Power supply design based on digital trigger

The development of power supply technology has integrated technologies from many fields such as electronics, power integration, automatic control, and computers [1-3]. The early linear power supply had a narrow adjustable output range and its application range was greatly limited. Since the 1980s, switching power supplies have been widely used in the fields of postal and telecommunications, military equipment, and transportation facilities due to their advantages of small size, light weight, and high efficiency [4]. This paper introduces a single-chip microcomputer as the core, a digital trigger is designed to trigger the thyristor, and the power supply voltage is changed by turning the thyristor on and off [5], and finally a DC power supply with continuously adjustable output voltage is realized. This power supply makes full use of the powerful functions of the single-chip microcomputer, conforms to the current trend of foreign DC power supply towards digitalization, meets the requirements of wide range and adjustable output voltage, and has a lot of room for expansion in software and hardware. With slight changes on the basis of this power supply, it can be used in many fields.

1 Hardware design of adjustable DC power supply

The hardware principle block diagram of the adjustable DC power supply is shown in Figure 1. The hardware part of the system includes a single-chip microcomputer module, a key-set voltage module, a three-phase half-controlled bridge, a digital trigger module, a data acquisition module, an alarm module and a display module.

1.1 Single-chip microcomputer module
From the perspective of economy and practicality, the system uses the AT89S51 single-chip microcomputer, which is the control core of the entire system. It processes the collected data and sends control signals and indication signals to other modules according to different needs. AT89S51 is used to implement the design of digital triggers to realize the alternating triggering of the three thyristors in the three-phase half-controlled bridge.
1.2 Three-phase half-controlled bridge
Since the system transformer adopts the Δ/Y connection method, this design adopts a three-phase half-controlled bridge composed of thyristors and diodes. In order to ensure the reliable operation of the circuit, sufficient and reasonable margin is left for the selection of thyristors. The protection of the circuit under overvoltage and overcurrent conditions is considered in the circuit. Since the voltage across the capacitor cannot change suddenly, RC components are added at both ends of the thyristor for overvoltage protection. Overcurrent protection is achieved by connecting fast fuses in three phases. The three-phase half-controlled bridge circuit is shown in Figure 2.

1.3 Digital trigger module
The digital trigger module plays a very important role in whether the three-phase half-controlled bridge can work normally and orderly. In order to separate the strong current and weak current in this system, a photocoupler is used for isolation. The isolation circuit is shown in Figure 3. The control signal output by the microcontroller is used to realize the orderly triggering of the three thyristors, as shown in Figure 4.

1.4 Data acquisition module
The data acquisition module sends the collected voltage signal to the single-chip microcomputer and compares it with the set voltage of the single-chip microcomputer to form a closed loop as shown in Figure 5. If the output voltage does not match the set voltage, the output voltage can be adjusted by software.
1.5 Alarm module
The alarm module is divided into sound alarm and light alarm. When the circuit is working normally, there is no sound prompt and the system green light is on. If the circuit does not work normally, the red light flashes and a "di di" warning sound is emitted.
1.6 Display module This system uses a liquid crystal display, including set voltage value and current current value. In addition, according to different system requirements, the corresponding prompt information can be displayed on this display module, which is a great improvement
over the previous LED display. 1.7 Keyboard setting voltage module The output voltage of this system can be set through the key module. A conventional 44 keyboard can be used. 2 Software design of adjustable DC power supply When the power supply starts working, a voltage value is first given manually, and this voltage value can be displayed on the display module. The voltage is sent to the AT89S51 microcontroller after analog-to-digital conversion . Through the voltage value, the microcontroller can read the trigger angle that meets the requirements. The output trigger module sends a trigger pulse to trigger the three-phase thyristor. The data acquisition module samples the output voltage and sends it back to the microcontroller after analog-to-digital conversion to compare with the given value. If the given voltage is different from the output voltage, fuzzy PID is used for parameter adjustment to stabilize the output voltage at the given voltage value. When the power supply is working normally, if the given voltage changes, the output voltage also changes with the given voltage, realizing a follow-up system.

This system uses a total of 4 interrupts, namely timer 0 interrupt, timer 1 interrupt, external interrupt 0, and external interrupt 1. The synchronous zero-crossing point is obtained by external interrupt 1, and the trigger angle is determined by the given voltage value sampled by external interrupt 0. Timer 1 and timer 0 trigger the three-phase thyristors according to the calculated trigger time in each cycle. This system also uses fuzzy self-tuning PID parameter subroutine, data conversion subroutine, digital filter subroutine, and delay program. The PID parameters are adjusted in the fuzzy self-tuning PID parameter subroutine, and the current sampling value that best matches the actual situation is obtained through an algorithm in the digital filter subroutine.
The main flow charts of the system are shown in Figures 6 and 7.

The DC power supply designed in this paper has a continuously adjustable output of 0-250 V. Compared with other thyristor devices, this power supply has a simple and reliable trigger circuit, stable output, and effectively isolates the strong and weak parts. At present, this power supply has been used in a ship power station simulation laboratory of a certain university. When the voltage is adjusted to between 180 and 220 V, the motor load can be driven by connecting the starting circuit and a stable DC voltage can be provided to the motor; when the motor drives the generator, the voltage regulation system can keep the system output DC voltage unchanged when the generator drives the load. In addition, this DC power supply can supply power to two motors at the same time and keep the voltage unchanged. By adjusting the excitation of each motor separately, the conditions for rough synchronous paralleling can be achieved (that is, the voltage of the unit to be paralleled is equal to the voltage of the running unit, and the frequency of the unit to be paralleled is equal to the frequency of the running unit), and the two generators are paralleled, thereby meeting the various requirements of ship power station experimental teaching.
References
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[2] Chen Nianjun, Hu Rongqiang, Bai Junjie. Design of output continuously adjustable switching power supply based on single-chip microcomputer control [J]. Electrical Applications, 2006, 25(4): 116-118.
[3] Wang Xilian, Wang Xudong. High-precision thyristor voltage linear digital triggering [J]. Power Electronics Technology, 2000, 34(3): 40-42.
[4] Huang Jun. Power Electronics Converter Technology [M]. Beijing: Machinery Industry Press, 1995.
[5] PILLAY P, KRISHNAN R. Modeling, simulation and analysis of permanent-motor drives, part I: the permanent-magnet synchronous motor drive [J]. IEEE Transactions on Industry Applications, 1989, 25(2): 265-273.