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Application of digital control technology of inverter power supply

inverter power supply uses advanced power electronic devices and high-frequency inverter technology to reduce the materials of traditional power frequency rectifier power supply by 80% - 90%, save energy by 20% - 30%, and improve the dynamic response speed by 2-3 orders of magnitude. At the same time, it is developing towards high-frequency, lightweight, modularization, intelligence and large capacity. Therefore, This paper analyzes the current situation and development trend of digital control technology of inverter power supply in detail, and introduces several control strategies of digital control of inverter power supply

1 development of digital control technology of inverter power supply

1.1 high performance inverter power supply and digital control technology

with the development of network technology, higher requirements are put forward for the network function of inverter power supply. High performance inverter power supply must meet: high input power factor, low output impedance; Fast transient response and high steady-state accuracy; High stability, efficiency and reliability; Low electromagnetic interference; This weakness seriously restricts the development and improvement of spray free materials. To realize these functions, digital control technology is indispensable

1.2 traditional inverter control technology

1.2.1 disadvantages of traditional inverter control technology

traditional inverter power supply is mostly analog control or a combination of analog and digital control system. Although plastic will be used more and more widely in automobiles, analog control technology has been very mature, but it has many inherent shortcomings: there are many components of control circuit, the circuit is complex, and the volume is large; If the flexibility is not enough and the hardware circuit is well designed, the control strategy cannot be changed; It is inconvenient to debug. Due to the differences in the characteristics of the devices used, the consistency of the power supply is poor, and the working point of the simulator drifts, resulting in the drift of the system parameters. It is difficult to realize the parallel connection of inverter power supply in analog mode, so the digital control of inverter power supply is the development trend and a hot spot in the research of modern inverter power supply

1.2.2 improvement of traditional inverter control technology

in order to improve the control performance of the system, the microprocessor was connected to the system through analog and digital (a/d) converters, and the digital control algorithm was realized in the microprocessor, and then the switching control signal was sent through the input, output port or pulse width modulation (PWM). The microprocessor can also display or transmit the collected working data of the power conversion device to the computer for storage. Some reference values used in the control can be stored in the memory of the microprocessor, and the circuit can be monitored in real time. The use of microprocessor improves the performance of circuit system to a great extent, but due to the limitation of microprocessor operation speed, in many cases, this microprocessor assisted circuit control system still needs analog control components such as operational amplifiers. In recent years, with the development of large-scale integrated circuits, modern programmable logic devices and digital signal processor (SP) technology, the full digital control of inverter power supply has become a reality. SP can read the output of the inverter in real time and calculate the PWM output value in real time, which makes it possible to apply some advanced control strategies to the control of the inverter, so that the harmonic generated when the nonlinear load changes dynamically can be dynamically compensated, and the output harmonic can reach an acceptable level

2 current situation of digital control technology of inverter power supply

2.1 digital, intelligent and networking control technology of inverter power supply

with the emergence of special sp for motor control and the general development of control theory, the control technology of inverter power supply is developing towards full digitalization, intelligence and networking, and the digital control technology of inverter power supply has undergone a great leap. The advantage of digital control of inverter power supply is that the hardware circuits of various control strategies are basically consistent. To realize various control strategies, there is no need to change the hardware circuits, just modify the software, which greatly shortens the development cycle, and some new complex control strategies can be applied. The consistency between various power supplies is very good, which provides a foundation for the further development of inverter power supply, And it is easy to form a large-scale inverter parallel operation system with high reliability

2.2 difficulties in the digital development of inverter power supply

digitization is the main direction of the development of inverter power supply, but the following problems still need to be solved:

a) the output of inverter power supply should track a given signal that changes according to the sinusoidal law, which is different from the constant value control of general on-off power supply. Under closed-loop control, the time difference between the given signal and the feedback signal is reflected in the obvious phase difference, which is related to the load, which brings difficulties to the design of the controller

b) the output filter of the inverter power supply has a great influence on the model of the system. The fluctuation amplitude of the input voltage and the nature and size of the load often have a large range of changes, which increase the complexity of the control object and significantly increase the high-order, uncertainty and nonlinearity of the control object model

c) for digital PWM, there is an out of control range of switching cycle. Generally, the width of this pulse is determined at the beginning of each switching cycle or at the end of the previous cycle. Even if the system changes at this time, the pulse width can only be adjusted in the next switching cycle, so now the digital control of inverter power supply has attracted widespread attention

3 digital control technology of inverter power supply

digital control method of inverter power supply has become a hot spot in the field of power supply research. Corresponding to digitization, a variety of discrete control methods have emerged, including digital proportional integral derivative (PI) regulator control, deadbeat control, digital sliding structure control, fuzzy control and various neural network control, So as to effectively promote the development of inverter control technology

3.1 digital PI control

digital PI control has been widely used because of its simple parameters and easy tuning. When the inverter adopts analog-digital PI control, if only the instantaneous value feedback of output voltage is used, its dynamic performance and performance under nonlinear load will not be satisfactory; If the instantaneous current value of the output filter inductor or output filter capacitor is introduced into the feedback, its performance will be greatly improved. However, the huge analog control circuit reduces the reliability of the control system, the debugging is complex, and it is not easy to set. The emergence of digital signal processing chip makes this problem be solved quickly. Nowadays, various compensation measures and control methods can be easily applied to the digital PI control of inverter power supply. The controller parameters are easy to modify and the debugging is simple

however, the application of digital PI control algorithm to the control of inverter power supply inevitably produces some limitations: on the one hand, the sampling quantization error of the system reduces the resolution of the algorithm, making the accuracy of PI regulator worse; On the other hand, the sampling and calculation delay make the controlled system a system with pure time delay, which makes the design of PI controller difficult and the stability reduced. With the development of high-speed SP and high-speed a/, digital PI control technology will be further applied in the control of inverter power supply

3.2 sliding mode variable structure control

the most remarkable feature of sliding mode variable structure control (svsc) is that it is insensitive to parameter changes and external disturbances, that is, it has strong robustness, coupled with its inherent switching characteristics, so it is very suitable for power converters with closed-loop feedback control

the discrete sliding mode control based on microprocessor makes the inverter output waveform have better transient response, but the steady-state performance of the system is not very ideal. The discrete sliding mode control system with feedforward control improves the transient performance and steady-state accuracy (see Figure 1), but if the system is overloaded, the burden of sliding mode controller will become very heavy. Self correcting discrete sliding mode control can solve this problem

Figure 1 discrete sliding mode control system with feedforward control

the controller of the inverter is composed of a parameter adaptive linear feedforward controller and a nonlinear sliding mode controller (see Figure 2). The sliding mode controller generates control force only when the load changes the output voltage. The steady-state control force is mainly provided by the feedforward controller. The switching surface (hyperplane) of the sliding mode controller is designed according to the optimization criteria

Figure 2 self correcting discrete sliding mode control system

3.3 deadbeat control

deadbeat control is a control method based on circuit equations. The basic idea of its control is to divide the output sinusoidal reference wave into several sampling periods at equal intervals. According to the starting value of the circuit in each sampling period, the action time of square wave pulse symmetrical about the center of the sampling period is calculated with circuit theory, The value of the load output at the end of the sampling period. The size of this output value is related to the polarity and width of the square wave pulse. Properly controlling the polarity and width of the square wave pulse can make the output on the load coincide with the output reference waveform at the end of the sampling period. By continuously adjusting the polarity and width of square wave pulse in each sampling period, the output with small harmonic distortion can be obtained on the load. Therefore, even at a very low switching frequency, deadbeat control can ensure the quality of the output waveform, which can not be achieved by other control methods. However, it also has limitations: due to the delay of sampling and calculation time, the duty cycle of the output pulse is greatly limited; It is sensitive to the changes of system parameters, such as power supply voltage fluctuations and load changes, and the robustness of the system is poor

for the influence of sampling and calculation delay, one method is to reduce the duty cycle limitation caused by calculation delay by modifying the output pulse mode; Another method is to predict the system state in advance through the state observer, and use the observed value to replace the actual value for control, so as to avoid the impact of sampling and calculation delay on the system. In order to improve the robustness of the system, one method is to use the load current prediction method to reduce the impact of load variation on the power output, but the actual improvement is limited; Another feasible method is to identify the system parameters, so as to determine the controller parameters in real time, so as to achieve good control effect. However, the computational complexity and storage capacity of system identification are very large, and it is difficult for general microprocessors to complete it in a very short time, so it is unlikely to realize it. Therefore, there is no better method to solve the problem of poor robustness of deadbeat control. It is precisely because the shortcomings and limitations of deadbeat control in power control are still difficult to solve, so the application of deadbeat control in industry needs to be further studied

3.4 repetitive control

the inverter adopts repetitive control to overcome the periodic distortion of output waveform caused by rectifier nonlinear load, which is usually combined with other PWM control methods. The idea of repetitive control is to assume that the fundamental waveform in the previous cycle will repeat at the same time in the next fundamental cycle. The controller determines the required correction signal according to the error of the given signal and the feedback signal, and then superimposes this signal on the original control signal at the same time in the next fundamental cycle, so as to eliminate the repetitive distortion in the subsequent cycles

the repetitive control system is shown in Figure 3 (a), (b). Ur in Figure 3 is the given voltage signal; UD is voltage disturbance signal; P (z) is the control object; Uo is the actual output of voltage. cycle