature sensor circuit [page]

3.3 Switching tube control signal generation circuit

In order to accurately control the voltage output of the switching circuit, this system uses pulse width modulation control to adjust the working state of the switch tube. 8155 divides the high-frequency pulse signal of the single-chip microcomputer into a suitable switching pulse signal as the counting pulse and gate signal of 8155. The single-chip microcomputer compares the given value with the signal collected by the sensor to generate an error signal. According to the voltage control algorithm, 8155 is set to generate square wave signals with different duty cycles (0-90%), and the switch adjustment circuit outputs the set voltage through the photoelectric coupler. The output end of 8155 is optically isolated from the switching circuit to prevent the interference signal from the switching power supply circuit from affecting the normal operation of the single-chip microcomputer system. In view of the high precision and fast adjustment characteristics of the output voltage of the controlled switching circuit, an improved PID control algorithm can be used, which makes the voltage adjustment fast, the overshoot small, and the performance stable.

3.4 Monitoring and protection system

In order to make the intelligent voltage-stabilized power supply work reliably and safely, this system is equipped with multiple monitoring and protection systems, mainly including overheat protection, overcurrent protection and short circuit protection. The overheat protection is controlled by interruption. The single-chip microcomputer system detects the operating temperature of the switch circuit and the output current of the power supply through the temperature sensor and the current sensor. If the temperature and current exceed the given value, the single-chip microcomputer system cuts off the switch circuit excitation signal and starts the sound and light alarm. The single-chip microcomputer uses voltage and current dual detection for short circuit protection. The short circuit protection is activated only when the voltage is very low and the current is very large.

3.5 Keyboard and Display Circuit

The keyboard and display part of the intelligent voltage-stabilized power supply are installed on the instrument operation panel, which consists of 8-digit LED digital tubes, 3 LED indicator lights and 16 keys, of which 4-digit digital tubes display the power supply voltage, 4-digit digital tubes display the current, and 3 lights are used as alarm displays. The keyboard and display circuit are connected to 8031 ​​through the 8155 interface circuit.

4 Software Design

The system software consists of a main program, two interrupt service programs and a subroutine, which controls the intelligent voltage-stabilized power supply to work in an orderly manner.

The main program flow chart is shown in Figure 4.

Figure 4 Main program flow chart

During the initialization process, first reset each port of 8031, then read the data stored before the last shutdown from EEROM, control the switch circuit, and display it. After the initialization is completed, open the interrupt. If there is an interrupt request, respond, otherwise perform data sampling and read the given value, and then perform data processing. If a short circuit or overcurrent occurs, call the alarm protection subroutine. If no short circuit or overcurrent occurs, reset the pulse width according to the voltage control algorithm to excite the switch circuit. The two interrupt service programs are the overheat detection protection alarm program and the keyboard setting program, and the subroutine is the protection alarm program.

5 Conclusion

The intelligent voltage-stabilized power supply designed in this paper can not only be used as a conventional scientific research experimental power supply, but also can generate a continuously changing output voltage through software programming, and can be used as a power supply for testing the performance (such as zero drift, dynamic distortion, etc.) of electronic products (such as audio amplifiers, etc.), thereby achieving multiple uses of one machine and improving the efficiency of instrument use.