MPPT Proposal 248.pdf

8/10/2019 MPPT Proposal 248.pdf

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Solar Maximum Power Point Tracker

ECE4007 Senior Design Project

Section L01, Solar Jackets Maximum Power Point TrackerProject Advisor, Dr. Whit Smith

Andrew Matteson, Team Leader

Giancarlo ValentinTravis Seagart

Ingrid Rodriguez

Submitted

February 21, 2011

Georgia Institute of TechnologyCollege of Engineering

School of Electrical and Computer Engineering


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SJ_MPPT (ECE 4007 L01) 2

Table of Contents

Executive Summary .................................................................................................................................. 31. Introduction ........................................................................................................................................... 4

1.1 Objective ......................................................................................................................................... 4

1.2 Motivation ....................................................................................................................................... 4

1.3 Background ..................................................................................................................................... 51.3.1 Switched-Mode Power Supplies ............................................................................................... 51.3.2 MPPT Algorithms ..................................................................................................................... 61.3.3 Synchronous Rectifier Technology ........................................................................................... 6

2. Project Description and Goals .............................................................................................................. 73. Technical Specifications ....................................................................................................................... 8

3.1 Physical Specifications .................................................................................................................... 83.1.1 Solar Array ................................................................................................................................ 93.1.2 Battery ....................................................................................................................................... 93.1.3 Temperature ............................................................................................................................ 10

3.1.4 Physical Characteristics .......................................................................................................... 10

3.2 Functional Specifications .............................................................................................................. 103.2.1 Microcontroller ....................................................................................................................... 103.2.2 Additions to SPAM functionality ........................................................................................... 11

4. Design Approach and Details ............................................................................................................. 124.1 Design Approach ........................................................................................................................... 12

4.1.1 Solar Cell Characteristics ........................................................................................................ 134.1.2 The Maximum Power Point .................................................................................................... 144.1.3 Implementing MPPT ............................................................................................................... 144.1.4 Algorithm ................................................................................................................................ 154.1.5 Software Development Approach ........................................................................................... 17

4.1.6 Voltage and Current Measurement ......................................................................................... 18

4.1.5 Temperature Measurement ..................................................................................................... 184.1.6 Synchronous Rectifier ............................................................................................................. 18

4.2 Codes and Standards ..................................................................................................................... 194.3 Constraints, Alternatives, and Tradeoffs ....................................................................................... 20

4.3.1 Algorithm Selection ................................................................................................................ 204.3.2 Number of MPPT Circuits per PCB ....................................................................................... 204.3.3 Diode vs. Synchronous Rectifier ............................................................................................ 20

5. Schedule, Tasks, and Milestones ........................................................................................................ 215.1 Schedule and Milestones ............................................................................................................... 215.2 Division of Labor and Accountability ........................................................................................... 22

5.3 Critical Path Method ..................................................................................................................... 22

6. Project Demonstration ........................................................................................................................ 237. Marketing and Cost Analysis.............................................................................................................. 24

7.1 Marketing Analysis ....................................................................................................................... 247.2 Cost Analysis ................................................................................................................................. 24

8. Summary ............................................................................................................................................. 269. References ........................................................................................................................................... 27APPENDIX A ......................................................................................................................................... 30


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Executive Summary

The SJ_MPPT group is requesting $39,740 to provide the Georgia Tech Solar Jackets racing

team with a Maximum Power Point Tracker (MPPT) system. This system will be integrated into the

electrical system of a car that will eventually cross the Australian continent in the 2011 World Solar

Challenge. The SJ_MPPT group will provide a robust system capable of adapting to changes in

sunlight and temperature so that it can deliver maximum power to the cars batteries.

The design will improve upon the initial prototype of the SPAM Fall 2010 group. Target goals

include an increase in efficiency (exceeding 85%), reduced physical dimensions (10.5 in. x 7.5 in for a

board containing two MPPT circuits), the addition of a functional RS-485 interface, and the

implementation of extensive safety mechanisms to protect the hardware and the driver in case of

malfunction.

The final design will be implemented on a printed circuit board (PCB). Each board will consist

of two MPPT circuits controlled by one microcontroller, and will be priced at a cost of $3973 per

board. The associated equipment costs are estimated to be $374 (per unit), and labor costs are

estimated to be $3600. The design will be tested extensively so that it is field-ready and can be mass

produced. Despite customization for the Solar Jackets, the design process will allow for modifications

that may eventually be used to interface the modules with different solar cells and different battery

configurations. The design can thus be customized for near universal applicability in the field of solar-

powered vehicles.


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Solar Maximum Power Point Tracker

1. Introduction

The Solar Jackets Maximum Power Point Tracker (SJ_MPPT) team is requesting $39,740 to

build a maximum power point tracker (MPPT) system for the Solar Jackets racing team. The team will

continue the work of the Solar Power Array Management (SPAM) team from Fall 2010.

1.1 Objective

SJ_MPPT will improve upon the design completed by the SPAM design team from Fall 2010,

who have provided SJ_MPPT with a working prototype with extra components, a complete yet

unverified printed circuit board layout, and thorough documentation. Before this design can be

implemented in the Solar Jackets car, it needs to undergo optimization for efficiency and size, as well

as testing for resiliency. The finished product will be a set of circuit boards implementing MPPT that

can withstand the conditions which the car will face in the competition.

1.2 Motivation

The Georgia Tech Solar Jackets racing team is designing a solar-powered car to compete in the

2011 World Solar Challenge. Systems incorporating solar energy depend on MPPTs to optimize the

electrical operating point of the solar array for maximum power extraction. The previous team to work

on this project determined that the Solar Jackets team already has an analog MPPT, but it lacks

documentation, and that a custom design would enable the group to create a smaller, digitally

controlled power-switching circuit [1]. The SPAM team designed and built a working prototype;

SJ_MPPT will improve this design and make it deliverable to the Solar Jackets for integration into the

car. Improvements will include enhancing the algorithm, optimizing the hardware for minimal power

consumption, compacting the design, ensuring proper ranges of allowable operating conditions, and

implementing a serial interface which will allow the MPPT units to communicate with other systems in

the car.


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1.3 Background

1.3.1 Switched-Mode Power Suppl ies

Switched Mode Power Supplies (SMPSs) can be classified into distinct types according to

whether the input and output waveforms are AC or DC [2]. Special focus is placed on DC-to-DC

converters due to their ability to serve as MPPTs.

Electronic switch-mode DC-to-DC converters convert one DC voltage level to another by

temporarily storing the input energy and then releasing it at a different voltage. The energy is stored in

magnetic or electrical storage elements (such as inductors and capacitors). These components are

controlled by a transistor switch driven by an external signal. The signal is controlled by an algorithm

programmed into an embedded control unit. The frequency and duty cycle of this signal regulates the

circuit's impedance and output voltage [3]. Taken together, the input signal, the switching algorithms,

the storage elements, and the circuit configuration are the four parameters that characterize a given

SMPS.

The main operating principle of DC-to-DC SMPSs is the tendency of an inductor to resist the

changes in current induced by the transistor switch. In order to resist, the inductor will either absorb

energy (charge) or release energy (discharge). The discharge voltage is proportional to the rate of

current change, and not to the original charging voltage. This mechanism allows an output voltage to

be different from the input voltage.

SMPSs are available in a wide range of sizes and forms. Austria Microsystems produces

integrated circuit implementations of DC-to-DC converters such as the AS1326B at a unit price of

$1.79 [5]. For consumers, SMPSs are targeted towards specific applications. For example, Nady

Systems manufactures a 48V microphone SMPS (SMPS-1X) for $19.99 [6]. Similarly, Samsung

Electronics' BN94-02071 is a television set SMPS with a unit price of $70.16 [7]. Commercial SMPSs

are classified by their voltage range and conversion efficiency. For example, the AS1326B (up to 96%

efficiency) has an input ranging from 0.7V to 5.0V while its output varies from 2.5V to 5.0V.


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1.3.2 MPPT Algorithms

There are many algorithms used in digital MPPT systems; they vary in cost, complexity, speed,

accuracy, and efficiency. The accuracy of an algorithm indicates its ability to find the true maximum

power point (MPP), and the speed metric indicates how fast it arrives at a stable point. Algorithms

also differ in how much power they consume relative to the amount of power being transferred through

their circuitry; this is determines the efficiency [8].

The SPAM team decided to use an algorithm called perturb and observe (P&O), which

affects the operating point of solar cells by adjusting the duty cycle of a SMPS [1]. The algorithm

continuously determines its direction of adjustment based upon the effect of the previous adjustment

on the power output. Another algorithm, known as incremental conductance, incrementally searches

for an operating point at which dP/dV = 0 (whereP is the power output from the solar cells) [8]. This

algorithm requires more sophisticated processing capability than P&O, yet achieves approximately the

same level of accuracy and efficiency [9]. Other options include fully analog algorithms of varying

complexity, as well as more advanced digital algorithms using artificial intelligence techniques such as

neural networks and fuzzy logic [8].

1.3.3 Synchronous Rectif ier Technology

Synchronous rectification is a technique for improving the efficiency, thermal performance,

power density, manufacturability, reliability, and cost of power supply systems. These improvements

are done by replacing diodes with actively-controlled switches such as transistors; most-often power

MOSFETs (metal-oxide-semiconductor field-effect transistors).

An asynchronous power converter uses a field-effect transistor (FET) and Schottky diode as its

switches. When the FET turns on, energy is delivered to the output inductor and the load. When the

FET turns off, the current in the inductor commutates to the Schottky diode. Provided the load current

is higher than half the ripple current of the output inductor, the converter operates in a continuous


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conduction mode. The Schottky diode is selected by its forward voltage drop and reverse leakage

current characteristics. As output voltages drop, the diodes forward voltage is more significant, which

reduces the converters efficiency. Physical limitations prevent the forward voltage drop of diodes

from being reduced below approximately 0.3V. In contrast, the on resistance, RDS(ON), of MOSFETs

can be lowered, either by increasing the size of the die or by using discrete devices in parallel.

Consequently, a MOSFET used in place of a diode can have a significantly smaller voltage drop at a

given current than the diode [4].

2. Project Description and Goals

The SJ_MPPT groups goal is to create a smart power-switching network to maximize the

power extracted from a solar array to charge a lithium-ion battery array for the Solar Jackets. The

system will incorporate the following items:

Two boost (step-up) power-switching circuits

One microcontroller

RS-485 network connectivity

Voltage, current, and temperature sensors

The SJ_MPPT team anticipates the project costing $374 in materials for the prototype and $94

per board for the final design. This does not include labor costs, but does include initial fixed costs

involved with prototyping a design. The design will be equipped with the following features:

Monitoring of input and output voltage and current as well as board temperature

Capability to report these measurements over the RS-485 network

Continuous circuit adjustments to achieve maximum efficiency

Ability to shut down or throttle gracefully to prevent component failure

Physical switches to isolate the MPPT from the solar array and battery array in an

emergency


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3. Technical Specifications

3.1 Physical Specifications

The specifications for the circuit design have been chosen based on the electrical conditions

under which the final product will operate. The allowable input voltage and current for the SMPS

circuit are directly dependent on the open circuit voltage (VOC) and short circuit current (ISC) of the

solar cells being used by the Solar Jackets. Likewise, the output voltage and current are dependent

upon the specifications of the batteries being used in the car. The electrical specifications are currently

the most important. The maximum operating temperature is determined by the components used in the

circuit, and the size and shape of the PCB are dependent upon the capabilities of Georgia Techs

milling machine. Specifications for a single circuit board (containing two SMPS circuits), as well as

specifications for the solar cells and batteries can be found below in Tables 1 through 5.

Table 1.System Specifications

Component Specification

Input Characteristics 0 - 14.8 V0 - 8.4 A

Output Characteristics ~96 V0 - 2.5 A

Operating Temperature -40 - 85 *C

Maximum Physical Dimensions 10.5 x 7.5

Microcontroller PIC18F4321

MPPT Algorithm Perturb and Observe

Switching Power Circuitry Boost (Step-Up)


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3.1.1 Solar Ar ray

Table 2 illustrates the electrical properties associated with the solar cells used by the Solar Jackets.

These specifications determine the range of inputs the MPPT circuit must sustain.

Table 2.Solar Cell* Electrical Specifications [10]

Attribute Specification

Maximum Voltage .654 V (VOC)

Optimal Operating Voltage .541 V (VMP)

Maximum Current 9.18 A (ISC)

Optimal Operating Current 8.25 (IMP)

Maximum Power 4.47 W (Pmax)

* Solar cells will be stringed together in arrays of 28 (as specified by the Georgia Tech Solar Jackets)

3.1.2 Battery

The battery specifications are outlined in Table 3. These specifications determine the output range

of the MPPT circuit.

Table 3.Battery Electrical Specifications+[11]

Attribute Specification

Battery Type Lithium-Ion(Headway LiFePO4 38120S)

Maximum Charge Current 6C(60 A)

Inter Impedance < 6 m

Normal Voltage 3.2 V

Maximum Charge Voltage 3.65 + 0.05V

Discharge stop voltage 2.0V

Work Temperature charge: 0~45C

discharge: -20~60C+Individual batteries will be arranged in modules of 30 cells


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3.1.3 Temperature

The temperature specifications are outlined in Table 4. ; temperature is crucial to proper

functionality of the MPPT circuit.

Table 4.Acceptable Temperature Ranges

Component Temperature Range (

C)

LMC6484IN -40 to 85 [12]

ACS715 -40 to 150 [13]

MAX487CPA+ -40 to 85 [14]

TC1413NXPA -65 to 150 [15]

STW75NF20 -50 to 150 [16]

LM7805ACT -40 to 125 [17]

PIC18F4321 -40 to 125 [18]

3.1.4 Physical Characteri stics

Based on space constraints inside the solar car, the dimensional parameters are shown in Table 5.

Table 5.External Connections and Dimensions

Parameter Connector / Size

Maximum PCB Dimensions 10.5 x 7.5

+12V Common Coaxial Power Connector

Solar Array Connector Locking 2-Pin Molex

Battery Array Connector Locking 2-Pin Molex

RS485 Connection Dual RJ11

3.2 Functional Specifications

3.2.1 Microcontroller

The SJ_MPPT team will begin development using the same model of microcontroller (MCU)

the SPAM team used in the prototype. Specifications for this model of microcontroller are listed

below in Table 6 [1].


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Table 6.PIC18F4321 Microcontroller Specifications [18]

Component Specification

Physical Data Transfer Up to 44 pins; 36 I/O ports

Program Memory 8 kB

Maximum Clock Speed 40 MHz

Analog-to-Digital (A/D) Resolution 10-Bit, 13-Channel

PWM Modules 1 CCP; 1 ECCP

PWM Resolution 10-Bit

Programming Interface C compiler Optimized Architecture

Operating Voltage Range 2.0 V- 5.5 V (4.2 V preferred)

Material Safety Lead Free/RoHS Compliant

3.2.2 Additions to SPAM functionali ty

Adaptive duty cycle delta: duty cycle adjustment amount will change dynamically

Serial Interface: take action according to the following commands received via RS-485

interface:

o Turn on:begin running P&O algorithm and switching the power supply

o Turn off: stop switching the power supply; leave the switch in open-circuit state.

o Report Status: Send a packet of state information including:

Input current and voltage

Output current and voltage

Temperature

Duty cycle and duty cycle delta

On / off

Slow shut down in response to certain output voltage or temperature levels, which will be

determined by testing.

Emergency Shut Down in response to any measured parameters exceeding allowable values as

listed in Table 1.


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4. Design Approach and Details

4.1 Design Approach

SJ_MPPT will initially develop a single board implementation of the MPPT circuit. Once this

design has been produced it will be tested to determine whether or not it meets the specifications set

forth by the Solar Jackets. Once these specifications have been met, SJ_MPPT will be responsible for

reproducing the design to satisfy the demands of the Solar Jackets. It is expected that at least ten

reproductions of the circuit will be necessary to service the 307 solar cells. This number will also

depend upon the need for additional boards to be used as replacements in case of emergency.

Figure 1.Block diagram illustrating the basic layout of the MPPT design.

The SJ_MPPT will be composed of two power switching networks embedded on a single PCB

board, as illustrated in Figure 1. In contrast to the Fall 2010 SPAM version, these two networks will


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be serviced by a single microprocessor. The advantages of using two networks on a single board are

twofold. First, it allows for the sharing of processor resources in a way that reduces the amount of

overall computing power required. Secondly, the implementation of such a compact design reduces

the production costs without compromising the systems abilities.

4.1.1 Solar Cell Characteri stics

A current-voltage (I-V) curve, Figure 2, shows the possible combinations of current and

voltage output of a photovoltaic (PV) device. A PV device, such as a solar module (for this

implementation a module will consist of 28 solar cells in series), produces its maximum current when

there is no resistance in the circuit, this maximum current is known as the short circuit current and is

abbreviated ISC. When the module is shorted, the voltage across the solar module is zero.

Figure 2.Current-voltage curve of a photovoltaic device.

Conversely, the maximum voltage occurs when there is a break in the circuit. This is called the

open circuit voltage (VOC). Under this condition the resistance is infinitely high and there is no

current, since the circuit is incomplete.

These two extremes in load resistance, and the whole range of conditions in between them, are

depicted on Figure 2. Current, expressed in amps, is on the (vertical) y-axis. Voltage, in volts, is on

the (horizontal) x-axis.


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The I-V of a PV device curve is based on the device being under standard conditions of

sunlight and device temperature. It assumes there is no shading on the device. Standard sunlight

conditions on a clear day are assumed to be 1,000 watts of solar energy per square meter (1000 W/m2

or 1 kW/m2). This is sometimes called one sun, or a peak sun. Less than one sun will reduce the

current output of the PV device by a proportional amount [19].

4.1.2 The Maximum Power Poin t

The power available from a photovoltaic device at any point along the curve is the product of

current and voltage at that point and is expressed in watts. This means that maximizing either current

or voltage will cause the power to be zero. There is a point on the knee of the curve at which

maximum power is extracted, known as the maximum power point. The array can be made to

operate at the maximum power point by changing the impedance to the value given by Equation 1:

Zmp = Vmp/Imp (Equation 1)

4.1.3 Implementing MPPT

The SMPS power extraction unit will consist of a toroidal inductor, a switching transistor, a

diode (or synchronous rectifier) and a PIC18F4321 microcontroller. The PIC18F4321 was chosen due

to the availability of a wide range of supporting documentation, low power consumption, and sufficient

computing power. The PIC microcontroller will provide the pulse-width modulation (PWM) signal to

drive the transistors switching. Current and voltage sensors will provide the PIC with the data

necessary to perform the MPPT algorithm.

One feature offered by the PIC18F4321 is an auto-shutdown feature, which allows an

external condition to cause the PWM output to default to some configurable voltage state. This would

allow for very fast emergency shut-off in the event of dangerous voltage levels. Unfortunately, the

PIC18F4321 only offers this feature for one of its two PWM outputs, and using the feature in the

design would require it on both outputs. If it is determined that the auto-shutdown feature is essential


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to the safe operation of the circuit, then the team will switch to a microcontroller which offers this

feature on multiple PWM outputs, such as the PIC18F45K22 [20].

4.1.4 Algorithm

Like the SPAM team, the SJ_MPPT team has decided that the P&O algorithm is the best option

for this design. Research done by both teams suggests that, when compared with other algorithms,

P&O is simultaneously one of the simplest and most accurate options [1], [9]. This algorithm works

by slightly perturbing the impedance of the circuit connected to the solar cells. Depending on the

effect the previous adjustment had on the power output from the cells, the algorithm either changes the

direction of its perturbations or continues in the same direction as the previous iteration. The

perturbations are accomplished by changing the duty cycle of the SMPS [9].

The SJ_MPPT team will be adding several features to the functionality of the design. The

microcontroller will be able to receive messages over an RS-485 interface and take appropriate action.

This will allow an external controller to tell the MPPT to stop and start the algorithm as well as report

its status. In addition, the algorithm will incorporate an adaptive duty cycle delta technique, which

means that the amount by which it adjusts the duty cycle (the duty cycle delta) will change

depending upon how far the present operating point is from the maximum power point. There are

several variations on this technique that need to be tested in order to optimize it. For instance, it may

be that the use of only two or three different deltas may be the more efficient than, for instance,

adjusting the delta slightly on every iteration. Research has been already been done on this technique,

such as in [21].

Figure 3 shows a flow chart of the algorithm. It includes the main P&O logic, message

handling, and delta adjustment. The slow shutdown feature is not shown here; testing and presently

unspecified parameters will dictate how and when that should happen. The logic for delta adjustment

may change significantly based on testing and optimization.


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Figure 3. Flow chart demonstrating the algorithm.


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4.1.5 Software Development Approach

An incremental approach to software development will be used. For building and testing the

P&O algorithm, the following steps will be taken:

1. Run SPAMs code on isolated Qwik & Low board.

2. Test SPAMs code with prototype MPPT circuitry.

3. Optimize code for speed; reorganize and rewrite as needed.

4. Begin experimenting with adaptive duty cycle delta techniques.

5. Test and optimize adaptive duty cycle code based on how quickly the algorithm arrives at the

maximum power point, as well as how stable it is once this point is found.

The implementation of the RS-485 interface will proceed as follows:

1. Design a circuit featuring two microcontrollers communicating with each other through RS-485

transceiver. Using this setup as opposed to, for example, using a single transceiver and by

simulating and observing RS-485 signals by some other means, will allow proper debugging of

both incoming and outgoing signals, under true RS-485 electrical specifications.

2. Write new interrupt-driven code that will handle the receiving and sending of messages and

debug it using the test circuit from (1).

3. Specify final RS-485 protocol, in collaboration with other Solar Jackets electrical teams.

4. Implement actions taken in response to RS-485 commands, as specified in section 3.2.2 [14].

5. Test these commands and their responses with MPPT circuitry.


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4.1.6 Voltage and Cur rent M easurement

For maximum power point tracking to be implemented, the voltage and current at the output of the

solar array must be measured and provided to the microcontroller. The voltage is measured and converted

into a digital value directly by the microcontrollersbuilt-in analog to digital converters. The current is

measured using Allegro current sensing integrated circuits. These ICs measure the current and convert it

to an analog voltage that is easily readable by the microcontroller. It is also possible to measure the

current using a resistor network, but this method is not as precise as measuring it directly.

4.1.5 Temperatur e Measur ement

Continuously monitoring board temperature is is key to preventing component failure.

Temperature is monitored using a pair of thermistors that output a voltage that linearly corresponds to

the temperature of the sensors. This voltage can be read in by the microcontroller with the analog to

digital converters. The microcontroller monitors the temperatures and can throttle back the output or

shut down completely to maintain safe temperature levels.

4.1.6 Synchronous Recti fi er

Physical limitations prevent the forward voltage drop of diodes from being reduced below

approximately 0.3V. As output voltages drop, the diodes forward voltage is more significant, which

reduces the converters efficiency. In contrast, the on resistance, RDS(ON), of MOSFETs can be

lowered, either by increasing the size of the die or by using discrete devices in parallel. Consequently,

a MOSFET used in place of a diode can have a significantly smaller voltage drop at a given current

than the diode [4].

With SPAMs project as a starting point, SJ_MPPT will transition from a diode to a rectifier

circuit. Since it has been established that the diode works, and it is a much simpler circuit, the team is

integrating the option to use the rectifier into the design by offering parallel paths for current to flow,


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as shown in Figure 4. This way, the option will still be open after the units have been tested in the car,

under actual operating conditions.

Figure 4.Illustration of parallel diode and synchronous rectifier.

4.2 Codes and Standards

The MPPT should be integrated into a solar-powered car compatible with the Technical

Regulations for the 2011 World Solar Challenge. Electrical isolation surrounding the battery pack

where the voltage will exceed 32 V is essential to ensure that it is impossible for any occupant of the

vehicle to touch live wires in order to comply with rule C.1.1 [22]. The system should enable the

driver to isolate the battery pack and solar array. Also, an emergency control must be provided outside

the vehicle according to rules C.2.1, C.2.2 and C.3.1 [1], [22].

The Solar Jackets have specified that a 12V power supply will be available to power the

components on the board; these include the PIC18F4321 microcontroller [18]. In the case the 12V

power supply is not available, rule E.5 states commercially-available instruments, computers and

digital multimeters may use ancillary batteries provided that the battery is internal to the instrument.

In that case, the PIC18F4321 microcontroller could be powered with an internal 3 V coin battery.


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4.3 Constraints, Alternatives, and Tradeoffs

4.3.1 Algori thm Selection

There is abundant literature on the comparison of MPPT algorithms. The P&O algorithm is the

most widely used, and regarded as among the most efficient and simple algorithms, especially when

optimizations are made [9]. Implementing a more complex algorithm would require much more

sophisticated computing power, but would offer only marginal increases in speed and accuracy [9].

Many of the more complex algorithms rely on frequent tuning of parameters, whereas P&O is highly

adaptive and works with no knowledge of the hardware on which it runs [8]. On the other side of the

spectrum, there is no reason to consider implementing a less complex (and less accurate) algorithm,

since P&O is relatively easy to implement.

4.3.2 Number of MPPT Circui ts per PCB

The choice to use a PIC18F4321 in the design was made in conjunction with the decision to

limit each PCB to two MPPT circuits. Each board will have a single microcontroller which will output

PWM signals to two different MPPT circuits, as shown in . The PIC18F4321 hardware is limited to

exactly two PWM outputs with different duty cycles, and the PCB size limitations leave enough room

to reasonably fit two MPPT circuits on one board. Switching to three or four would introduce the need

to account for higher temperature levels and strict compliance to PCB layout rules, and would require a

more sophisticated microcontroller. If the Solar Jackets racing team subsequently imposes a tighter

size requirement, these options will be considered.

4.3.3 Diode vs. Synchr onous Rectif ier

The constant voltage drop of a standard p-n junction diode is typically between 0.7V and 1.7V,

causing significant power loss in the diode. Electrical power depends on current and voltage: the

power loss increases proportionally with both current and voltage. MOSFETs have a constant, very

low resistance when conducting, known as on-resistance (RDS(ON)). The voltage drop across the


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transistors resistance is then much lower, meaning a reduction in power loss and a gain in efficiency.

However at high currents, the drop can exceed that of a diode.

5. Schedule, Tasks, and Milestones

5.1 Schedule and Milestones

Table 7 shows major tasks with their expected duration and completion dates. A more detailed

schedule can be found in Appendix A.

Table 7.Projected tasks and their associated duration.

Task Name Duration Start Date Finish Date

Circuit Tasks

Prototype of SPAMs Design 8 days Feb 21, 2011 Mar 3, 2011

Design and Simulation 8 days Mar 3, 2011 Mar 15, 2011

Prototype Circuit 19 days Mar 15, 2011 Apr 9, 2011PCB Tasks

Learn PCB Software 3 days Feb 21, 2011 Feb 24, 2011

Prototype Design 10 days Feb 28, 2011 Mar 12, 2011

Final Design 8 days Mar 25, 2011 Apr 6, 2011

PIC Tasks

Load and Run SPAMs code 3 days Feb 21, 2011 Feb 24, 2011

Adaptive Delta Method 4 days Feb 24, 2011 Mar 2, 2011

Specify Protocol 3 days Mar 2, 2011 Mar 5, 2011

Design RS-485 test circuit 3 days Mar 7, 2011 Mar 10, 2011

Test RS-485 3 days Mar 10, 2011 Mar 15, 2011Add RS-485 to algorithm 3 days Mar 15, 2011 Mar 18, 2011

Finalize PIC Code 5 days Mar 18, 2011 Mar 25, 2011

Lessons learned 1 day Mar 25, 2011 Mar 26, 2011

Demonstration

Final Product Testing 6 days Apr 8, 2011 Apr 16, 2011

Final Presentation

Assessment 2 days Apr 29, 2011 Apr 30, 2011

Presentation 1 day Apr 29, 2011 Apr 30, 2011

Demonstration 1 day May 2, 2011 May 3, 2011

Report 4 days May 3, 2011 May 7, 2011


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5.2 Division of Labor and Accountability

Table 8 shows major tasks with the division of labor and accountability for each task.

Table 8.Division of labor and accountability.

Task Name Andrew Travis Giancarlo IngridCircuit Tasks

Prototype of SPAMs Design

Design and Simulation

Prototype Circuit

PCB Tasks

Learn PCB Software

Prototype Design

Final Design

PIC Tasks

Load and Run SPAMs code

Adaptive Delta Method

Specify Protocol

Design RS-485 test circuit

Test RS-485

Add RS-485 to algorithm

Finalize PIC Code

Lessons learned

Demonstration

Final Product Testing

Final Presentation

AssessmentPresentation

Demonstration

Report

5.3 Critical Path Method

Figure 5 shows the Critical Path Method (CPM) which shows task dependencies, precedence and

their associated times.


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Figure 5.Illustration of CPM.

6. Project Demonstration

Function of the final and prototype designs will be demonstrated both outdoors using solar cells

and indoors using a power supply to verify function over a wide range of conditions. In both cases the

output of the MPPT board will be connected to a 96V lithium-ion battery array with similar

characteristics to the final design to be used by the Solar Jackets. The outdoor test will be used to

demonstrate that the design can function correctly using solar cells in an environment with varied

levels of sunlight. The indoor demonstration will be used to verify the efficiency of the design, the

functionality over a wide range of voltages, and the correct operation of safety mechanisms. The

indoor demonstration will also show the ability of the device to accept and respond to commands sent

over the RS-485 network.

The following characteristics will be displayed during the demonstration:

Voltage and current from the solar array for both tests.

Voltage and current output for the indoor test (see Figure 6).

Computed efficiency.

Proper response to all RS-485 commands.


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Figure 6. Block diagram of the indoor test setup that includes voltmeters and ammeters.

7. Marketing and Cost Analysis

7.1 Marketing Analysis

Due to the specific needs associated with powering a solar electric vehicle, the Solar Jackets

team requires a customized MPPT with cost and efficiency beyond what can be provided by existing

commercial technology. For this reason, the MPPT system will be designed to meet the specific needs

of the Solar Jackets. In particular, the design will be informed by the Solar Jackets choice of solar

cells and batteries. This design will also consider the spatial and financial constraints that typically

affect a solar car racing team such as the solar jackets. In this sense, it is possible that, despite its high

degree of customization, the SJ_MPPT system can form the basis for a commercial product targeted to

solar-powered race cars.

7.2 Cost Analysis

The total cost of engineering is listed in Table 9. The estimated hours and number of engineers

required for each task are listed, along with the total cost associated with each task.


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Table 9. Anticipated Engineering Labor Costs

Engineering Costs

Task Hours Engineers Cost ($50/hr)

Circuit Design 25 2 $2,500.00

Circuit Simulation 20 2 $2,000.00

Learn PCB Software 10 1 $500.00PCB Design 45 1 $2,250.00

Program and Optimize Algorithm 45 1 $2,250.00

Implement RS-485 30 1 $1,500.00

Test Final Design 20 4 $4,000.00

Place parts and solder all units 15 4 $3,000.00

Demonstration & Preparation 10 4 $2,000.00

Final Presentation & Preparation 10 4 $2,000.00

Lecture 20 4 $4,000.00

Meetings 50 4 $10,000.00

Totals Engineer-hours: 720 $36,000.00

Due to the availability of materials from previous MPPT design, the cost of the initial prototype

was lower than what the market value of each component would predict. Table 10 illustrates the

components necessary for constructing a single board prototype.

Table 10. Cost Analysis [23]

Components Team Cost Market Cost

Synchronous Rectifier x 2 $6 $6

Toroid Inductor Provided $1

Schottky Diode (x10) $2 $2

PIC Mircrocontroller Provided $2

MPLAB ICD 2 Programmer Provided $200

PCB $35 $66

Test Solar Cells Provided $80

MOSFET x 2 $6 $6

Resistors (x20) Provided $6

RS 485 Cables Provided $5

TOTAL $49 $374


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8. Summary

At this stage, there are three areas of simultaneous development inside SJ_MPPT. The first one

involves the construction of a prototype power-switching supply based on the existing SPAM designs.

This process will provide SJ_MPPT with a detailed understanding of the successes and shortcomings

of this previous design. Once the assessment is complete, alternative implementations will be gradually

integrated in order to determine to what extent the existing components can be replaced by more

efficient ones. In particular, a synchronous rectifier is being considered as a potential replacement for

the standard diode. Replacing the diode with a synchronous rectifier, which has a lower power

dissipation, will allow for higher efficiency of the circuit.

Another area of activity involves the optimization of the microcontroller code that implements

the power tracking algorithm. Modifications of this code will be tested with the prototype circuit to

determine what improvements result from potential variations. In particular, the implementation of

non-blocking code will be considered in order to ensure that execution of tasks competing for shared

resources are not indefinitely postponed by mutual exclusion.

The third and final area of activity is concerned with the design of potential printed circuit

board layouts (PCB) for use in a final design. Even though PCB layouts may vary depending on the

results of the prototyping experiments, preliminary constraints and design decisions can be made based

on these initial designs. As the project progresses, these three processes will gradually merge into the

final product.
http://www.google.com/url?q=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FThread_(software_engineering)&sa=D&sntz=1&usg=AFQjCNGcnQZXqS5X5k8Ti09dcGKW78ks9ghttp://www.google.com/url?q=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FThread_(software_engineering)&sa=D&sntz=1&usg=AFQjCNGcnQZXqS5X5k8Ti09dcGKW78ks9g


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9. References

[1] M. Calotes et al., Solar Power-Switching Network, Solar Power Array Management, Georgia

Institute of Technology, Georgia, Proposal, September 2010.

[2] A. Pressman, K. Billings and T. Morey, Switching Power Supply Design , 3rd ed., New Delhi:

McGraw-Hill Professional, 2009, p. 22.

[3] S. Salivahanan, N. Kumar, and A. Vallavaraj,Electronic Devices and Circuits, 2nd ed., New

Delhi: McGraw-Hill, 2008, p. 14.

[4] R. Selders Jr., Synchronous Rectification in High-Performance Power Converter Design

Power Designer: Expert tips, tricks, and techniques for powerful designs,No. 112, March,

2008. [Online]. Available: http://www.national.com/analog/power/designer [Accessed: Feb. 2,

2011].

[5] Austria Microsystems, AS1326, austriamicrosystems.com, January, 2011. [Online].

Available:http://www.austriamicrosystems.com/eng/Products/Power-Management/DC-DC-

Step-up-Converters/AS1326/ [Accessed February 3, 2011].

[6] Amazon.com, Inc. Nady SMPS-1X Phanom Power Supply amazon.com, February 2011

[Online] http://www.amazon.com/Nady-SMPS-1X-Phantom-Power-Supply/dp/B000XUUXB8

[Accessed February 3, 2011].

[7] Part Store Samsung BN9402071A PartStore.com , October 2010 [Online]

http://www.partstore.com/Part/Samsung/Samsung/BN9402071A/New.aspx [Accessed:

February 2, 2011]

[8] T. Esram and Patrick L. Chapman, Comparison of Photovoltaic Array Maximum Power

Point Tracking Techniques,IEEE Transactions on Energy Conversion, vol. 22, no. 2,

June, pp. 439-437, 2007.


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[9] D. P. Hohm and M. E. Ropp, Comparative Study of Maximum Power Point Tracking

Algorithms,Progress in Photovoltaics: Research and Applications, vol. 11, issue 1, Feb., pp.

47-61, 2002.

[10] F. Zimbardi. (2011, February 16). Solar Cell Info - Solar Jackets [Online]. Available e-mail:

[email protected]

[11] Headway Headquarters. BatteryPacks: H-38120S [Online]. Available:

http://stores.headway-headquarters.com/-strse-1/Headway,-batteries,-EV,-solar/Detail.bok

[Accessed Feb. 20, 2011].

[12] National Semiconductor, CMOS Quad Rail-to-Rail Input and Output Operational Amplifier,

LMC6484 datasheet, August 2000.

[13] Allegro MicroSystems, Automotive Grade, Fully Integrated, Hall Effect-Based Linear Current

Sensor IC with 2.1 kVRMS Voltage Isolation and a Low-Resistance Current Conductor,

ACS715 datasheet, 2010.

[14] Maxim Integrated Products, Low-Power, Slew-Rate-Limited RS-485/RS-422 Transceivers,

MAX487CPA+ datasheet, September 2009.

[15] Microchip Technology, 3A High-Speed MOSFET Drivers, TC1413/TC1413N datasheet,

2003.

[16] STMicroelectronics, N-channel 200V Low gate charge STripFET Power MOSFET,

STW75NF20 datasheet, March 2007.

[17] Fairchild Semiconductor, 3-Terminal 1A Positive Voltage Regulator, LM78XX/LM78XXA

datasheet, April 2010.

[18] Microchip Technology, 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and

nanoWatt Technology, PIC18F43231 Family datasheet, 2007.


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19] D. Darling, I-V Curve (of a photovoltaic device) in Encyclopedia of Alternative Energy and

Sustainable Living. [online document], 1999. Available:

http://www.daviddarling.info/encyclopedia/AEmain.html [Accessed: Feb. 16, 2011].

[20] Microchip Technology, 28/40/44-Pin, Low-Power, High-Performance Microcontrollers with

nanoWatt XLP Technology, PIC18(L)F2X/4XK22 Family datasheet, 2010.

[21] A. Pandey, N. Dasgupta, and A.K. Mukerjee, High-Performance Algorithms for Drift

Avoidance and Fast Tracking in Solar MPPT System,IEEE Transactions on Energy

Conversion, vol. 23, no. 2, June, pp. 681-688, 2008.

[22] C. Selwood, 2011 World Solar Challenge Technical Regulations, Feb, 2011 [Online].

Available:http://www.worldsolarchallenge.org/participants/regulations.

[23] Futurlec, Future, The Electrical Components and Semiconductors Superstore, 2011. [Online].

Available:http://www.futurlec.com/Components.shtml.
http://www.worldsolarchallenge.org/participants/regulationshttp://www.futurlec.com/Components.shtmlhttp://www.futurlec.com/Components.shtmlhttp://www.worldsolarchallenge.org/participants/regulations


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APPENDIX A

GANTT CHART


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Documents

MPPT Proposal 248.pdf