Design of digital controlled DC regulated power supply based on ATmega16

Since the late 1990s, with the demand for higher system efficiency and lower power consumption, the technological update of telecommunications and data communication equipment has promoted the development of DC/DC power converters in the power supply industry towards higher flexibility and intelligence. The DC regulated power supply designed in this paper is mainly composed of a single-chip microcomputer system, a keyboard, a digital tube display, an indicator light and an alarm circuit, a detection circuit, a D/A conversion circuit, a DC voltage stabilization circuit, etc. Among them, the digital control power supply uses a keyboard to set the output voltage and the alarm threshold in two ways, fast and slow. The output is output by the single-chip microcomputer through the D/A control drive module to output a stable voltage. At the same time, the voltage stabilization method is controlled by a single-chip microcomputer. The single-chip microcomputer samples the output voltage through A/D and compares it with the set value. If there is a deviation, the output is adjusted. If it exceeds the limit, an alarm signal is output and the current is cut off. During the working process, the working state of the regulated power supply (various working states such as output voltage and current) is driven by the single-chip microcomputer output to display on the LCD, and the keyboard is controlled for dynamic logic switching. With the single-chip microcomputer as the core, an intelligent high-precision simple DC power supply is designed. The power supply adopts digital adjustment and has high output accuracy. It is particularly suitable for various occasions with high precision requirements. It has the following obvious advantages: (1) higher intelligence and more perfect performance; (2) flexible control and convenient system upgrade; (3) improved reliability of the control system and easy standardization.

1 Basic principles of DC regulated power supply

The DC power supply circuit is generally composed of a power transformer, a rectifier filter circuit and a voltage stabilization circuit. As shown in Figure 1. The voltage stabilization circuit often uses a three-terminal voltage regulator. The application circuit is shown in Figure 2. As long as the positive input voltage U1 is added to the input terminal of the LM7805 and the common terminal of the LM7805 is grounded, its output terminal can output the chip's nominal positive voltage U2. In practical applications, in addition to connecting large-capacity filter capacitors between the input and output terminals and the ground, it is usually necessary to connect a small-capacity capacitor to the ground at the root of the chip lead pin. C1 is used to suppress self-excited oscillation, and C2 is used to narrow the high-frequency bandwidth of the chip and reduce high-frequency noise. As shown in Figure 2. 2 Implementation scheme of CNC constant voltage source

The traditional DC regulated power supply is adjusted by the coarse adjustment band switch and the fine adjustment potentiometer, and the voltage value is indicated by the potentiometer. This kind of regulated power supply has the disadvantages of non-intuitive reading, easy wear of the potentiometer, low precision, difficult to adjust, complex potential structure, and large size. The digital control DC power supply based on the single chip control not only realizes the function of DC voltage regulation, but also does not have the above disadvantages.

2.1 Design requirements

Output voltage range: 0.0 V ~ 9.9 V;
output voltage adjustment method: step, step value is 0.1 V;
display method: LCD1602 liquid crystal display;
monitor the output voltage value of D/A.

2.2 Digital control power supply scheme

Figure 3 shows the design block diagram of the digital control power supply, and its output voltage value is controlled by the keyboard. The voltage value to be output is input into the single chip microcomputer in a stepping manner through the keyboard. Here, the voltage is output by the PWM analog voltage of the single chip microcomputer. The display circuit can be used to display the output voltage value and the adjustment process of the keyboard circuit. If the output voltage requirements are not met, a voltage amplifier will need to be added. After the LM324 linear conversion, the required voltage value is obtained. In addition, the actual output voltage value of the monitoring voltage is sampled, and the sampled value is sent back to the single-chip microcomputer through the A/D sampling port of the single-chip microcomputer for processing and display. In this digital control power supply, the AVR chip is used to complete the system control key input judgment, voltage value display and various digital controls of external chips. 3 Digital control part ATmega16 is a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISC structure; the data throughput rate is as high as 1 MIPS/MHz, which can alleviate the contradiction between the system power consumption and processing speed; it has 4 channels of PWM and 8 10-bit ADC. The D/A of this system selects the commonly used DAC0832. When it is connected to the single-chip microcomputer, the circuit and program are simple. It only needs to directly connect the data line of the single-chip microcomputer to the input end of DAC0832. The connection and periphery of each pin are shown in Figure 4. 3.2 Voltage amplifier circuit The operational amplifier usually works in a closed-loop state. By connecting the amplifier circuit of the operational amplifier to a certain feedback circuit and external components, various mathematical operations can be realized. There are various forms of operational amplifier feedback circuits. Different feedback circuits and different input methods can form operational amplifier circuits for various purposes. Figure 6 is a proportional operational circuit with the input signal added to the inverting input terminal. R1 is the input terminal resistor, and Rf is the feedback resistor. It feeds back the output voltage to the inverting input terminal in the form of parallel negative feedback. In order to output Uo=0 when the input signal Ui=0, the resistor selection should satisfy R2=R1//Rf. This ensures that the external resistance of the inverting input terminal and the non-inverting input terminal of the operational amplifier is equal, so that it is in a symmetrical and balanced state, so as to eliminate the influence of the bias current of the operational amplifier on the output voltage. Therefore, R2 is called a balancing resistor. From the two important conclusions of the ideal operational amplifier, it can be seen that Ii≈0 and U+≈U-. The current I1 passing through R1, that is: If=I1, and because the input terminal of the operational amplifier is grounded, U+=0, so U+≈U-, that is, when the in-phase terminal is grounded, U+=0, the potential of the inverting input terminal U-≈0, it is an ungrounded "ground", called "virtual ground". The existence of "virtual ground" is an important feature of the operation circuit in the closed-loop working state. 4 Software Design The control program is written in C language, compiled under the ICCAVR platform, and downloaded to the chip using the Ssangyong download software. When the key is pressed, the voltage can be adjusted, the maximum adjustable voltage is 1 V, and the step is 0.1 V. In the process of key addition and subtraction, the voltage displayed by the LCD module changes up and down. When the key is not activated, the PWM analog output voltage of the single-chip microcomputer is output through the secondary filter circuit, and linearly amplified to obtain the same voltage as the displayed voltage value. 4.1 Program Design Flowchart The design flowchart is divided into three parts, namely the main program flowchart, keyboard scanning flowchart and keyboard control flowchart. The main program flow chart is shown in Figure 8. 4.2 Debugging After preparation, the transformers are powered on and tested to see if they meet the design requirements. The items to be checked include the output voltage range, the step adjustment value within the entire output voltage range, whether the output voltage matches the preset voltage, and the accuracy of the digital voltmeter function. The power supply of the CNC power supply system is provided by a DC regulated power supply, which is provided by the ±15 V power supply and 5 V power supply of the hardware circuit. The voltage test results are shown in Table 1. The above are the voltage test results. Since the resolution of PWM is 0.2, its error range can be limited to about 0~0.2 V, and errors within this range are allowed. Therefore, the monitoring voltage is basically consistent with the output voltage. Because the PWM output is 8 bits, the resolution = PWM duty cycle/250, then when the duty cycle value changes by 1, its voltage changes by 0.02 V, and then the op amp amplifies the voltage by 0.04 V. Therefore, the voltage change accuracy can be achieved to 0.04 V. This system uses the high-performance AVR microcontroller ATmega16 chip and the 8-bit precision D/A converter DAC0832 as the core components, and utilizes the 5 V voltage characteristics of the common and output terminals of the commonly used three-terminal voltage regulator LM7805 to finally realize the digital display of output voltage and current values, achieving the expected goal.