Design and implementation of digital adjustable switching power supply based on infrared remote control

Switching power supplies are widely used in electronic communications, military equipment, transportation, industrial equipment and other fields due to their small size, light weight and high efficiency. It is a power supply that uses modern power electronics technology to control the time ratio of the switch tube to open and close to maintain a stable output voltage. Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs. Compared with linear power supplies, the cost of both increases with the increase of output power, but the growth rate is different. The cost of linear power supplies is higher than that of switching power supplies at a certain output power point, which is called the cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is also constantly developing, and the cost reversal point is increasingly moving towards the low output power end, which provides a broad development space for switching power supplies. This article introduces a switching power supply with 89S52 microcontroller as the core, MOSFET as the main switch tube, and a method of combining PWM regulation to make the output voltage continuously adjustable.

Overall analysis of switching power supply design

1 Principle of Switching Power Supply
The switching power supply controls the switch tube through the circuit to conduct high-speed on and off, converts the direct current into high-frequency alternating current and provides it to the transformer for transformation, thereby generating one or more sets of voltages required. The switching transformer can be made very small, and the working temperature is not very high, and the cost is very low. It can be roughly divided into two types: isolation and non-isolation. The isolation type must have a transformer, but the non-isolation type may not necessarily have one.

The working process of the switching power supply is:
① The AC power input is rectified and filtered into DC;
② The switch tube is controlled by a high-frequency PWM (pulse width modulation) signal, and the DC voltage is added to the primary of the switching transformer;
③ The secondary of the switching transformer induces a high-frequency voltage, which is supplied to the load after rectification and filtering;
④ The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.
When the AC power is input, it generally passes through something like a choke to filter out the interference on the power grid, and also filters out the interference of the power supply to the power grid; when the power is the same, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements for the switching tube; the secondary of the switching transformer can have multiple windings or one winding has multiple taps to obtain the required output; generally, some protection circuits should be added, such as no-load, short circuit, etc., otherwise the switching power supply may be burned.

2 Block diagram of this design
This design takes MCU as the core, and the input is the grid voltage. Through the friendly keyboard LCD interaction method, the continuously adjustable power output is completed. The grid voltage is converted into a DC voltage through the rectifier and filter in the input circuit and input into the high-frequency converter. It is converted into a high-frequency pulse square wave voltage through the high-frequency rectifier and filter in the output circuit, and then converted into a DC voltage to supply the load. The block diagram of the entire system design is shown in Figure 1.

Figure 1 System Block Diagram

System Hardware
DesignAT89S52 is a low-power, high-performance CMOS 8-bit microcontroller with 8K programmable Flash memory. With it as the core, the main components of the system are: signal conditioning module for acquisition, pulse width output signal driver module, overcurrent and overvoltage protection module, human-computer interaction interface module, sound and light alarm module, RS232 module. Among them, the use of infrared remote control module is a major feature of the system. The system structure is shown in Figure 2.

Figure 2 System hardware structure

The signal conditioning circuits for the output voltage, output current, input voltage, etc. are mainly composed of precision operational amplifiers. Through differential and proportional operational amplification, the common-mode interference of the signal is eliminated, and the collected signal is conditioned into a standard 0-5V voltage signal, which is input to the microcontroller for processing through the ADC. The PWM output drive circuit mainly amplifies the PWM signal output by the 89S42 microcontroller. Overcurrent and overvoltage protection are mainly composed of traditional relays and combinational logic circuits. When the microcontroller control circuit detects output overvoltage, overcurrent or input overvoltage, the microcontroller outputs the switch quantity to the combinational logic circuit to perform corresponding emergency processing.

The human-computer interaction interface is a very important part of the power supply system. Considering the commercialization and productization needs of the switching power supply, a 128×64 LCD display unit is used. The data of the microcontroller can be sent to the LCD screen through the serial port for display. In addition, keyboard input and infrared remote control input are used to enable interaction between the product and the user in harsh environments. The system circuit diagram is shown in Figure 3.

Figure 3 X51 system circuit

High-frequency conversion circuit design
This switching power supply needs to have the function of adjustable output voltage. Because it uses a combination of PWM and PFM regulation methods, a suitable high-frequency conversion circuit must be selected. During the design, a double MOSFET is used to form a flyback half-bridge high-frequency conversion circuit, as shown in Figure 4.

Figure 4 High frequency conversion circuit

The high-frequency transformer T in this circuit has a primary winding connected to the DC power supply after industrial frequency rectification through two field-effect transistors, which are turned on and off at the same time. When the field-effect transistor is turned on, it stores energy; when it is turned off, the magnetic field energy is converted into electrical energy to supply the load. This circuit is suitable for applications with fixed frequency, variable frequency, complete and incomplete energy transfer. Other industrial frequency rectification and filtering circuits and high-frequency rectification and filtering circuits all use circuit forms commonly used in ordinary switching power supplies.

Figure 5 Software design

System software design
1 Multitasking design
The software design adopts a real-time multitasking embedded operating system (Small RTOS51 System) to control the system operation through efficient time slice management.
2 System flow chart
The operation flow of the entire system is shown in Figure 6.

Figure 6 Software Flowchart