Research on Switching Power Supply Based on Digital Control

0 Introduction

Switching power supplies are known as high-efficiency and energy-saving power supplies. Traditional switching power supplies use analog control technology, using components such as comparators, error amplifiers, and analog modulators to adjust the power supply output voltage. The analog control method is only suitable for switching power supplies with high frequency, low power, and few functions. It also has disadvantages such as complex control circuits, many components, and difficulty in modifying the control circuit once it is formed, which is not conducive to the integration and miniaturization of switching power supplies. The digital control technology of switching power supplies can solve these problems better. This article introduces the digital control technology of switching power supplies, and gives two models of switching power supplies based on single-chip microcomputer control and DSP control. The advantages and disadvantages of the two and their application scenarios are analyzed and compared. Now they are introduced as follows.

1 Comparison of analog control and digital control of switching power supply

1.1 Types and characteristics of switching power supply analog control

The analog control method of switching power supplies has been used for decades, and a series of control methods have been formed, which can be roughly divided into three types: pulse width modulation (PWM), pulse frequency modulation (PFM) and pulse frequency modulation (PFM).

Figure 1 shows a pulse width modulation (PWM) switching power supply. The AC220V input voltage is rectified and filtered to become a DC voltage U, which is then chopped by the power switch tube VT and stepped down by the high-frequency transformer T to obtain a high-frequency rectangular wave voltage. Finally, the required DC output voltage U0 is obtained through the output rectifier filter VD and C2. The error amplifier and PWM comparator form a closed-loop regulation system. This analog control circuit requires a lot of space because it uses many components. The values ​​of these components themselves will also change with the use time, temperature and other environmental conditions, and have a negative impact on the stability and response ability of the system, which is not conducive to the testing and maintenance of the analog system. In addition, the control response characteristics of analog control are determined by the values ​​of discrete components, so it is impossible to provide the most optimized control response for all power values ​​or load points.

1.2 Characteristics and applications of digital control of switching power supplies

The so-called digital control of power supply, also known as "processor inside the loop", means that the controller can execute all system control algorithms in the digital domain. It must compare two digital strings to generate pulse width to drive the power switch, instead of using traditional analog PWM comparators. It converts all analog system parameters into digital signals, calculates the control response using these data in the digital domain, and then transmits the newly generated control information to the system.

The digital control power supply system has the following characteristics:

(1) An intelligent switching power supply system with a digital signal processor (DSP) or a single-chip microcomputer as the core and a digital power driver and PWM controller as the control object.

(2) The “integrated digital power supply” technology is used to achieve an optimized combination of analog components and digital components in the switching power supply.

(3) High integration, which realizes the monolithic integration of the power system and integrates a large number of discrete components into one chip or a group of chips.

(4) It can give full play to the advantages of digital signal processors and microcontrollers, so that the designed digital power supply can achieve high technical indicators.

This technique can be used in applications where the load is constant for a long time, allowing the power supply to run at a high frequency, such as power factor correction, uninterruptible power supplies, multiple chemical battery translation, and motor control; it can also be used in other applications such as PMUs in mobile phones and PDAs that use several configurable PWM cores and control, diagnostic, and interface circuits. Subcircuits or peripherals in the run-time control circuit can provide the most appropriate operating voltage for their current state to save energy. Digital power control can make the regulator more sensitive.

2 Solution of switching power supply based on digital control technology

Combining current digital control technology and popular power management modes, we propose the following two solutions.

2.1 Switching power supply based on single chip microcomputer control

With the rapid development of electronic technology and the further improvement of ultra-large-scale integrated circuit design, single-chip microcomputer technology has also developed rapidly. It has been widely used in intelligent instruments, industrial detection and control, power electronics, automotive electronics, mechatronics, etc., and has achieved great results. Using a single-chip microcomputer as the control core, the design method is easy to master, and the requirements for the single-chip microcomputer are not high, and the cost is relatively low. This solution uses a single-chip microcomputer to sample through an external A/D conversion chip, calculates and adjusts the data obtained after sampling, and then transmits the result to the PWM chip after D/A conversion, realizing the indirect control of the single-chip microcomputer on the switching power supply. Its principle structure is shown in Figure 2.

Among them: the single-chip microcomputer uses MCS51; the A/D converter uses the TLC2543 chip, which uses a serial port as its interface. Compared with the parallel port, this method has the characteristics of simple interface, easy expansion, and small size. The interface of TLC2543 is a typical SPI interface. When it is connected to the MCS51 single-chip microcomputer, its hardware circuit is very simple. However, since MCS51 does not have a standard SPI interface, the TLC2543 can only be operated in the program by imitating the SPI operation mode, so the program is more complicated. The D/A converter uses the TLC5615 chip to connect to the MCS51, and the interface also uses a serial mode. The "watchdog" provides a power-on reset signal for the microcontroller. When the program is disordered or the voltage is abnormal, the internal "watchdog" circuit is activated to force the microcontroller to reset and make the program start from the beginning. It has a 512-byte EPROM storage unit to store various important data in case the important data is lost after communication. It has an external serial port, which connects to RS-485 or RS-232 through level conversion to realize signal transmission between the switching power supply and the host computer. The LCD and keyboard interface circuits realize human-computer exchange.

Although the control circuit of the digital power supply based on the single-chip microcomputer is relatively complex and there is a certain delay, which has a certain impact on the dynamic performance and voltage regulation accuracy of the power supply, this method does not have high requirements for the single-chip microcomputer, the cost is relatively low, and the design method is easy to master. Therefore, it can be used in situations where the voltage regulation accuracy and dynamic performance requirements are not very high.

2.2 Switching power supply based on digital signal processing control

Using high-performance DSP digital chips to directly control the power supply can simplify the design of the control circuit. In addition, these chips have relatively high sampling speed (the 10-bit A/D converter inside TMS320LF2407 only takes 500ns to complete an A/D conversion) and operation speed, which can quickly and effectively implement various complex control algorithms. The principle structure of the switching power supply based on DSP control is shown in Figure 3.

Among them: DSP adopts the currently popular TMS320LF2407, which is mainly used for digital PID calculation; the complex programmable logic device CPLD generates a digital PWM waveform to control the main power converter according to the result of DSP calculation, thus avoiding the double pulse phenomenon and half-frequency phenomenon in the analog PWM controller and realizing the complete digitalization of PWM control; the AID conversion circuit is used for the collection of voltage, current, temperature and other data. The chip can use TLC5540 chip or TLC2543 chip. The voltage and other signals collected by this AID conversion circuit enter the DSP through the lower eight bits of the data bus and are compared with the standard sine wave signal. When it is detected that the output voltage amplitude is higher than the standard sine wave signal, the duty cycle is reduced proportionally, thereby realizing the adjustment of the output sine wave and amplitude of the switching power supply. The DSP can also expand the LCD and keyboard for human-computer exchange through the interface circuit and communicate data through the serial port RS-485 or RS-232.

This kind of switching power supply based on digital signal processing has a complex DSP chip structure, high cost, and relatively difficult to master DSP control technology. However, due to the high sampling and computing speeds of the chip, it can quickly and effectively implement various complex control algorithms, achieve effective control of the power supply, and has high dynamic performance and voltage regulation accuracy. Therefore, this method will play an important role in the digital control technology of switching power supplies in the future.

3 Conclusion

The switching power supply adopts full digital control technology, which can effectively reduce the size of the power supply, reduce costs, and greatly improve the reliability of the equipment and the adaptability to users. It is a trend in the development of switching power supplies in the future. At present, it has been well applied in the field of communications. This article gives two solutions for switching power supplies based on digital control technology, which are suitable for different environments. We can choose different solutions based on factors such as sampling speed, operation speed and control algorithm complexity.