Implementation of FPGA Control for Multilevel Boost Converter used

2010 2nd IEEE International Symposium on Power Electronics for Distributed Generation Systems

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Implementation of FPGA Control for Multilevel Boost Converter used for PV Applications Hamdy Radwan, Mostafa Mousa, IEEE student member, Mahrous Ahmed, IEEE Member, and Mohamed Orabi, IEEE Senior Member APEARC, South Valley University, Aswan City, Egypt [email protected] period. It is possible to create, implement, and veri1t a new design. A configuration program stored in internal static memory cells of the XILINX FPGA is written by VHDL programming language that has been designed and optimized for describing the behavior of digital systems; VHDL has many features appropriate for describing the behavior of electronic components ranging from simple logic gates to complete microprocessors and custom chips. Features of VHDL allow electrical aspects of circuit behavior (such as rise and fall times of signals, delays through gates, and functional operation) to be precisely described. The utilization of FPGAs instead of other architectures was mainly based on four factors: the acceleration of the design or parts of it, the flexibility of reconfiguration hardware, the reduction of costs, and the energy consumption. These factors had a different impact on each application area [1]. Several MPPT techniques have been proposed, during the last decades. They range from conventional methods, from simple hill-climbing algorithms (P&O, MP&O and EPP), to fuzzy logic and neural network algorithms, The hill climbing algorithm [2-4] is widely used in practical PV systems because of its simplicity and because it does not require prior study or modeling of the source characteristics and can account for characteristics’ drift resulting from ageing, shadowing, or other operating irregularities, but its performance is poor compared to Artificial Intelligent methods, Recently, the increasing performance and cost reduction of digital circuits have made possible their applications for power converter control Comparison with other digital signal processors, Field Programmable Gate Array (FPGA) based systems could provide a number of run-time advantages over the sequential machines such as a microcontroller. Moreover, with concurrent operation, it is executed continuously and simultaneously which faster than DSP. Thus the FPGA has been applied for high speed switching circuit to reduce equipment-sizing [5] specially in the implementation of MPP, FPGA features are utilized [6, 7]. The paper is organized as follows: Section II shows the system configuration of the proposed system. In section III a multilevel boost converter is analyzed .The MPPT control is discussed in section

Abstract--- This paper presents the implementation of a Maximum Power Point Tracker (MPPT) for photovoltaic (PV) applications by using multilevel boost converter and FPGA Board. The control algorithm for extracting maximum power from the cell is proposed by means of the VHDL code and implemented using Xilinx XC3S400 FPGA Board. In this work, a practical implementation of the real-time Estimate Perturb and Absorb algorithm for maximum power point tracking (MPPT) control in a PV system has been developed. The Developed implementation has the advantage of simple programming with high performance even with low resolution ADC and low cost current sensor. The proposed technique has been validated through detailed experimental work. Index Terms-- Photovoltaic (PV), FPGA, multilevel boost converter, maximum power point tracking, simple sensor.

I. INTRODUCTION Renewable energy sources such as solar energy are acquiring more significance, due to shortage and environmental impacts of conventional fuels. The photovoltaic (PV) system for converting solar energy into electricity is in general costly and is a vital way of electricity generation only if it can produce the maximum possible output for all weather conditions The PV array has a highly non-linear current-voltage characteristic varying with the irradiance and temperature that substantially affects the array power output. The maximum power point tracking (MPPT) control of the PV system is therefore critical for the success of a PV system. P&O MPPT algorithm which implemented by XILINX FPGA have been considered extensively in the literature. Field programmable gate arrays (FPGA's) are standard integrated circuits that can be programmed by a user to perform a variety of complex logic functions. The high level of integration available with these devices (currently up to 500,000 gates) means that they can be used to implement complex electronic system. Furthermore, there are many advantages due to the rapid design process and reprogrammable function. XILINX FPGA enables to produce prototype logic designs right in a short 1 The authors would like to thank STDF project for their support during this work.

978-1-4244-5670-3/10/$26.00 ©2010 IEEE

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IV. Section V shows the experimental results. Finally Finally, conclusions are presented in section VII.

III. MULTILEVEL BOOST CONVERTER TOPOLOGY DESIGN Figure 3 illustrates a multilevel boost converter which combines the boost converter and the switched capacitor function to provide an output of several capacitors in series with the same voltage and self balanced voltage, The major advantages of thi this topology are: (i) continuous input current, (ii) a large conversion ratio with low duty cycle and without a transformer. It can be built in a modular way and more levels can be added without changing the main circuit; it provides several self sel balanced voltage levels and only one switch is necessary [[9]. Here, a multilevel boost converter is designed based on the specification shown in Table. 2.

II. PV CELL MODEL AND SIMULATION The simplest equivalent circuit of a solar cell is a current source in parallel with a diode. The output of the current source is directlyy proportional to the light falling on the cell. The diode determines the I  V characteristics of the cell. Increasing sophistication, accuracy and complexity can be introduced to the model by adding in turn: • Temperature dependence of the diode saturation current I . • Temperature dependence of the photo current I . • Series resistance R  , which gives a more accurate shape between the maximum power point and the open circuit voltage. • Shunt resistance R  in parallel with the diode. • Either allowing the diode quality factor n to become a variable parameter (instead of being fixed at either 1 or 2) or introducing two parallel diodes (one with A =1, one with A =2) with independently set saturation currents [8]. [ For this research work, a model of moderate complexity ity was used. The model includes temperature dependence of the photo--current I and the saturation current of the diode I . A series resistance R  was included, but not a shunt resistance. A single shunt diode was used with the diode quality factor sett to achieve the best curve match. The circuit diagram for the solar cell is shown in Fig. 1. The studied system configuration is shown in Fig. 2. This system consists of a solar array array. The type of the solar cell that is used is SP85P and the Electrical Characteristics is shown in T Table. 1.

The transfer function of the conventional boost converter is

 



(1)



But in the multilevel converter the transfer function of it is ∗

  

(2)

It can be noted from the above equations 1 and 2 that the MLBC has a higher conversion ratio compared with the conventional converter based on the number of level used. Table 1: Electrical lectrical characteristics of the PV module. Rated Power (Pmax) Voltage at Pmax (Vmp) Current at Pmax (Imp) Short circuit current (Isc) Open circuit voltage (Voc)

85W 17.6V 4.83A 5.14A 21V

Table 2: Design specification of the MLBC. MLBC Specification Input Voltage (Vs) Input Current (Is) Output Voltage (Vo) Output Current (Io) Maximum Output Power (Pmax) Switching Frequency (f) Duty Cycle (D)

Fig. 1 The circuit model diagram of the solar cell.

20-30V 20 0-5A(< 5A(< 5% ripple) 250 250-320V 0-2A(