Implementation of DC-DC Converter for MPPT by Direct Control Method

Implementation of DC-DC Converter for MPPT by Direct Control Method

D. D. Gaikwad

Electronics & Telecommunication Engineering KIT COE, Kolhapur

Kolhapur, India

M. S. Chavan

Electronics & Telecommunication Engineering KIT COE, Kolhapur

Kolhapur, India

Abstract This paper presents implementation of DC/DC converter for maximum power point tracking (MPPT) based on direct control method for TL494 control circuit for solar array power generation systems. The main difference of the proposed system to present MPPT systems which eliminates second control loop i.e. proportionalintegral control loop and finding of the effect of simplifying the control circuit. The TL494 is a constant frequency and variable duty cycle pulse width modulation (PWM) control IC for DC/DC converter; it has two error amplifier and dead band control pin (DTCON) which can control directly duty cycle of by DTCON pin of applying a adjustable voltage level from 0V to 3.3V maximum respectively vary duty cycle from 100% to minimum resultant that output power from input varies. By using proper algorithm method and controller the proposed system is able to track MPPs accurately, rapidly and less oscillation at MPP. The two high gain error amplifiers used for to maintain output voltage and current limit purpose.

The proposed system was developed and tested successfully on a small dc variable power instead of using programmed MPPT microcontroller in the laboratory. Experimental results indicate the feasibility and improved functionality of the system for MPPT application.

Keywordsphotovoltaic, topologies in dc/dc converter, TL494 based converter.

1. INTRODUCTION

   Now days, the world is growing to developing renewable power generation system, clean and practically inexhaustible, and interdisciplinary research is continuously work in order to sustain the development of existing conversion technologies and the innovative ones [1], [2]. Photovoltaic (PV) offers an environmentally friendly source of electricity power generation, of which the fuel is sunshine, and clean pollution free. Solar panel is the fundamental energy conversion component of photovoltaic (PV) systems. It has been used in many applications, such as aerospace industries, electric vehicles, grid connected system equipment, etc. Practically the power delivered from the PV panels depends on many external factors, such as insolation (incident solar radiation) levels, temperature, and load condition.

   Thus, electrical power output usually increases linearly with the insolation and decreases with the cell / ambient temperature. However, adopting a maximum power point tracking (MPPT), the photovoltaic systems power transfer efficiently and reliability can be improved significantly as it can continuously maintain the operating point of the solar panel at the MPP all the times shown in Fig.1.[3].

   Fig.1 Power-voltage characteristics of photovoltaic module at different irradiance levels

   In practice, there are two methods to extraction of generated power in medium- and large-scale PV systems. They are sun tracking and maximum power point (MPP) tracking or both. Due to economic reasons and response a uses of MPP tracking is a popular technique today and for that purpose need dc/dc converter. Generally dc/dc converter is used as a constant source of power supply, change in load and line regulation. DC/DC converter this paper used for another purpose it will operate at maximum PV voltage / power which is based on which algorithm used. TL494 based dc-dc converter is more suitable for any type of algorithm for MPPT system.

   The available maximum power point trackers uses various types of control circuit or logic to search MPP point, thus to operate the converter circuit to extract available maximum power all the times. It needs to track the maximum power point (MPP) for a successful PV system. Maximum power point tracker (MPPT) uses DC/DC converter between PV and load. Various types of dc/dc converters are available based on isolated and Non-isolated converters. MPPT uses two loops one for maintaining constant switching frequency and another for maintaining tracking algorithm. For that purpose special programmed microcontroller and additional circuitry and PWM control technique require for efficient. This paper proposes a TL494 based push-pull isolated converter which requires less components and facilities

   provided to control duty cycle of PWM only for small change in voltage level at DTCON pin to reduce one of the loop.

2. MATERIAL AND METHODS

   1. Topologies

      There are various topologies are used as DC-DC converter. They are isolated or non-isolated topologies. Isolation transformer used in small-sized high-frequency electrical which provides the benefits of light weight, DC isolation, and between input and output, and we obtain step up or down of output voltage by change of transformer turns ratio. In PV applications, the grid-tied systems use these types of topologies when electrical isolation is preferred for safety reasons. Non-isolated topologies do not have isolation transformers. They are almost always used in DC motor drives. These topologies are further categorized into three types:

      Step down (Buck) Step up (Boost) and

      Step up & down (Buck-Boost).

      The buck topology is used where step-down requires. In PV applications, for battery charging process the buck type converter is usually used. The boost topology is used for stepping up the voltage. The grid-tied systems use a boost type converter to step up the output voltage to the utility level before the inverter stage.

   2. Control Techniques

      The MPPT algorithm gives information MPPT controller how to find the operating voltage by varying duty cycle. Then, job of MPPT controller to operate the PV panel voltage to a required level and maintain it. There are several methods often used for MPPT.

      1. PI control

         MPPT takes measurement of PV voltage and current, and then tracking algorithm calculates the reference voltage (Vref) where the PV operating voltage should move next.

      2. Direct control

         This control method is simpler and uses only one control loop, and it performs the directly adjustment of PWM duty cycle within the MPP tracking algorithm. The way how to adjust the duty cycle is totally based on which algorithm is preferred?

      3. Output sensing control

         The system usually requires another set of sensors for the output to detect the over voltage and over-current condition of load. This output sensing method measures the power change of PV at the output side of converter and uses the duty cycle as a control variable. This control method employs the P&O algorithm to locate the MPP.

   3. MPPT Methods

      There are a various number of algorithms that are able to track MPPs. Some of them are simple, such as those based on voltage and current feedback, and some are more complicated, such as perturbation and observation (P&O) or the incremental conductance (IncCond) method. [5][6]. On the other hand, some MPPTs are more rapid and accurate and, thus, more impressive, which need special design and familiarity with specific subjects such as fuzzy logic [7] or

      neural network [8] methods. MPPT fuzzy logic controller shave good performance under varying atmospheric conditions and exhibit better performance than the P&O control method [8]

      TABLE I. COMPARISION OF MPPT TECHNIQUES

      MPPT Techniques	Speed	Complexity	Reliability	Implementatio
      Fractional Isc	Medium	Medium	Low	Digital / Analog
      Fractional Voc	Medium	Low	Low	Digital / Analog
      IncCond	Varies	Medium	Medium	Digital
      P & O	Various	Low	Medium	Digital / Analog
      Fuzzy Logic	Fast	High	Medium	Digital

   4. Direct Method

   Normally MPPT systems have two independent control loops to control the MPPT. The first control loop contains the constant switching frequency and another MPPT algorithm with proportional (P) or Pintegral (PI) controller. By use of any one of the algorithm method to generate an error signal to adjust duty cycle, which is will operate at the MPP; however, it is not operate at most of the operating points. For ease of operation, and ease of design, inexpensive maintenance, and low cost made controllers very popular in most linear systems.

   However, the MPPT of standalone PV system is a nonlinear control problem due to the nonlinearity nature of voltage and current characteristics and unpredictable environmental conditions, and hence, PI controllers do not generally work well [7].

   In this paper, the direct control method is selected. The PI control loop, and constant frequency switching loop is eliminated, and the PWM of duty cycle is adjusted directly from algorithm controller. TL494 based converter has inbuilt two high gain error amplifier. One error amplifier used for maintain constant output voltage and another to limit output overcurrent. The objective of this paper is to eliminate the first control loop and to show that complicated MPPT methods do necessarily obtain the best results.

3. DESIGN OF PROPER CONVERTER

   When proposing an MPPT tracker, the aim of work first to choose and design a highly efficient and fast response converter, MPPT is supposed to operate as the main part system. Normally high frequency switching-mode power supplies are well designed.

   Among all the topologies available, buckboost converters operates either higher or lower output voltage compared with the input voltage. This paper proposed the push pull buck-boost converter configuration is higher frequency isolated dcdc converters used. It can also provide a better efficiency and isolation. Thus, the proper converter is employed in designing of MPPT controller.

   The power circuit of this paper consists of a Push pull converter based on TL494 control circuit. The control tasks involve measuring the analog voltage and current of the PV

   module using current and voltage sensors, the analogue information from sensors convert them to digital using an ADC, process the present information in a programmed microcontroller with defined algorithm, and generates a valid analogue voltage for adjusting duty cycle of converter at maximum power level and the main program continues to track the MPPs.

   This paper choose push-pull high frequency converter using TL494 as PWM generator. The output from programmed Mpp algorithm Microcontroller applied to Pin 4 of TL494 dead time control (DTCON), this varies duty cycle which is function of algorithm. For Design of proper converter we select the solar panel voltage of 75Wp, 1000W/m2.

   Design of Push Pull Converter using TL494

   1. Ferrite Core transformer Design

      To design of high frequency push pull converter first step to design a ferrite core center tap transformer. The proposed converter is a ferrite core based center tap transformer the turns calculations considered during design of converter Table II shows electrical parameters considered while design of dc/dc converter.

      TABLE II. ELECTRICAL PARAMETERS

      Vin(min) minimum input voltage	13.5 Volts
      Vin(max) maximum input voltage	20.5 Volts
      Vin(nom) nominal voltage	17 Volts
      Switching frequency	22000Hz
      Vout Maximum output voltage	28Volts
      Iout Maximum output current	4 Amp

      To design a high frequency ferrite core transformer.

      voltage to transformer = 0.98 * 13.5V = 13.23V. So, voltage ratio (secondary: primary) = 28V:13.23V = 2.1

      Hence voltage ratio (secondary: primary) = 2.1, turns ratio (secondary: primary) must also be 2.1 as turns ratio (secondary: primary) = voltage ratio (secondary: primary). Turns ratio is designated by N. So, in our case, N = 2.1.

      Npri = 11

      Nsec = N * Npri = 2.1 * 11 Nsec = 22

   2. PWM Circuit And Feedback Control Circuit

   For the generation of PWM signal and feedback control signal use TL494 control IC. TL494 control IC is Complete Pulse-Width Modulation (PWM) Power-Control Circuitry, Uncommitted Outputs for Single-Ended or Push-Pull Application. Fig. 2 show a practical push pull converter, interface between PV module and the Load used as power stage. The power circuit of the proposed system consists of a push-pull converter, TL494, and feedback components. The output of TL494 in push pull mode inverted by using NOR gate input is connected to MOSFET gate terminal. The control of the switching is done using the voltage and current feedback control circuit and also from DTCON pin which is control from MPP algorithm. Output power changes 100% to minimum for change of voltage at DTCON pin 0 to 3.3V adjust. The design of proposed push pull converter used components selected from design equations are referred from its datasheet by manufacturer Texas Instruments. Table III shows a list of used components.

   TABLE III. COMPONENT LIST

   Item Quantity Reference Part

   1	2	C1,C2	2200UF
   2	1	C3	2200UF,50V
   3	1	C4	100UF
   4	1	C5	104
   5	2	C6,C8	1UF
   6	1	C7	103
   7	1	C9	10U
   8	1	C10	2200U
   9	1	C11	22OOU
   10	2	C15,C12	1000uf
   11	1	C13	0.01uf
   12	1	C14	CAP
   13	2	D1,D8	1n4007
   14	2	D3,D2	IRF3205/TO
   15	4	D4,D5,D6,D7 DIODE SCHOTTKY
   16	2	U1,U4 LM7808/TO
   17	1	U2 4001
   18	1	U3 TL494I/SO

   The formula for calculating the number of required primary and secondary turns as:

   .10.8

   Npri =

   4.. .

   ———(1)

   Npri means number of primary turns; Nsec means number of secondary turns

   Where,

   Vin (nom) Nominal Input Voltage.

   f The operating switching frequency in Hertz. Bmax Maximum flux density in Gauss.

   1.25 for ETD39.

   19 1 L1 1mh INDUCTOR

   Now, obtain the values of all required parameters for calculation Npri the number of required primary turns.

   From above equation Npri = 11

   The output of our DC-DC converter is 28V considered. Transformer output must be 28V at all input voltages, i.e. input from 20.5V to all the way down to 13.5V. For the PWM controller, we will take maximum duty cycle to be 98%.

   20 2 R1,R7 10

   22	1	R3	RESISTOR
   23	1	R4	51K
   24	1	R5	200
   25	2	R8,R9	4k7
   26	1	R10	1K
   27	1	R13	50K
   28	2	R16,R14	O.1E,5W
   29	1	T1	ETD39

   21 5 R2,R6,R11,R12,R15 10K

   At minimum input voltage (when Vin = Vinmin), duty cycle will be maximum. Thus duty cycle will be 98% when Vin = 13.5 = Vinmin. At maximum duty cycle = 98%,

   JP1

   2

   1

   2200UF,50V

   C1 C2 C3

   PV_INPUT

   U1 LM7808/TO

   GND

   1 VIN

   VOUT 3

   D1

   1n4007

   R1

   14

   10 IRF3205/TO

   TRANSFORMER POWER

   2

   C5 104

   C4 100UF

   R2

   R3

   U2A

   1

   3

   2

   7

   14

   4001

   D2 1 T1

   2

   16

   2

   15

   1 3 14

   13

   3 3

   4

   iout

   3

   R5 200

   R4 51K

   v out

   10K

   U2B

   5

   4

   6

   7

   4001

   5 12

   D3 6 11

   1 7 10

   2

   8 9

   2 R6

   1

   10K

   R8 4K7

   16 In2+

   U3

   In2- 15

   FBK 3

   In1- 2

   OC

   Ref Out

   DTC

   1

   In1+

   CT RT

   12

   VCC

   C1 8

   GND E1 E2

   11

   C2

   TL494I/SO

   R7 10

   D4 DIODE SCHOTTKY L1

   dcout

   JP2

   C6

   1uf

   U4

   R9

   4k7

   13

   C8 1UF

   J1 1

   C9 10U

   R12 10K

   C7

   5

   6

   1

   103

   14

   4

   R10 1K

   R13 50K

   2

   D5

   7

   9

   10

   C10 2200U

   D6

   D7

   R14

   C11 22OOU2

   1

   R15 10K

   R11 v out 10K

   1

   2

   3

   DC OUT

   LM7808/TO

   GND

   1 VIN

   VOUT 3

   DTCON

   2

   C12 C13

   D8

   3

   1n4007

   power

   iout

   R16

   O.1E,5W C14

   dcout

   1000uf

   0.01uf

   C15

   1000uf

   CAP

   Fig 2. Experimental DC-DC Converter

4. EXPERIMENTAL SETUP

To verify working and performance of the proposed dc/dc converter shown in Fig. 2, a prototype of the isolated push pull converter and voltage and current limit control circuit was implemented. The variable small dc source is used at place of mppt algorithm or controller, it provide the control signals for the TL494 based converter so as to control the PWM signal directly.

The Fig. 3 shows the practical closed-loop system of MPPT system, which includes the PV module, the DC-DC boost converter, and the MPPT algorithm. To test the system operation, the situation of changing voltage at pin 4 of TL494 dead time control (DTCON pin). The change of PWM duty cycle respectively converter output power varies. Voltage level is varying between 0 to 3.3V levels. Table III shows the practical values we have tested on various voltage levels at pin 4 DTCON of TL494.

Fig. 3 shows the practical closed-loop system of MPPT

Fig 5, 6, 7 shows the duty cycle change when change in DTCON voltage. Duty cycle will vary by voltage on pin DTCON of TL494. Digital storage oscilloscope is connected to CH-1 at output of gate circuit and CH-2 at pin 4, which are absolutely the desired output power. The purpose of this paper is directly control dc-dc converter by MPPT algorithm controller.

TABLE IV.

PWM CHANGE ON DTCON VOLTAGE LEVEL

Fig. 4 Graph of duty cycle Vs DTCON reference voltage at pin 4 of TL494

TABLE V.

VARIOUS LOAD CONDITIONS INPUT OUTPUT PARAMETERS

Fig. 5 for DTCON voltage Maximum

Fig. 6 for DTCON 1.50V

Fig. 7 for DTCON 0.490V

CONCLUSION

In this paper, a Dc to Dc Converter for MPPT for direct control method was employed, and the needed another control loop was eliminated. The proposed system was constructed, and the working of the suggested direct control concept was proven. The obtained results during the practical experiments, it was confirmed that, with a well-designed system including a proper converter and selecting an efficient, the implementation of MPPT is simple and can be easily constructed to achieve an acceptable efficiency level of the PV modules. The results shown in table IV and Fig. 4, 5, 6 and 7 demonstrates that the proposed control system is capable of tracking the PV array maximum power all the time by use of any algorithm and thus improves the efficiency of the PV system and reduces low power loss and system cost.

ACKNOWLEDGMENT

The authors would like to thank the authorities of KIT Collage of Engineering, Kolhapur for providing facilities to carry out the research work.

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