Solar Power Array Management for the Solar Racing Team Mark Calotes Ginah Colón Alemneh Haile Nidhi Joshi Michael Lu School of Electrical and Computer Engineering GT Solar Jackets December 13, 2010 Georgia Institute of Technology

Project Overview Design a system to charge car batteries with the energy received from solar cells To be used for the Solar Jackets’ solar car in the World Solar Challenge Estimated cost: $151.58

Design Goals Implement boost converter to extract power from solar array Implement feedback with microcontroller to extract maximum power Create a compact PCB board with circuit and microcontroller Implement RS-485 communication to transfer information

Overall System Diagram

Technical Specifications CharacteristicSpecification Input Voltage Range0 to 6 V Input Current Range0 to 5.5 A Operating Frequency15 kHz External Supply Required12 V Supply Input SourcesPower Supply, Solar Array Output LoadsResistor, Battery Average Efficiency84.6%

Absolute Maximum Limits ParameterMaximum Value Input Current10 A Input Voltage12 V Output Current10 A Output Voltage14 V Maximum Switching Frequency20 kHz

Overall Circuit Diagram

Microcontroller Input Protection Voltage on microcontroller pin  Minimum: -0.6 V  Maximum: 5.6 V Protects Microcontroller

Circuit Protection External User Control  Switch at Input  Switch at Output Over Current Protection  Automotive Fuse at Input  Cylindrical Fuse at Output

Preliminary Test Circuit

Preliminary Circuit Output Voltage

Issues and Solutions Current sensor output voltage dropped upon connection to microcontroller – Use op-amp between sensor and microcontroller Output voltage failed to boost – Verify gate driver output Voltage regulator heated up – Check for faulty components that draw too much current

Circuit Future Work Adapt system to new solar array and 96 V battery output – Components to Modify Schottky Diode Inductor Fuses Current Sensor Increase efficiency by choosing lower resistance components and active rectification

MCU Key Benchmarks and Specifications ComponentSpecificationBenchmark Testing PWM/Duty CycleFrequency: 15 kHz Step Size: 0.5% Used oscilloscope to measure duty cycle and frequency with based on known input voltages TX/RX SerialSends measured input components to console once per half-second Viewed console for updated outputs and used MATLAB program to verify valid measurements of all parameters Integer conversion to ASCII for serial and LCD output One ASCII character assigned per digit of any integer with knowledge of total digits in number Verified from both LCD display on Qwik&Low board and serial console, which must be programmed digit-by-digit A/D Conversion13 analog channels to accept input data; scales value from corresponding to 0-V dd Accurate voltage measurements displayed on LCD display and serial based on known voltages from DC Power supply

Microcontroller Issues and Concerns Current Issues – MCU PWM pins from prototype have burned out due to excessive current drawn by MOSFET base – RS-485 communication to other devices work in progress – Only functional for one PWM, not two independent modules – Can measure output voltage, but no conditional checking for circuit protection coded yet – Performance quality expensive (59% program memory utilization) and size results in some lost output data

MPPT Safeguards Voltage scaled from so we had to readjust the scale to be from 0-5 – V = Vr/1023 * 5 Duty cycle stays between 5% and 95% – This keeps the PWM from breaking Voltage must stay below 5 V – This keeps the micro-controller from breaking

PIC Microcontroller: Future Work Obtain more test data from circuit to improve MPPT implementation Develop and integrate RS-485 communication protocol in code Optimize code for less memory consumption Develop more objective way to view input data and change duty cycle, e.g. through a moving averager

Onboard or remote computer can check on each board’s status Status includes voltage, current, power, and whether the PWM is increasing or decreasing RS-485 Connection

Printed Circuit Board

PCB Design Laid out using DesignSpark PCB One MPPT module Toggle switches for solar cells and battery Automotive fuse for solar cells Cylindrical fuse for battery RS-485 communication

PCB Issues & Resolutions Two-module board too large to print on campus – Remove one module Pins improperly connected in schematic – Remove trace and reconnect with flying leads Drill hole sizes incorrect – Re-drilled by Bob House

Future PCB Modifications Print two-module board off campus Use more accurate PCB footprints Use larger soldering pads for easier soldering Place switches, fuses, and peripherals along one side of board for ease of access Consolidate components to minimize board size

Cost Analysis Total cost: $ Circuit Component Cost: $17.56 Sensors: $10.50 Protection: $15.45 Connectors/Communication: $7.14 Prototyping: $100 Some parts were provided by the lab and therefore were free.

Modifications Required for Solar Jackets for Spring 2011 Finalize Solar Array Characteristics Modify Power Switching Circuit to Accept Higher Voltage and Current Values Optimize MPPT algorithm and Memory Utilization PCB with 2 MPPT Modules RS 485 communication with Multiple MPPT boards