Study on Fuzzy Control PID Design of Switching Power Supply and Simulation with MATLAB

1 Introduction

The switching power supply is a DC regulated power supply controlled by a switch mode. It is widely used in various electronic devices for its small size, high efficiency and light weight. Most of the control parts of the switching power supply are designed and work according to analog signals, and their anti-interference ability is not very good, and the signal is distorted. Due to the rapid development of computer technology, the control and processing of digital signals have shown more and more advantages: it is easy to process and control by computers, avoid distortion of analog signal transmission, reduce interference of stray signals, and facilitate software debugging, etc. Digital PID control has emerged. It makes the switching power supply develop in the direction of digitalization, intelligence and multi-function. 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 precise model, and approximate processing is often used. In addition, its power supply system and load changes are uncertain, so it is often difficult to change the parameters of the PID regulator using the above-mentioned analog or digital PID control method, and the control effect is not ideal. Fuzzy control, which has been developed recently, is a human-like intelligent control method. It does not rely on the mathematical model of the controlled object and is easy to use human experience and knowledge to implement control. It is very suitable for some complex and variable systems or systems with uncertain structures that are difficult to describe with accurate mathematical models. It has strong robustness [1], especially for complex objects that cannot be determined. This paper proposes the design and simulation of fuzzy control of switching power supply based on this idea.

2 Circuit structure and control strategy

2.1 Main circuit structure

The main circuit topology is shown in Figure 1. The 220V single-phase AC power is rectified by full-wave uncontrolled rectification to obtain a DC voltage of about 300V, and then the required high-frequency pulsating voltage is obtained by the half-bridge high-frequency converter composed of IGBT, and the required DC voltage is obtained by rectification. Since the power of this power supply is not large (output 45V, 10A), the main circuit adopts an IGBT half-bridge topology structure, which can prevent magnetic bias and save costs. The main frequency is selected as 20kHz to make the IGBT work in the best state. In the figure, L1 and C1 are used to suppress differential mode noise, and L2, C2, and C3 are used to suppress common mode noise. The calculation of each parameter is omitted.

Figure 1 Main circuit schematic diagram

2.2 PID control circuit design

The main circuit and PWM generation circuit of the switching power supply are modeled in the MATLAB environment, as shown in Figure 3. For the convenience of simulation, some filtering, protection and absorption circuits are omitted in the figure. Figure 2 is the simulation schematic diagram of the PWM subsystem. Due to the nonlinearity of the switching power supply, there is a lot of approximation when abstracting its model. Based on the above model, the transfer function of the PID controller is designed by applying the traditional design method:

It can be seen in the subsequent analysis that the control performance of this controller is not ideal when the power supply voltage or load changes suddenly.

Figure 2 PWM subsystem

Figure 3 Conventional PID control model of switching power supply

2.3 Design of Fuzzy Controller

The structure of the fuzzy controller is shown in Figure 4, in which Ug is the given voltage, Ur is the feedback voltage, e and ec are the deviation and the rate of change of the deviation, E and EC are the deviation and the rate of change of the deviation after fuzzification. The output U of the fuzzy controller is defuzzified to u, and then controls the main circuit of the switching power supply through the PWM circuit.

Figure 4: Switching power supply model

The input and output language variables both take 7 language values, namely: PB, MB, PS, ZE, NS, NM, NP. The membership functions used to describe the fuzzy subsets on the domain of each language variable are shown in Figure 5. The fuzzy control table obtained based on experience is as follows:

Figure 5 Membership functions of input and output variables

Fuzzy inference rules table

According to the above design, the switching power supply fuzzy control circuit based on MATLAB is shown in Figure 6.

3 Experimental Results

Based on the experiments on PID controller and fuzzy controller, the two control algorithms are compared.

Figure 6 Switching power supply fuzzy control circuit model

3.1 Comparison of dynamic characteristics during the start-up phase

The first half of Figure 7 (t: 0-0.15) is a comparison of the output characteristics of the PID controller and the fuzzy controller at startup. The curve is measured when the load is 8.5A and the output voltage setting value is 38V. It can be seen from the figure that the overshoot of fuzzy control is much smaller than that of PI control. Fuzzy control can prevent the system from having a large overvoltage at startup, which is very beneficial to the normal operation of the equipment.

Figure 7 Starting characteristics and dynamic characteristics of operating point changes

3.2 Comparison of dynamic characteristics when parameters change

Since the voltage required by the load of the switching power supply is adjustable, that is, the operating point is variable, the second half of Figure 7 (t: 0.15-0.3) shows the dynamic characteristics of the PID controller and the fuzzy controller when the operating point changes (voltage: 38V→28V, current: 8.5A→6.4A). It can be seen from the figure that when the operating point changes, the dynamic process of fuzzy control is faster and without oscillation than that of PID control.

4 Conclusion

Fuzzy control is a nonlinear control, and its control rules are based on human experience. It has good robustness and overshoot suppression capabilities for nonlinear objects such as switching power supplies. Introducing fuzzy control strategies in switching power supplies will create a new situation for the development of switching power supplies.

The innovation of this paper is to propose a design and MATLAB simulation method for fuzzy control PID of switching power supply.

References

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