LunaLight PCB Rev 2.1

LunaLight PCB Rev 2.1

Mike Deagen (MATE)IME 458

May 25, 2012

Fully Integrated Solar-Rechargeable LED Lantern and Cell Phone Charger

The LunaLight is the result of a multidisciplinary senior project in collaboration with the non-

profit organization One Million Lights

LunaLight debut in Kenya (April 2012)

A solar-rechargeable LED lantern and cell phone charger has three sub-circuits

IC Type Purpose

Battery Management Take the input power from the solar panel and safely charge the rechargeable batteries within the lantern

LED Driver Provide a constant current to the LEDs at the appropriate forward voltage

DC-DC Boost Increase the potential of the 3.7V Li-ion battery to meet the 5V USB standard voltage

Battery Management LED Boost Driver & DC-DC Boost for Cell Phone Charger

LT3652MSOP-12 packageCost: $4.36

MC34063SOIC-8 packageCost: $0.45

Rev 1.0 of the PCB had wires leading to the batteries, the LEDs, and the switch

The goal for the second revolution was to reduce overall assembly time by integrating

all the electronics onto a single board

Other GoalsUse mostly SMT components

Multiple brightness settings with a slide switchLi-ion battery instead of Nickel Metal Hydride

Ports and Switches

LEDs

Battery Holder

I modeled the thermal management to see if the LED junction could rise above Tmax = 150°C

Heat Generated = 0.84 WTotal Est. θ = 102°C/WΔT = 86°CIf T∞ = 25°C, Tjunc = 121°C

81% of Tmax

I used reflow soldering technique for SMT components (front side) and manual soldering

for thru-hole components (back side)

91% isopropyl rubbing alcohol removed the solder flux residue

The basic functions of the PCB were confirmed with simple verification tests

Illuminance from 12” away“HI” = 473 lux“LO” = 94 lux

Solar Li-ion battery chargingVbatt_chg = 3.96 V

Ichg = 212 mA

Testing the LED voltage drop over time indicated a junction temperature of 50°C

Temperature coefficient of voltage = -4.0 mV/°C [CREE XP-E datasheet]

For two LEDs in series, temperature coefficient of voltage = -8.0 mV/°C

0 200 400 600 800 100012001400 16001800 20005.6

5.65

5.7

5.75

5.8

5.85

5.9

5.95

6

time (s)

LEDVoltage

(V)

∆ 𝑇=∆𝑉

−8.0𝑚𝑉 /℃=5.75𝑉 −5.95𝑉−8.0𝑚𝑉 /℃

∆ 𝑇=−200𝑚𝑉−8.0𝑚𝑉 /℃=25℃

𝑇=𝑇 ∞+∆𝑇=25+25=𝟓𝟎℃

However, the temperature coefficient of voltage is

probably not linear!

Overall, I think the design was successful, and future LunaLight teams should consider

implementing a fully integrated PCB

Questions?