lunalight pcb rev 2.1 mike deagen (mate) ime 458 may 25, 2012 fully integrated solar-rechargeable...
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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
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 1000120014001600180020005.6
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