MPPT based Solar Photo-Voltaic Module

A mini project submitted in partial fulfillment of the requirements for the Degree of

BACHELOR OF TECHNOLOGYIn

Electrical Engineering By

Durgesh Pandey(1101020019)

Bhalendu Tiwari(1101020015)

Avinash Pathak(1101020012)

Prateekchhit Pandey(1101020033)

Under the supervision ofMr. Ashish Tripathi

United college of engineering and research

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TABLE OF CONTENTS

S.no. Name of content Page no.

1. Abstract -------------------------- 4

2. Introduction -------------------------- 5

3. Literature survey -------------------------- 6

4. Objective -------------------------- 7

5. Solar energy -------------------------- 7

6. Distribution of solar radiation -------------------------- 7

7. Photovoltaic cell -------------------------- 8

8. Photovoltaic array -------------------------- 9

9. Photovoltaic module -------------------------- 9

10. Modeling of solar cell -------------------------- 10

11. Effect of variation of solar radiation -------------------------- 11

12. Effect of variation of temperature -------------------------- 13

13. MPPT algorithms -------------------------- 14

13.1 An overview of MPPT -------------------------- 14

13.2 Different MPPT techniques -------------------------- 14

13.3 Implemented methods -------------------------- 14

13.3.1 Perturb and observe method -------------------------- 14

13.3.2 Incremental conductance method -------------------------- 15

13.4 Other methods -------------------------- 15

13.4.1 Parasitic capacitance -------------------------- 15

13.4.2 Voltage control maximum point -------------------------- 16

Tracker

13.4.3 Current controlled maximum power -------------------------- 16

Point tracker

14. Buck converter -------------------------- 17

14.1 Inductor selection -------------------------- 18

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14.2 Output capacitor selection -------------------------- 19

14.3 Input capacitor selection -------------------------- 21

14.4 Diode selection -------------------------- 22

14.5 MOSFET selection -------------------------- 23

15. Software options -------------------------- 25

15.1 Reasons of using PSPICE -------------------------- 25

16. PSPICE -------------------------- 26

17. PSpice circuit model -------------------------- 26

18. Simulation results -------------------------- 29

19. Component used -------------------------- 33

19.1 Resistor -------------------------- 33

19.2 Capacitor -------------------------- 33

19.3 PWM -------------------------- 33

19.4 BJT -------------------------- 33

19.5 MOSFET -------------------------- 34

19.6 Free wheeling diode -------------------------- 34

19.7 Zener diode -------------------------- 34

20. References -------------------------- 36

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1. ABSTRACT

Renewable energy sources play an important role in electricity generation. Various

renewable energy sources like wind, solar, geothermal, ocean thermal and biomass can be

used for generation of electricity and for meeting our daily energy needs. Energy from

the sun is the best option for electricity generation as it is available everywhere and is

free to harness. On an average the sunshine hour in India is about 6hrs annually also the

sun shine shines in India for about 9 months in a year. Electricity from the sun can be

generated through the Solar Photovoltaic Modules (SPV). The SPV comes in various

power output to meet the load requirement. Maximization of power from a solar photo

voltaic module (SPV) is of special interest as the efficiency of the SPV module is very

low.

A maximum Power Tracker is used for extracting the maximum power from the SPV

module. The present work describes the Maximum Power Point Tracker (MPPT) for the

SPV module connected to a battery which is used as a load. A Microcontroller is to be

used for control of the MPPT algorithm. Maximum power point tracking (MPPT) is used

in photovoltaic (PV) systems to maximize the photovoltaic array output power,

irrespective of the temperature and irradiation conditions and of the load electrical

characteristics. A new MPPT system is to be developed, consisting of a Buck-type dc/dc

converter, which is controlled by a microcontroller-based unit. The resulting system has

high-efficiency, lower-cost and can be easily modified to handle more energy sources

(e.g., wind-generators).

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2. INTRODUCTION

Solar panels generate power by using the photovoltaic effect: electrons are transferred

between different energy bands in the atom by means of irradiation. The solar panel has a

characteristic p-v characteristic where a global maximum is present. This means that for a

different operating point of the solar panel, a different output power is obtained. The

maximum power is obtained when the solar panel operates at the voltage where the

global maximum of the p-v characteristic is present. Therefore, only for one specific

operating point, the maximum power output is obtained from the solar panel. This point

in the p-v characteristic is called the Maximum Power Point (MPP). This MPP changes

when the irradiation and temperature changes or when the solar panel is partially shaded.

To track the constantly changing MPP a device is needed, this device is called the

Maximum Power Point Tracker (MPPT). The MPPT consists of two main parts, a

microcontroller to track the MPP and a converter to convert the generated voltage to a

desired level for the load. An algorithm runs on the microcontroller to track the MPP.

There are a lot of different algorithms to track the MPP, but they all do not work in fast

changing levels of irradiance or when the solar panel is partially shaded. This is a

problem for us, so it is important for us that the MPP is tracked in an environment where

there are fast changing levels of irradiance and the solar panel is partially shaded. In this

thesis, we explore different options to solve these problems. Our goal is to implement the

most efficient algorithm that works in fast changing levels of irradiance and when the

solar panels are partially shaded. Especially the efficiency of the algorithm is important,

because we want to make an MPPT with a very high efficiency. Furthermore, the

implementation complexity of the algorithm should not be too high and it must be

executable on a microcontroller.

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3. LITERATURE SURVEY

The topic of solar energy utilization has been looked upon by many researchers all

around the globe. It has been known that solar cell operates at very low efficiency and

thus a better control mechanism is required to increase the efficiency of the solar cell.

In this field researchers have developed what are now called the Maximum Power

Point Tracking (MPPT) algorithms.

Mummadi Veerachary has given a detailed report on the use of a SEPIC converter in

the field of photovoltaic power control. In his report he utilized a two-input converter

for accomplishing the maximum power extraction from the solar cell.

M. G. Villalva in his both reports has presented a comprehensive method to model a

solar cell using Simulink or by writing a code. His results are quite similar to the

nature of the solar cell output plots.

P. S. Revankar has even included the variation of sun’s inclination to track down the

maximum possible power from the incoming solar radiations. The control mechanism

alters the position of the panel such that the incoming solar radiations are always

perpendicular to the panels.

M. Berrera has compared seven different algorithms for maximum power point

tracking using two different solar irradiation functions to depict the variation of the

output power in both cases using the MPPT algorithms and optimized MPPT

algorithms.

Ramos Hernanz has successfully depicted the modeling of a solar cell and the

variation of the current-voltage curve and the power-voltage curve due the solar

irradiation changes and the change in ambient temperature.

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4. OBJECTIVE

The basic objective would be to study MPPT and successfully implement the MPPT

algorithms either in code form or using the Simulink models. Modeling the converter and

the solar cell in Simulink and interfacing both with the MPPT algorithm to obtain the

maximum power point operation would be of prime importance.

5. SOLAR ENERGY

It is a non-conventional type of energy. Solar energy has been harnessed by humans since

ancient times using a variety of technologies. The secondary solar-powered resources

such as tidal wave and wind power, hydro power and biomass, are account for most of

the readily available non-conventional type of energy source on earth. But only a small

fraction of the available solar energy is used. Solar powered electrical generation relies

on photovoltaic system and heat engines. Solar energy's uses are limited by human

creativity. To harvest the solar energy, the most common way is to use photovoltaic

panels which will receive photon energy from sun and convert it into electrical energy.

Solar technologies are broadly classified as either passive solar or active solar energy

depending on the way the solar energy may detain, convert and distributed. Active solar

techniques include the use of PV panels and solar thermal collectors to strap up solar

energy. Passive solar techniques include orientation and selecting materials with

favorable thermal mass or light dispersing properties and design spaces that naturally

circulate air .Solar energy has a vast area of application such as electricity generation for

distribution, heating ,water pumping, lightening building, crop drying etc.

6. DISTRIBUTION OF SOLAR RADIATION

The solar radiation receives by earth is about 174 peta watts (PW) of incoming solar

radiation at the upper atmosphere orbit and Approximately 30% is reflected back to space

and only 89 PW is absorbed by oceans and land masses. The spectrum of solar light at the

Earth's surface is generally spread across the visible and near-infrared reason with a small

part in the near-ultraviolet. The total solar energy absorbed by Earth's atmosphere, oceans

and land masses is approximately 3,850,000 EJ per year.

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7. PHOTOVOLTAIC CELL

In 1954 Bell labs Chopin, Fuller, Pearson fabricated PV cell with efficiency of 6%.In

1958 PV cell was used as a backup power source in satellite Vanguard-1. This extended

the life of satellite for about 6 years .A photovoltaic cell is the basic device that converts

solar radiation into electricity which is made of semiconductor materials, such as silicon.

For solar cells, a thin semiconductor wafer is specially treated to form an electric field,

positive on one side and negative on the other. When light energy strikes the solar cell,

electrons are knocked loose from the atoms in the semiconductor material. If electrical

conductors are attached to the positive and negative sides, forming an electrical circuit,

the electrons can be captured in the form of an electric current that is, electricity. This

electricity can then be used to power a load. A PV cell can either be circular or square in

construction.

1. Photovoltaic cell

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8. PHOTOVOLTAIC ARRAY

The power that one module can produce is not sufficient to meet the requirements of

home or business. Most PV arrays use an inverter to convert the DC power into

alternating current that can power the motors, loads, lights etc. The modules in a PV array

are usually first connected in series to obtain the desired voltages, the individual modules

are then connected in parallel to allow the system to produce more current.

2. Photovoltaic array

9. PHOTOVOLTAIC MODULE

Cells are arranged in a frame to form a module. The several PV cells are connected in

series (for high voltage) and in parallel (for high current) to form a PV module for desired

output. Separate diodes may be needed to avoid reverse currents, in case of partial or total

shading, and at night. The p-n junctions of mono-crystalline silicon cells may have

adequate reverse current characteristics and these are not necessary. Reverse currents

waste power and can also lead to overheating of shaded cells. Solar cells become less

efficient at higher temperatures and installers try to provide good ventilation behind solar

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panels .Each PV cell is rated for 0.5 –0.7 volt and a current of 30mA/cm2. Based on the

manufacturing process they are classified as:

Poly crystalline: efficiency of 12%.

Amorphous: efficiency of 6-8%

Life of crystalline cells is in the range of 25 years

Whereas for amorphous cells it is in the range of 5years

10. MODELLING OF SOLAR CELL

A solar cell is the building block of a solar panel. A photovoltaic module is formed by

connecting many solar cells in series and parallel. Considering only a single solar cell; it

can be modeled by utilizing a current source, a diode and two resistors. This model is

known as a single diode model of solar cell. Two diode models are also available but

only single diode model is considered here.

3. Single diode model of a solar cell

In this model we consider a current source (I) along with a diode and series resistance

(Rs). The shunt resistance (RSH) in parallel is very high, has a negligible effect and can

be neglected.

The output current from the photovoltaic array is

I=Isc – Id

Id= Io (eqVd/kT - 1)

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Where Io is the reverse saturation current of the diode, q is the electron charge, Vd is the

voltage across the diode, k is Boltzmann constant (1.38 * 10-19 J/K) and T is the junction

temperature in Kelvin (K)

From eq. 3.1 and 3.2

I = Isc – Io (eqVd/kT - 1)

Using suitable approximations,

I = Isc – Io (eq((V+IRs)/nkT) - 1)

Where, I is the photovoltaic cell current, V is the PV cell voltage, T is the temperature (in

Kelvin) and n is the diode ideality factor In order to model the solar panel accurately we

can use two diode model but in our project our scope of study is limited to the single

diode model. Also, the shunt resistance is very high and can be neglected during the

course of our study.

an

4. P-V I-V curve of a solar cell at given temperature d solar irradiation

11. EFFECT OF VARIATION OF SOLAR IRRADIATION

The P-V and I-V curves of a solar cell are highly dependent on the solar irradiation

values. The solar irradiation as a result of the environmental changes keeps on

fluctuating, but control mechanisms are available that can track this change and can alter

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the working of the solar cell to meet the required load demands. Higher is the solar

irradiation, higher would be the solar input to the solar cell and hence power magnitude

would increase for the same voltage value. With increase in the solar irradiation the open

circuit voltage increases. This is due to the fact that, when more sunlight incidents on to

the solar cell, the electrons are supplied with higher excitation energy, thereby increasing

the electron mobility and thus more power is generated.

5. Variation of P-V curve with solar irradiation

6. Variation of I-V curve with solar irradiation

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12. EFFECT OF VARIATION OF TEMPERATURE

On the contrary the temperature increase around the solar cell has a negative impact on

the power generation capability. Increase in temperature is accompanied by a decrease in

the open circuit voltage value. Increase in temperature causes increase in the band gap of

the material and thus more energy is required to cross this barrier. Thus the efficiency of

the solar cell is reduced.

7. Variation of P-V curve with temperature

8. Variation of I-V with temperature

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13. MAXIMUM POWER POINT TRACKING ALGORITHMS

13.1 An overview of Maximum Power Point Tracking

A typical solar panel converts only 30 to 40 percent of the incident solar irradiation into

electrical energy. Maximum power point tracking technique is used to improve the

efficiency of the solar panel.

According to Maximum Power Transfer theorem, the power output of a circuit is

maximum when the Thevenin impedance of the circuit (source impedance) matches with

the load impedance. Hence our problem of tracking the maximum power point reduces to

an impedance matching problem.

In the source side we are using a boost convertor connected to a solar panel in order to

enhance the output voltage so that it can be used for different applications like motor

load. By changing the duty cycle of the buck converter appropriately we can match the

source impedance with that of the load impedance.

13.2 Different MPPT techniques

Different algorithms help to track the maximum power point of the solar pv module

automatically. The various algorithms used are:

a) Perturb and Observe.

b) Incremental Conductance.

c) Parasitic Capacitance.

d) Voltage Based Peak Power Tracking.

e) Current Based peak power Tracking

13.3 Implemented Method

13.3.1 Perturb and Observe method - In this algorithm a slight perturbation is

introduced in the system. Due to this perturbation the power of the module changes. If the

power increases due to the perturbation then the perturbation is continued in that

direction. After the peak power is reached the power at the next instant decreases and

hence after that the perturbation reverses.

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When the steady state is reached the algorithm oscillates around the maximum point. In

order to keep the power variation small the perturbation size is kept very small. The

algorithm is developed in such a manner that it sets a reference voltage of the module

corresponding to the maximum voltage of the module. A Microcontroller then acts

moving the operating point of the module to that particular voltage level. It is observed

that there some power loss due to this perturbation also the fails to track the power under

fast varying atmospheric conditions. But still this algorithm is very popular and simple.

13.3.2 Incremental conductance method

The incremental conductance can determine that the MPPT has reached the MPP and

stop perturbing the operating point. If this condition is not met, the direction in which the

MPPT operating point must be perturbed can be calculated using the relationship between

dl/dV and - I/V. This relationship is derived from the fact that dP/dV is negative when the

MPPT is to the right of the MPP and positive when it is to the left of the MPP. This

algorithm has disadvantages over perturb and observe in that it can determine when the

MPPT has reached the MPP, where perturb and observe oscillates around the MPP. Also,

incremental conductance can track rapidly increasing and decreasing irradiance

conditions with higher accuracy than perturb and observe. One disadvantage of this

algorithm is the increased complexity when compared to perturb and observe method.

13.4 Others Method

13.4.1 Parasitic capacitances: - The parasitic capacitance method is a refinement of

incremental conductance method that takes into account the parasitic capacitances of the

solar cells in the PV array. Parasitic capacitance uses the switching ripple of the MPPT to

perturb the array. To account for the parasitic capacitance, the average ripple in the array

power and voltage, generated by the switching frequency, are measured using a series of

filters and multipliers and then used to calculate the array conductance. The incremental

conductance algorithm is then used to determine the direction to move the operating point

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of the MPPT. One disadvantage of this algorithm is that the parasitic capacitance in each

module is very small, and will only come into play in large PV arrays where several

module strings are connected in parallel. Also, the DC-DC converter has a sizable input

capacitor used filter out small ripple in the array power. This capacitor may mask the

overall effects of the parasitic capacitance of the PV array.

13.4.2 Voltage control maximum point tracker: - It is assumed that a maximum power

point of a particular solar PV module lies at about 0.75 times the open circuit voltage of

the module. So by measuring the open circuit voltage a reference voltage can be

generated and feed forward voltage control scheme can be implemented to bring the solar

pv module voltage to the point of maximum power. One problem of this technique is the

open circuit voltage of the module varies with the temperature. So as the temperature

increases the module open circuit voltage changes and we have to measure the open

circuit voltage of the module very often. Hence the load must be disconnected from the

module to measure open circuit voltage. Due to which the power during that instant will

not be utilize.

13.4.3 Current control maximum power point tracker:- The maximum power of the

module lies at the point which is at about 0.9 times the short circuit current of the

module. In order to measure this point the module or array is short-circuited. And then by

using the current mode control the module current is adjusted to the value which is

approx 0.9 times the short circuit current. The problem with this method is that a high

power resistor is required which can stain the short-circuit current. The module has to be

short circuited to measure the short circuit current as it goes on varying with the changes

in isolation level.

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14. BUCK-CONVERTER

9. Buck Converter Power Stage

Step down (buck) switching converters are integral to modern electronics. They can

convert a voltage source (typically 8 V to 25 V) into a lower regulated voltage (typically

0.5 V to 5 V). Step down converters transfer small packets of energy using a switch, a

diode, an inductor and several capacitors. Though substantially larger and noisier than

their linear-regulator counterparts, buck converters offer higher efficiency in most cases.

Despite their widespread use, buck-converter designs can pose challenges to both novice

and intermediate power-supply designers because almost all of the rules of thumb and

some of the calculations governing their design are hard to find. And though some of the

calculations are readily available in IC data sheets, even these calculations are

occasionally reprinted with errors. In this article, all of the design information required to

design a buck converter is conveniently collected in one place.

Buck-converter manufacturers often specify a typical application circuit to help engineers

quickly design a working prototype, which in turn often specifies component values and

part numbers. What they rarely provide is a detailed description of how the components

are selected. Suppose a customer uses the exact circuit provided. When a critical

component becomes obsolete or a cheaper substitute is needed, the customer is usually

without a method for selecting an equivalent component.

This article covers only one step down regulator topology — one with a fixed switching

frequency, pulse width modulation (PWM) and operation in the continuous-current mode

(CCM). The principles discussed can be applied to other topologies, but the equations do

not apply directly to other topologies. To highlight the intricacies of step-down converter

design, we present an example that includes a detailed analysis for calculating the various

17

component values. Four design parameters are required: input-voltage range, regulated

output voltage, maximum output current and the converter's switching frequency.

14.1 INDUCTOR SELECTION

Calculating the inductor value is most critical in designing a step down switching

converter. First, assume the converter is in CCM, which is usually the case. CCM implies

that the inductor does not fully discharge during the switch-off time. The following

equations assume an ideal switch (zero on-resistance, infinite off-resistance and zero

switching time) and an ideal diode:

Where fSW is the buck-converter switching frequency and LIR is the inductor-current ratio

expressed as a percentage of IOUT (e.g., for a 300-mAp-p ripple current with a 1-A output,

LIR = 0.3 A/1 A = 0.3 LIR).An LIR of 0.3 represents a good tradeoff between efficiency

and load-transient response. Increasing the LIR constant — allowing more inductor ripple

current — quickens the load-transient response, and decreasing the LIR constant —

thereby reducing the inductor ripple current — slows the load-transient response.  

Peak current through the inductor determines the inductor's required saturation-current

rating, which in turn dictates the approximate size of the inductor. Saturating the inductor

core decreases the converter efficiency, while increasing the temperatures of the inductor,

the MOSFET and the diode. You can calculate the inductor's peak operating current as

follows:

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These equations yield a calculated inductance of 2.91 µH (LIR = 0.3). Select an available

value that is close to the calculated value, such as a 2.8 µH, and make sure that its

saturation-current rating is higher than the calculated peak current (IPEAK = 8.09 A).

Choose a saturation-current rating that's large enough (10 A in this case) to compensate

for circuit tolerances and the difference between actual and calculated component values.

An acceptable margin for this purpose, while limiting the inductor's physical size, is 20%

above the calculated rating.

Inductors of this size and current rating typically have a maximum dc resistance range

(DCR) of 5 mΩ to 8 mΩ. To minimize power loss, choose an inductor with the lowest

possible DCR. Although data sheet specifications vary among vendors, always use the

maximum DCR specification for design purposes rather than the typical value, because

the maximum is a guaranteed worst-case component specification.

Selected inductor = 1 mH.

14.2 OUTPUT CAPACITOR SELECTION

Output capacitance is required to minimize the voltage overshoot and ripple present at the

output of a step-down converter. Large overshoots are caused by insufficient output

capacitance, and large voltage ripple is caused by insufficient capacitance as well as a

high equivalent-series resistance (ESR) in the output capacitor. The maximum allowed

output-voltage overshoot and ripple are usually specified at the time of design. Thus, to

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meet the ripple specification for a step down converter circuit, you must include an

output capacitor with ample capacitance and low ESR.

The problem of overshoot, in which the output-voltage overshoots its regulated value

when a full load is suddenly removed from the output, requires that the output capacitor

be large enough to prevent stored inductor energy from launching the output above the

specified maximum output voltage. Output-voltage overshoot can be calculated using the

following equation:

Rearranging Eq. 2 yields:

Where CO equals output capacitance and ΔV equals maximum output-voltage overshoot.

Setting the maximum output-voltage overshoot to 100 mV and solving Eq. 3 yields a

calculated output capacitance of 442 µF. Adding the typical capacitor-value tolerance

(20%) gives a practical value for output capacitance of approximately 530 µF. The

closest standard value is 560 µF. Output ripple due to the capacitance alone is given by:

ESR of the output capacitor dominates the output-voltage ripple. The amount can be

calculated as follows:

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Be aware that choosing a capacitor with very low ESR may cause the power converter to

be unstable. The factors that affect stability vary from IC to IC, so when choosing an

output capacitor, be sure to read the data sheet and pay special attention to sections

dealing with converter stability.

Adding the output-voltage ripple due to capacitance value (the first term in Eq. 4) and the

output-capacitor ESR (the second term in Eq. 4) yields the total output-voltage ripple for

the step-down converter:

A decent step down converter usually achieves an output-voltage ripple of less than 2%

(40 mV in our case). For a 560-µF output capacitance, Eq. 5 yields 19.8 mΩ for the

maximum calculated ESR. Therefore, choose a capacitor with ESR that's lower than 19.8

mΩ and a capacitance that's equal to or greater than 560 µF. To achieve an equivalent

ESR value less than 19.8 mΩ, you can connect multiple low-ESR capacitors in parallel.

14.3 Input Capacitor Selection

The input capacitor's ripple-current rating dictates its value and physical size, and the

following equation calculates the amount of ripple current the input capacitor must be

able to handle:

Plots ripple current for the capacitor (shown as a multiple of the output current) against

the input voltage of the buck converter (shown as a ratio of output voltage to input

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voltage). The worst case occurs when VIN = 2VOUT (VOUT/VIN = 0.5), yielding IOUTMAX /

2 for the worst-case ripple-current rating.

The input capacitance required for a step-down converter depends on the impedance of

the input power source. For common laboratory power supplies, 10 µF to 22 µF of

capacitance per ampere of output current is usually sufficient. Given the design

parameters you can calculate the input-ripple current as 3.16 A. You then can start with

40 µF in total input capacitance and can adjust that value according to subsequent test

results.

Tantalum capacitors are a poor choice for input capacitors. They usually fail “short,”

meaning the failed capacitor creates a short circuit across its terminals and thereby raises

the possibility of a fire hazard. Ceramic or aluminum-electrolytic capacitors are preferred

because they don't have this failure mode.

Ceramic capacitors are the better choice when pc-board area or component height is

limited, but ceramics may cause your circuit to produce an audible buzz. This high-

pitched noise is caused by physical vibration of the ceramic capacitor against the pc

board as a result of the capacitor's ferroelectric properties and piezo phenomena reacting

to the voltage ripple. Polymer capacitors can alleviate this problem. Polymer capacitors

also fail short, but they are much more robust than tantalums, and therefore are suitable

as input capacitors.

14.4 DIODE SELECTION

Power dissipation is the limiting factor when choosing a diode. The worst-case average

power can be calculated as follows:

Where VD is the voltage drop across the diode at the given output current IOUT MAX.

(Typical values are 0.7 V for a silicon diode and 0.3 V for a Schottky diode.) Ensure that

the selected diode will be able to dissipate that much power. For reliable operation over

the input-voltage range, you must also ensure that the reverse-repetitive maximum

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voltage is greater than the maximum input voltage (VRRM ≥ VINMAX). The diode's

forward-current specification must meet or exceed the maximum output current (i.e.,

IFAV ≥ IOUTMAX).

14.5 MOSFET SELECTION

Selecting a MOSFET can be daunting, so engineers often avoid that task by choosing a

regulator IC with an internal MOSFET. Unfortunately, most manufacturers find it cost

prohibitive to integrate a large MOSFET with a dc-dc controller in the same package, so

power converters with integrated MOSFETs typically specify maximum output currents

no greater than 3 A to 6 A. For larger output currents, the only alternative is usually an

external MOSFET.

The maximum junction temperature (TJMAX) and maximum ambient temperature

(TAMAX) for the external MOSFET must be known before you can select a suitable

device. TJMAX should not exceed 115°C to 120°C and TAMAX should not exceed 60°C.

A 60°C maximum ambient temperature may seem high, but step down converter circuits

are typically housed in a chassis where such ambient temperatures are not unusual. You

can calculate a maximum allowable temperature rise for the MOSFET as follows:

Inserting the values mentioned above for TJMAX and TAMAX into Eq. 7 yields a

maximum MOSFET temperature rise of 55°C. The maximum power dissipated in the

MOSFET can be calculated from the allowable maximum rise in MOSFET temperature:

The type of MOSFET package and the amount of pc-board copper connected to it affect

the MOSFET's junction-to-ambient thermal resistance (ΘJA). When ΘJA is not specified in

the data sheet, 62°C/W serves as a good estimate for a standard SO-8 package (wire-bond

interconnect, without an exposed paddle), mounted on 1 in.2 of 1-oz pc-board copper.

There exists no inverse linear relationship between a ΘjA value and the amount of copper

23

connected to the device, and the benefit of decreasing the ΘJA value quickly dwindles for

circuits that include more than 1 sq in. of pc-board copper. Using ΘJA = 62°C/W in Eq. 8

yields a maximum allowable dissipated power in the MOSFET of approximately 0.89 W.

Power dissipation in the MOSFET is caused by on-resistance and switching losses. On-

resistance loss can be calculated as:

Because most data sheets specify the maximum on-resistance only at 25°C, you may have

to estimate the value of on-resistance at TJHOT. As a rule of thumb, a temperature

coefficient of 0.5%/°C provides a good indicator for maximum on-resistance at any given

temperature. Thus, the hot on-resistance is calculated as:

Assuming the on-resistance loss is approximately 60% of the total MOSFET losses, you

can substitute in Eq. 10 and rearrange to yield Eq. 11, the maximum allowable on-

resistance at 25°C:

Switching losses constitute a smaller portion of the MOSFET's power dissipation, but

they still must be taken into account. The following switching-loss calculation provides

only a rough estimate, and therefore is no substitute for evaluation in the lab, preferably a

test that includes a thermocouple mounted on P1 as a sanity check.

Where CRSS is the reverse-transfer capacitance of P1, IGATE is the peak gate-drive

source/sink current of the controller and P1 is the high-side MOSFET.

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Assuming a gate drive of 1 A (obtained from the gate driver/ controller data sheet) and a

reverse-transfer capacitance of 300 pF (obtained from the MOSFET data sheet), Eq. 11

yields a maximum RDS(ON)25°C of approximately 26.2 mΩ. Recalculating and summing

the on-resistance losses and the switching losses yields a net dissipated power of 0.676

W. Using this figure, you can calculate for the MOSFET a maximum temperature rise of

101°C, which is within the acceptable temperature range.

Necessary Parameters of the Power Stage

The following four parameters are needed to calculate the power stage:

1. Input voltage: VIN = 25V approx.

2. Nominal output voltage: VOUT = 15 V approx

3. Maximum output current: IOUT(max) = 1.5A - 2A

4. Integrated circuit used to build the buck converter. This is necessary because some

parameters for the calculations must be derived from the data sheet. If these parameters

are known, the power stage can be calculated.

15. SOFTWARE OPTIONS

1. MATLAB

2. PSpice

3. Allspice

4. Simulink

15.1 REASON OF USING PSPICE

1. PSpice allows multiple plots to be viewed simultaneously, such as voltage, power,

etc. Also, specific points, such as a voltage at a certain time, can be selected and

marked on the output plot in PSpice

2. PSpice contains libraries full of specific components with manufacturer

specifications. These components are included so the user may obtain realistic

simulation results,

3. Very simple to represent any electrical circuit, in particular power-electronic

circuits and a wide library of commercial electric components are available.

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16. PSPICE

SPICE stands for Simulation Program with Integrated Circuit Emphasis. SPICE is an

analogue (originally) circuit simulator that was developed at the University of California

at Berkeley. PSpice is one of the many commercial SPICE derivatives, and has been

developed by MicroSim Corporation (now taken over by Cadence =>ORCAD).

PSpice's strong point is that it helps the user simulate the circuit design graphically on the

computer before building a physical circuit. Hence, the designer can make any necessary

changes on the prototype without modifying any hardware. As soon as the test design is

completed, PSpice can help one run a check on it before deciding to commit you to

building a hardware model. Hence, PSpice allows one to check the operability of the

circuit model in real life simulations to validate its viability (Nilsson & Riedel). Since all

the tests, designs and modifications are made over a terminal; the designer can save a lot

of money that would have otherwise been spent on the building of models and modifying

them. SPICE is used to fine tune the design process, not to replace it. Although

approximate “first-cut” circuit designs can often be made by hand, an exact analysis of

circuit behavior is sometimes required. A complicated IC design, for example, must be

perfected before it is actually fabricated, since fabrication to accommodate even minor

design changes is costly.

In such situations, SPICE can provide valuable assistance in testing a tentative design

before it is actually fabricated.

17. PSPICE Circuit Model

The following PSPICE model is made for the simulation of the circuit.

It consists of the Buck Converter, whose switching is given through a Common Emitter

amplifier. We use a Voltage Regulator LM7812 to give the Voltage to the Collector of

the amplifier. A Zener Diode D1N5245 (V=12V) is placed across the n-MOS IRFZ10.

n-MOS IRFZ10 has the rating of 60V, 10A.

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10. PSPICE Circuit Model for the Power Circuit

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The MOSFET allows current from the battery to pass through it, but when it allows

current to pass through it is governed by the pulse width modulator (PWM). The PWM

creates pulses, and the high section of these pulses turns on the MOSFET. The longer the

MOSFET is turned on, the more is the output voltage. Thus, by varying the duty cycle, it

is possible to vary the output voltage. Due to the properties of the load and the

characteristics of inductance, a freewheeling diode allows the output current to continue

even when it is not drawing any current from the source. This is illustrated in the timing

diagram below.

11. Load Current Waveform

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18. SIMULATION RESULT

The input voltage is 20V.

0Duty cycle is 0.2

The simulation result is shown in the following graph:

12. Simulation result for Duty Cycle 0.2

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Input Voltage 20V

Duty Cycle 0.8

13. Simulation result for Duty Cycle 0.8

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14.1 Wave form1: Vout2

14.2Waveform 2 : PWM input

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15.1 Waveform 1: Vout1

15.2 Waveform 2: PWM input

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19. COMPONENT USED

19.1 RESISTORS

A Resistor is a two-terminal electronic component designed to oppose an electric current

by producing a voltage drop between its terminals in proportion to the current, that is, in

accordance with Ohm's law:

V = IR

Resistors are used as part of electrical networks and electronic circuits. They are

extremely commonplace in most electronic equipment. Practical resistors can be made of

various compounds and films, as well as resistance wire (wire made of a high-resistivity

alloy, such as nickel/chrome).

19.2 CAPACITORS

Capacitors store electric charge. They are used with resistors in timing circuits because it

takes time for a capacitor to fill with charge. They are used to smooth varying DC

supplies by acting as a reservoir of charge. They are also used in filter circuits because

capacitors easily pass AC (changing) signals but they block DC (constant) signals.

19.3 PULSE WIDTH MODULATOR

The purpose of the pulse width modulator (PWM) is to provide a gating signal to the

MOSFET to turn it on and off. The PWM creates a square pulse whose duty cycle (time

in high state divided by its period) is varied so as to control the output voltage.

19.4 BJT

The current output of the PWM was not enough to properly turn on the MOSFET, so we

added a high current BJT to amplify the current from the PWM. The BJT used is

Q2N2222a.

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19.5 MOSFET

The MOSFET in our circuit acts as a switch. It allows current to flow through it for

certain periods of time. These periods are controlled by the PWM current waveforms that

flow to the gate of the MOSFET. The MOSFET conducts for the high portion of the

gating signal, and does not conduct for the low portion of the gating signal. The higher

he duty cycle of these input waves, the longer the MOSFET acts as a closed switch,

connecting the source to the load. The MOSFET used is IRFZ10.

19.6 FREE WHEELING DIODE

The freewheeling diode has a unique function in the circuit. It ensures that the output

voltage during each "off" time, allotted by the MOSFET, is equal to 0 V. It achieves this

by acting as a sink for the load. That is, when the MOSFET stops conducting, the current

stored in the inductance discharges itself through the load and the diode. The observed

effect is that the load continues operation despite drawing no current from the battery.

The diode used is MUR805.

19.7 ZENER DIODE

The MOSFET in our design is to be protected. It has Zener diodes to limit the voltage

levels between the gate and the source on the MOSFET.

MOSFETS have safe operating ranges for voltage levels that should never be breached.

Zener diodes function to limit the voltage levels between the gate and the source on the

MOSFET, providing a layer of safety for our circuit. The zener diode configuration

shown in the figure below illustrates this simple but very helpful safety addition.

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16. Zener Diode Connection for MOSFET

From our datasheets for our MOSFET, we found that for continuous operation, the gate

to source voltage should not exceed 20 Vdc. By using Zener diode D1N5245, we ensure

that the voltage from gate to source will not exceed 12 V. This safety measure keeps

these voltage levels in check, avoiding damage to the MOSFET.

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20. REFERENCES

1) Eftichios Koutroulis, Kostas Kalaitzakis, Member, IEEE, and Nicholas C.

Voulgaris, “Development of a Microcontroller Based solar Photovoltaic Maximum

Power Point Tracking Control System”.

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 16, NO. 1, JANUARY

2001

2) Chihchiang Hua and Chihming Shen. “Control of DC/DC Converters for Solar

Energy System with Maximum Power Tracking”.

3) Joe-Air Jiang1, Tsong-Liang Huang2, Ying-Tung Hsiao2*and Chia-Hong Chen2,

“Maximum Power Tracking for Photovoltaic Power Systems”.

Department of Bio-Industrial Mechatronics Engineering, National Taiwan University

Taipei, Taiwan 106, R.O.C. Department of Electric Engineering, Tamkang

UniversityTamsui, Taiwan 251, R.O.C.

4) Sachin Jain, Student Member, IEEE, and Vivek Agarwal Senior Member, IEEE

“A New Algorithm for Rapid tracking of Approximate Maximum Power Point in

Photovoltaic systems”. IEEE POWER ELECTRONICS LETTERS, VOL. 2, NO. 1,

MARCH 2004

5) Mohamad A. S. Masoum, Seyed Mahdi Mousavi Badejani, and Ewald F.

Fuchs.IEEE, “Microprocessor-Controlled New Class of Optimal Battery Chargers

for Photovoltaic Applications”.

6) By Donald Schelle and Jorge Castorena, Jorge Castorena, Technical Staff, Technical

Staff, Maxim Integrated Products, Sunnyvale, Calif. "Buck-Converter Design

Demystified."

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7) http://www.ece.uvic.ca/ Project Work Buck Converter Developers: Jason Allan,

Eric Helander

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