Abstract: This paper proposes a model-free control method for switching power supplies. The non-modeling adaptive controller, also known as a model-free controller, breaks free from the constraints of PID control, pointing out a new direction for the development of switching power supplies. Keywords: switching power supply, PID control, model-free controller, PWM [b][align=center]Research of Switching Power Supply Based on Model Free Controller ZHANG Ke, QI Xing-Guang (Shandong Institute of Light Industry, Shandong, Jinan, 250353[/align][/b] Abstract: This paper presents a method of designing a model-free switching power supply. The model-free adaptive controller, also named non-modeling adaptive controller, eliminates the concepts of PID, pointing out a new direction for the development of switching power supplies. Key words: switching power supply, PID control, model-free controller, PWM 1. Introduction With the rapid development of power electronics technology, power electronic equipment is becoming increasingly intertwined with people's work and lives, and all electronic equipment relies on a reliable power supply. Switching power supplies utilize modern power electronics technology to control the on/off time ratio of switching transistors to maintain a stable output voltage. Switching power supplies are generally composed of a pulse width modulation (PWM) control IC and MOSFETs. The control section of a switching power supply is mostly designed and operates based on analog signals, resulting in poor anti-interference capabilities. Due to the rapid development of computer control technology, digital signal processing and control have shown significant advantages: easier computer processing and control, greatly improved design flexibility, and convenient software debugging, leading to the emergence of PID control. It has enabled the switching power supply to develop towards digitalization, intelligence, and multi-functionality. This undoubtedly improves the performance and reliability of the switching power supply. However, since the switching power supply itself is a nonlinear object, it is quite difficult to establish its accurate model. Approximation is often used. Moreover, the power supply system and load changes are uncertain. Therefore, it is often difficult to make the parameters of the PID regulator change accordingly when using the above-mentioned analog or digital PID control methods. The control effect is not ideal. The recently developed modelless control [1] is a control method with broad prospects. It does not rely on the mathematical model of the controlled object and integrates modeling and control. This is very suitable for some complex, variable or structurally uncertain systems that are difficult to describe with an accurate mathematical model. It improves the control system of the switching power supply and not only meets the requirements of high performance and high reliability of the switching power supply. 2. Working principle of the switching power supply The principle block diagram of this switching power supply is shown in Figure 1. The grid voltage is converted into DC voltage by the rectifier and filter in the input circuit and input to the high-frequency converter. The high-frequency converter converts the input DC voltage into a high-frequency pulse square wave voltage. The pulse square wave is converted into DC voltage by the high-frequency rectifier and filter in the output circuit and supplied to the load. [align=center] Figure 1 Working principle of the switching power supply[/align] The control loop, centered on a microcomputer and supported by control software, samples the output voltage and current of the switching power supply, compares them with given data, and then adjusts and controls the inverter, changing the conduction frequency or on/off time of the MOSFETs to stabilize the output and monitor the operating status of the switching power supply. 3. Hardware System Composition of the Switching Power Supply The control system of the switching power supply can select different microprocessors according to the actual situation of the project. Its block diagram is shown in Figure 2. The power-on/reset circuit provides a stable power supply and reset function to the microprocessor. The output voltage feedback is used to adjust the output voltage value and maintain its stability; the current feedback circuit has a similar function to the voltage feedback. The digital tube display circuit and keyboard input circuit realize the human-computer interaction function. The PWM output drive circuit outputs pulses to control the on and off of the switching transistors. When the output voltage is higher than the required voltage, the width of the output pulse decreases, thereby reducing the output voltage; when the output voltage is lower than the required voltage, the width of the output pulse increases, thereby increasing the output voltage. [align=center] Figure 2[/align] 4. Model-Free Control Principle 4.1 Overview of Model-Free Control In control law design, it is generally necessary to establish a mathematical model of the dynamic system. Classical methods require that this mathematical model must be established in advance, or at least its structure must be determined in advance. Moreover, the more accurate the model, the better. In the design of model-free control laws, the limitation of the requirement that the mathematical model of the control law be established as accurately as possible in advance is broken. Our modeling procedure is carried out in conjunction with feedback control. The initial mathematical model may be imprecise, but it must be ensured that the designed control law has a certain degree of convergence. The model-free control law we designed is to model and control at the same time. After obtaining new observation data, we model and control again. This continues, so that the mathematical model obtained each time gradually becomes more accurate, and thus the performance of the control law is improved accordingly. We call this procedure the integrated procedure of real-time modeling and feedback control. 4.2 Integrated Approach of Modeling and Adaptive Control In the references, the following general model is proposed: y(k)-y(k-1)=φ(k-1)[u(k-1)-u(k-2)] (4-1) Without loss of generality, we assume here that the time delay of the controlled dynamic system S is 1, y(k) is the one-dimensional output of system S, and u(k-1) is the P-dimensional input. φ(k) is a characteristic parameter, which is estimated online using a certain identification algorithm, and k is the discrete time. We will see that φ(k) has obvious mathematical and engineering significance in the integrated identification and control process of real-time identification and real-time feedback correction. 4.3 Real-time modeling and feedback control integration Specifically, our framework for integrated modeling and feedback control is as follows: (1) Based on the observation data and the generalized model y(k)-y(k-1)=φ(k-1)[u(k-1)-u(k-2)], the estimated value φ(k-1) of φ(k-1) is obtained using an appropriate estimation method. (2) To find the predicted value φ*(k) of φ(k-1) one step ahead, a simple method is to take φ*(k)= φ*(k-1). When seeking the control law, we still denote φ*(k) as φ(k). (3) Apply the control law to system S to obtain a new output y(k+1). Thus, a new set of data {y(k+1), u(k)} is obtained. Based on this new set of data, repeat (1), (2) and (3) to obtain new data {y(k+2), u(k+1)} and so on. As long as system S meets certain conditions, under the action of this procedure, the output y(k) of system S will gradually approach y[sub]0[/sub]. 4.4 Controller Program Design Currently, the controllers used in industrial production process control are mostly classic PID controllers and their variants. For systems with less severe coupling, the control effect of PID controllers is satisfactory, but for systems with severe coupling, PID controllers are powerless. Below, we compare the model-free controller with the PID controller based on the PID controller to illustrate that the model-free controller has better decoupling and anti-interference capabilities. Model-free control flowchart [align=center] Figure 3 Model-free control flowchart[/align] 5. Experimental Results Here, we simulate and compare the decoupling capabilities of the model-free controller and the PID controller. To ensure the fairness of the comparison, the parameters of both the model-free controller and the PID controller are tuned to their optimal state. The following system [1] (4-5) is controlled: The control results are shown in Figure 4 and Figure 5 [align=center] u(t) y(t) Figure 4 Simulation results of PID control u(t) y(t) Figure 5 Simulation diagram of model-free control[/align] The simulation results clearly show that both the model-free controller and the PID regulator have achieved good results in controlling nonlinear systems, but the model-free control method has a much stronger control capability for nonlinear coupling situations than the PID regulator. 6. Conclusion Model-free control is suitable for nonlinear control. Its control rules do not require the determination of a specific object model. It has good stability and anti-interference capability for the control of nonlinear objects such as switching power supplies. The introduction of model-free control strategy has opened up a broad space for the development of switching power supplies. References: [1] Han Zhigang, Design problems of model-free controllers [J]. Control Engineering, 2002, 5-3: 19-22. [2] Liu Wenjun, Luo Yufeng. Design and MATLAB simulation study of fuzzy control PID of switching power supply [J]. Microcomputer Information, 2006, 10-1: 29-30. [3] Zhou Zhimin, Zhou Jihai. Practical technology of switching power supply [M]. Beijing: People's Posts and Telecommunications Press, 2002. The innovation of this paper: introducing model-free control into the control of switching power supplies. Author Biography: ZHANG Ke (1982—), male, Postgraduate, major: intelligent control and measurement in industrial processes. Detailed Contact Information: Zhang Ke, Postgraduate Student, Class of 2006, School of Information and Control Engineering, Shandong University of Light Industry, University Science Park, Western New City, Jinan, Shandong Province, 250353, China. Tel: +86-13864168250 E-mail: [email protected]