Design Process Analysis & Evaluation Part I Example Design: Solar Candle by Prof. Bitar

Analysis & Evaluation Homeworks #4 & 5 Define Product Interfaces Define Product Changes HW#4 HW#5 1 Analyze 2 Design 3 Build 4 Test 5 Evaluate Analysis & Evaluation

The Customer Requirements Explicit –Cat Safe –Look Nice –Different Colors –Automatic –Six Hour Minimum Implicit –Low Cost –Reliable / Durable –Low Maintenance

Product Requirements Safety / Durability –Heavy Base (Batteries in Base) –Unbreakable Bulb (LED) –Secure to Window Sill, Sash or Window Pane –No Cords –Low Voltage (1.5V or 3V) Operation & Aesthetics –Traditional Look –Interchangeable Color LED’s –Flickering Option Low Operating Cost –Long Battery Life (Six Hour Minimum) –Rechargeable Battery –Solar Rechargeable –Photo Sensor or Timer

Modified System Block Diagram Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit LED Switching Control Mode Selection Photo Sensor Timer

Technical Research Areas Solar Cells – Availability, cost, size, shape, output voltage, output current, silicon types, spectrum sensitivity. Charge Controllers – Voltage Regulation, Current Regulation, etc. Rechargeable Batteries – NiCad, NiMH, Other?, Overcharging & Under-voltage Concerns, etc. Drive Circuits – DC/DC Boost Converters, Charge Pumps, etc. LED’s – Colors, Voltage Levels, Current Requirements, Intensities, Illumination Angles, etc. Photo Sensors – Types, Interface Requirements

What to Consider First? Start at the OUTPUT. What does our design have to do? Drive an LED. What current? What Voltage(s)? Take a look at LED specification sheets…

Agilent White LED Spec. Sheet Dissected

LED Specifications Absolute Maximum Ratings NOT TO BE EXCEEDED !!

LED Specifications Electrical Characteristics Normal Operating Conditions

LED Specifications – Curves ! Relative Intensity vs. Wavelength

LED Specifications – Curves ! Max. Forward Current vs. Temperature Stay well under this curve !!

LED Specifications – Curves ! Max. Forward Current vs. Voltage Check: 3.2V (Typical)

LED Specifications – Curves ! Spatial Radiation Pattern Check Viewing Angle !!

Mechanical Specifications

LED Drive Requirements “First Stakes in the Ground” Must supply a constant current of approximately 20mA. Must supply a minimum of 3.2V for white LED’s. Should work for other color LED’s, since they have lower voltage levels.

Customer Requirement Check NOTE: Each decision MUST map to a customer requirement !! Question: Which customer requirements are met by designing for an LED current of 20mA at a minimum of 3.2V? Answer: Multi-Color LEDS, Reliable, Durable, Low Maintenance

Update System Block Diagram Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

What to Consider Next? The Battery (or Batteries) –Battery Capacity: How long does the candle need to operate? (Six Hours Minimum) –Battery Voltage: What is the minimum battery voltage? ( 3.2V ) –Battery Availability: What rechargeable technologies will work in this application? (NiMH, NiCd, Lithium Ion, Sealed Lead Acid) –Battery Shape: Needs to fit in candle stem or base. –Battery Cost: I don’t want to spend a lot on batteries!

Battery Selection - Two Viable Options Major Factor? Price! Voltage/CellCapacitySizePrice (1)Price (1k) NiCd1.2V700mAhrAA NiMH1.2V1600mAhrAA Problem: Need at least 3 batteries in series to achieve 3.2V minimum.

Energy Considerations… Need: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: Joules) One NiCd: 700mAhrs x 1.2V = 840 mW hrs (ie: 3024 Joules) Conclusion: A single NiCd has more than enough energy to do the job ! (But the voltage will have to be “boosted”).

Battery Specifications

Battery Specification Discharge Curve

Battery Specification Charge Curve

Update System Block Diagram Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

What Next? LED Drive Circuit Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

Searching LED Driver IC’s… – –LED Drivers (1,319 IN STOCK!) Narrowing Search (down to 47) – –Step-Up (Boost) – –Vmin ≤ 0.8V (why?) – –Internal Switch – –Number of Outputs (1) – –Small Qty. Packaging (cut tape / tube) Case…avoid DFN, QFN, LLP, BGA

Possible LED Drivers ManufacturerPart NumberUnit Linear TechnologyLTC3490ES8#PBF$4.20 LT1932ES6#TRMPBF$4.17 Diodes/ZetexZXLD383ET5CT-ND$1.56

Diodes/Zetex - ZXLD383ET5CT

Typical Application Circuit

Update System Block Diagram Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd 700mAHr Zetex LED Driver 85% Eff. LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer