Converter Design for Photovoltaic System

Converter Design for Photovoltaic System

Swapnil Zadey

Assist. Professor, DMIETR,Sawangi,Wardha,M.S.

Abstract- The solar energy conversion system is an alternative for conventional power generating system. It has no running cost due to freely available and non polluting solar radiations. The voltage which is available from solar array is variable and to obtain a stable voltage from solar panels, DC-DC converters are required for constant power production. There are mainly three converters namely Buck, Boost and Buck- Boost converters which can be used for either increasing or decreasing the voltage.

This paper presents mobile charging circuit with a PV source. The circuit structure of the proposed system adopts buck converter combined PWM MPPT technique. In this research, buck converter is used as a charger for charging mobile battery. The input voltage can typically change from (12V) initially, down to (5V), and provide a regulated voltage within the range of the 4.5V required for the charging of mobile batteries.

KeywordsBuck converter, MOSFET, Mobile charging system, MPPT technique,PV system.

1. INTRODUCTION

   DC-DC converters are considered to be of great economical importance in todays society, and are perhaps one of the few electronic circuits that are commonly used in switching power supplies, generally are widely used at home solar systems to produce the desired output power[6]. More sophisticated applications require electronic converters to process the electricity from the PV device. These converters may be used to regulate the voltage and current at the load, control the power flow in grid- connected systems, and mainly track the maximum power point (MPP) of the device[10]. Therefore, this paper tries to study an electronic converter in PV systems, namely the Buck converter, and propose an easy way to electronic converter designers to calculate component values needed to realize it.

   1. DC-DC Converters

      There are three kinds of switching mode DC-DC converters, buck, boost and buck-boost. The buck mode is used to reduce output voltage, whilst the boost mode can increase the output voltage. In the buck-boost mode, the output voltage can be maintained either higher or lower than the source but in the opposite polarity. The simplest forms of these converters are schematically represented in Figure 1,2& 3. These converters consist of the same components, an inductor, L, a capacitor, C and a switch, which has two states u = 1 and u = 0. All converters connect to a DC power source with a voltage (unregulated),

      Vin and provide a regulated voltage, vo to the load resistor, R by controlling the state of the switch. In some situations, the load also could be inductive, for example a DC motor, or approximately, a current load, for example in a cascade configuration. For simplicity, here, only current and resistive loads are to be considered.

      Figure:1 Buck Converter

      Figure:2 Boost Converter

      Figure:3 Buck- Boost Converter

   2. Working Principles:

      The working principles of the these DC-DC converters can be explained as follows.

      In the buck mode, when the switch is on position 1, the DC source supplies power to the circuit which results an output voltage across the resistor. When the switch changes its position to 0, the energy stored in the inductor and capacitor will discharge through the resistor. Appropriately controlling the switching position can maintain the output voltage at a desired level lower than the source.

      In the boost mode, when the switch is on position 1, the circuit is separated into two parts: on the left,

      the source is charging the inductor, meanwhile the capacitor on the right maintains the output voltage using previously stored energy. When the switch changes its position to 0, both the DC source and energy stored in the inductor will supply power to the circuit on the right, hence boost the output voltage. Again, the output voltage can be

      maintained at desired level by controlling the switching sequence.

      Finally, for the buck-boost mode, switch positions 1 and 0 represents charging and discharging modes

      of the inductor. Appropriately controlling the switching sequence can result in output voltage higher or

      lower than the DC source. Since the inductor cannot

   3. Analytical expressions:

   The basic circuit of the Buck converter showed in Figure 1 has two energy storage elements: the inductor and capacitor which can be calculated in a second order differential equation. We start by the differential equation for the capacitor voltage, in the ON state, which given by:

   change the direction of current, the output voltage is opposite to the DC source.

   This paper provides the designing and simulation of Buck

   d 2V

   L C c

   dt 2

   L dVc V (t) V R dt c in

   converter. The reason to choose a Buck converter is that our load is a 4.5V mobile battery, where we will charge it with only one PV module which generates the most watts running at around 12 V. Thus, we need a DC/DC converter which takes a higher input voltage and converts it to a lower output voltage. This can only be done by the Buck converter. We will, first, start by a brief introduction to the functioning of a Buck converter, and then, write its basic

   Assumption: Vc ( t ) is constant, so the differential equation above can be simplified and can be written for the current through the inductor as:

   L dI L (t) V V

   dt in out

   For a steady state, and when tTON = T the inductor current attains it final

   equations.

   value: IL

   Vin Vout T I L L0

2. WORKING OF BUCK CONVERTER IN BRIEF

   1. Circuit description:

      The basic circuit of a Buck converter is shown below in

      Where IL0 is the initial value of the inductor current (just before the switch is turned on). The peak-to-peak current ripple IL is the difference between the final and initial value of IL and is given by:

      Figure 4. Two switches are used in this basic circuit, usually one controlled (MOSFET) and one uncontrolled

      IL

      IL

      • IL0

      Vin Vout T

      L

      .(1)

      (diode), to achieve unidirectional power flow from the input to theoutput. This circuit, also, uses one capacitor and one inductor to store and transfer energy from input to output. They, also, filter or smooth voltage and current[10].

      This proportionality between the current ripple and the duty cycle prove that IL can be controlled by .

      For OFF state, we have a first order differential equation (the inductor current flows through the diode):

      L dI L (t) V

      dt out

      The solution of the differential equation yields:

      L

      L

      L

      L

      I (t) Vout t I VL

      Now, for tTOFF = (1 )×T , the inductor current decreases to its minimum

      Value IL0 therefore:

      V

      V

      Vout

      Figure:4 Basic circuit of a Buck converter

   2. Circuit operation:

   We used inductors and capacitors as energy storage

   IL0 (1 ) t IL L

   So, we can obtain the equation of the current ripple:

   Von

   components to control the energy flow from the input to the output by continuously opening and closing the switch

   IL IL IL0

   (1 ) T (2)

   L

   which is usually an electronic device that operates in two states: When the switch is ON for a time duration T ( is the duty cycle), the switch conducts the inductor current and the diode becomes revers biased. This results in a positive voltage VL = Vin Vout across the inductor. This voltage causes a linear increase in the inductor current IL . When the switch is turned OFF , because of the inductive energy storage, IL continues to flow. This current now flows through the diode, and VL = Vout for a time duration (1 )T until the switch is turned ON again.

   By a simple comparison between (1) and (2) it is obvious that:

   Vout Vin

   in one-half cycle the capacitor will charged and the increase in charge is:

   .(3)

   and because Vc = Vout , the increase in capacitor voltage is:

   VOUT

   Q C

   Then from (2) and (3) we obtain:

   Figure:5 Inductor current waveform

   To obtain the average inductor current, we can use:

   VOUT

   1

   OUT

   OUT

   8L C f 2 V

   I L,avg

   Vout

   R

   In continuous conduction mode (CCM), IL,min = 0 ; (the minimum current can be

   zero at the time of switching), so from (12) we have:

   Therefore, the maximum and the minimum current through

   the inductor are: V V

   I out out (1 ) t

   IL,max IL,avg L

   2

   R 2L

   I Vout Vout (1 )t .

   Then if the desired switching frequency f and load

   L,max

   R 2L

   resistance R are established,

   the minimum inductor current required for CCM is:

   I IL,min IL,avg L

   2

   Lmin

   R (1 )

   2 f

   I Vout Vout (1 )t

   Likewise we can obtain:

   L,min

   R 2L R

   We can now with the same analysis obtain the maximum and minimum current

   Through the capacitor:

   fmin 2 f

   (1 )

   Ic,max

   IL

   2

   Vout (1 ) t

   2L

   2 f L Rmin (1 )

   Ic,min

   IL

   2

   Vout (1 ) t

   2L

   These equations are used for designing and simulating buck converter circuit.

   The waveform for the current through the capacitor is shown below:

   Figure:6 Capacitor current waveform

   From Figure 3, its clearly that the average current through the capacitor is zero, but

3. SIMULINK MODEL

   Simulink Model of PV array is shown in figure below which is then simulated to obtain result by using variable input radiation parameters:

   Figure:7: Simulink model of PV array

   These three modes of DC-DC converters have been uniformly implemented in the MATLAB/Simulink as

   show in Figure 8.

   .

   Figure 8. A uniform Simulink model of DC-DC converters.

   The input-output connections of the model is shown in Figure 9. The first input to the model is the switch

   Figure 9: Input and output connections of the DC-DC converter model.

   signal eight 1 or 0. The second one defines the DC source voltage and internal resistance. The third input is used to define the output current. The model has two outputs, the output voltage and the inductor current, which are the states of the system. The model can be configure with a number of parameters. These parameters are: the capacitance, C, inductance, L, the internal resistance of the capacitor and the inductor, RC and RL respectively. Three converter modes can be selected through the pull-down menu. One can also define either zero or non-zero value to the initial capacitor voltage by selecting or de selecting the zero capacitor voltage" option. Finally, the option Positive Inductor Current" defines whether the condition

   should be enforced or not.

4. SIMULATION RESULTS

   After simulating the above equations and above models, following results can be obtained:

   1. Current Vs Voltage of PV array

   2. Power Vs Voltage of PV array.

   3. Output of Buck Converter

5. CONCLUSION

   Thus in this paper, a buck converter is designed and simulated successfully to obtain the constant output of about4.5V integrated with mobile charging circuit to charge the mobile. From simulation results, it can be seen that by using buck converter, we were able to convert variable voltage and current into a constant one. As the solar panel provides variable current and voltage depending upon the solar radiations, we used PWM MPPT technique to obtain continuous maximum power output which is then given input to the buck converter. Our further research work will be in comparative study of converters namely Buck, Boost and Cuk converter.

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