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 November 1, 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: $222

Design Goals Maximum power point tracking Switching power supply Microcontroller programming Board/PC communication

Design Overview

Solar Array Material: CZ silicon (mono- crystalline) Predicted Efficiency: 17.8% 10 cells in series Testing outdoors at 2:30 p.m. on a sunny day V oc = 5.4 V I sc = 5.45 A

Individual Solar Cell I-V Curve

Battery HR9-12 Sealed Lead Acid Battery 12 V (6 cells in series)

Power Switching Circuit Layout

Preliminary Test Waveforms Input: 6V Output: 12 V

Control of Circuit Output voltage controlled by changing duty cycle of PWM that is used to switch MOSFET Frequency of MOSFET Switching PWM = 1 kHz Duty Cycle of MOSFET Switching PWM = 21%

Circuit Efficiency Efficiency = 85% Causes for low efficiency: MOSFET IRF 520: R on = 0.27 Ω Regular Diodes Inductor Series Resistance = Ω Solutions: Use MOSFET STW75NF20 with R on = Ω Diode with lower voltage drop (SBR10U45SP5 for solar cells)

Power MOSFETs and Gate Driver Need for lower rise time in MOSFET switching Losses occur during MOSFET switching Rise time limits maximum switching frequency Initial Testing MOSFET IRF 520: Rise Time = 73.3 μs Gate Driver: Higher switching voltage → lower rise time Lower input resistance

Inductors Inductor value chosen on the basis of maximum switching frequency Lower value of L → smaller components

Scaling of System for Final Solar Car Final system will use a battery at 96 V Main Issue: Final solar array characteristics unknown Component Characteristics Components that need to be changed for compatibility Inductor: Select depending on final array characteristic Modify range of frequency and duty cycle for circuit control ComponentKey Characteristics MOSFET STW75NF20200 V, 75 A Diode SBR10U45SP5V br = 45 V, Max. I = 10 A Current Sensor AC715Range = 0 to 20 A

PIC18F4321 Microcontroller 44-Pin MCU Important Pins: – 30, 31: Internal Oscillators – 11, 36: PWM Module – 1, 44: PC I/O Port Accurate Clock Speed: – DC – 40 MHz Low Supply Voltage: – 2.2 V – 5.5 V Max. Current Draw: – mA C compiler available Unreserved pins for power usage designated as I/O Pins

Qwik-and-Low Board (MCU On Board) Advantages: – LCD display information linked to I/O ports already – Built-in RS-232 serial port – GT-based compiler and debugging software Disadvantages: – Limited user of I/O pins due to default assignment – Unused board space; needs to be customized for S.P.A.M. design Current S.P.A.M. Progress: – Have initialized testing and editing template programs to become accustomed with the MCU – Specific I/O pins for algorithm implementation not yet specified

Perturb and Observe (P & O) Algorithm Implementation Goals: – Take current voltage (V) and current (I), then compare with most previous value – Incrementally increase duty cycle for greater voltage; decrease duty cycle for less voltage – Repeat searching continuously Algorithm Strengths: – Popular implementation; open-source code available – Effective in dynamically changing environments Algorithm Disadvantages: – Inevitable oscillation around the maximum power point (MPP) – May mistake a local maximum as the absolute maximum

Coding Progress and Problems Progress –Attained capability of using the Qwik-and-Low Board and PIC microcontroller to program prototypes –Besides algorithm, no C coded program available from group yet Problems/Setbacks –Unsure about how to proceed with programming MCU on a custom PCB board –Physical implementation significantly delayed due to time spent understanding MCU behavior and characteristics

Printed Circuit Board Small, compact PCB –Switching power supply –Microcontroller –Serial communication –Voltage- & current-measuring circuits Two modules per board –Maximize microcontroller I/O resources –Anticipate multiple solar sub-arrays

Future Work Schedule Finalize circuit components & design by 11/19/2010 –Minimize inductor & maximize efficiency Design printed circuit board by 11/24/2010 Program microcontroller by 12/1/2010 –Code MPPT Test that the circuit adjusts to changing light intensity by 12/8/2010

Circuit Components Current Sensor ACS715 Current Range = 1 to 20 A R = 1.2 mΩ Output Sensitivity = 133 to 185 mV/A MOSFET STW75NF V, 75 A, Ron = Ω Rise Time = 33 ns when Vgs = 10 V Diode SBR10U45SP5: Bypass Diode for Solar Panels Forward Voltage Drop = 0.42 V, Reverse Breakdown Voltage = 45 V, Max. Current = 10 A UC2714 MOSFET GATE DRIVER High Current Power FET Driver, 1.0 A Source / 2 A Sink Sources: Datasheets