PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 8/2012 187 Hoda POURHOSSEIN 1 , Assef ZARE 1 , Mohammad MONFARED 2 Islamic Azad University, Gonabad Branch, Islamic Republic of Iran (1), Ferdowsi University of Mashhad (2) Hybrid Modeling and PID-PSO Control of Buck-Boost Chopper Abstract. Due to its simplicity, low voltage stress, high reliability, low switch and inductor losses, and small inductor size, the Buck-Boost chopper has attracted a lot of attention in applications where it is necessary to step-up or step-down the DC voltage. In this paper a hybrid model of the Buck- Boost DC-DC converter using the PWA (piecewise affine) modeling framework is proposed, and then a PID controller is designed based on the PWA model. Finally, the particle swarm optimization (PSO) method is used to determine near optimum PID controller parameters. Designing the controller and analyzing the performance of the system based on the non-linear model are very difficult, so we used the PWA as an alternative solution. The proposed piecewise affine hybrid model lets decide about the control strategy and analyze the stability and performance of the closed loop control system using the classical control theory. Extensive simulations show the superiority of the PWA over the small signal linear model in prediction of the system behavior. Streszczenie. W artykule przedstawiono hybrydowy przekształtnik DC/DC typu Buck-Boost wykorzystujący modelowanie PWA ) piecewise affine). Wykorzystano algorytm mrówkowy PSO do optymalizacji parametrów. (Hybrydowe modelowanie i sterowanie PID-PSO przekształtnikiem Buck- Boost) Keywords: Hybrid system, Buck-Boost chopper, piecewise affine (PWA) approximation, PID-PSO controller. Słowa kluczowe: przekształtnik Buck-Boost, sterownik PID-PSO Introduction The theory of control includes controlling continuous systems. This theory is now combined with the digital computing science that comprises discrete models and the results of this combination are hybrid systems. As you can see in Fig. 1, hybrid systems are some kind of dynamic systems, in which the behavior of system is a combination of continuous dynamics, which is modeled with differential equations, and discrete dynamics, which is usually modeled with automatic machines. As a result, in recent years, the hybrid systems have attracted a lot of attentions [1]. Fig.1. The structure of hybrid systems Power electronic converters are the best choice for hybrid modeling and control; because the circuit parameters are continuous which lead to continuous dynamics and on the other hand, the switching behavior imposes discrete dynamics. Choppers are some kind of power electronic converters that are utilized in various industrial applications. Choppers can be classified according to their control variable into two classes [2]. In the first class, the switching frequency is fixed and the system is controlled by changing the switches duty cycles through adjusting the active duration of switches in one switching period. In the second class, which includes the variable frequency converters such as resonant converters, the system is controlled by changing the switching frequency [3]. In this paper a new member of fixed switching frequency choppers, named Buck-Boost chopper is considered. This converter has found a lot of attentions in applications where it is necessary to step-up or step-down the DC voltage, because of its simplicity, low voltage stress, high reliability, low switch and inductor losses, and small inductor size. Equations that characterize these converters are non-linear. Designing and analyzing controllers with non-linear equations are very complex. It is a common practice to use different linear approximation methods, because we want to use linear tools for the design and analysis. PWA method is recently proposed for overcoming to such a problem [4, 5]. In this paper, PWA hybrid technique is employed to model the converter and decide about the control strategy and analyze the stability and performance of the closed loop control system using the classical control theory. In the following sections, at first, the Buck-Boost chopper is introduced. Then the PWA approximation method is defined and successfully applied to this converter. Next, a voltage loop PID controller is designed based on the PWA model. Finally, the particle swarm optimization (PSO) method is used to determine near optimum PID controller parameters. The effectiveness of this modeling and controller design method for the Buck- Boost chopper is verified by simulations. Buck-Boost Chopper This converter is a new member of choppers also known as DC to DC converters which can simultaneously increase or decrease the input DC voltage. The duty cycle is modified according to the difference between the measured and reference values of the output voltage [6]. Fig.2 shows the Buck-Boost chopper together with the control system. The averaging state-space method is a generalized analysis tool which is readily applicable to either simple circuits or complex structures in order to achieve a continuous model. For the converter of Fig.2, there are two distinct switching states as follows [6]: 1. S conducts (0 < t < dT): (1) 1 [0 ] : 1 [ ] C C L in dV V dt C R a di V dt L           (2) 1 0 0 1 0 0 C C in L L dV V dt V RC i di L dt                                       