Solar Harvester
Michael Zwerg - April 3rd 2013
With ubiquitous computing getting more and more reality, it is a desire to have computing devices, which can operate on ambient energy. These so called energy harvesters collect small amount of energy and convert it to a form where it can be used by standard electronic components.
In this case a Solar cells is used as an energy source. The solar cell chosen for this project (Sanyo AM-1801) will generate a voltage of 3V with 18uA of current under a 200 LUX of light exposure (indoor use case). Theoretically the MSP430 will easily work with this kind of energy source, when sitting most of the time in low power mode. However in practical live the start-up of the device is the challenge. As soon as the solar cell generates enough voltage to overcome the brownout threshold, the device starts working in active mode and drain more current that can be delivered by the solar cell. Which means the device will not get to the point, where it can go to low power mode. Even with a buffer capacitor in parallel to the solar cell, the device will not start.
The solution to this problem is a TI high efficient buck converter with the part number TPS62120. The specialty of this converter is a build in cooperator, which prevent the start-up of the secondary power supply before the primary supply reached a voltage of 2.5V. The converter will than work down to 1.8V before it shuts down the secondary supply again. This feature can be used in conjunction with a buffer cap (330uF) on the primary supply. The solar cell will slowly charge the capacitor till the turn on voltage is reached. The converter is now starting up and providing the power for the MSP430 uController. All the energy to bring up the MSP430 (Band Gap, Power Supply, Boot Code, Init Code) will be delivered by the buffer cap, instead of from the solar cell. Then the MSP430 can go in LPM and the solar cell delivers enough energy to supply the converter and the MSP430.
This trick with the hysteresis on the buffer cap can now also be used by to operate the MSP430 in burst mode. A voltage divider on the primary supply is fed back to the MSP430 on a comparator input pin. This way the software can schedule some high current task (here blinking LED) to a point where the cap has stored enough energy to maintain the target voltage to the device (here 2.2V).
Energy Source: Solar Cell SANYO AM-1801
Cell Rating: 3.0V / 18.5uA @ 200LUX
Turn On Voltage (TPS62120): 2.5V
Turn Off Voltage (TPS62120): 1.8V
Output Voltage: 2.2V
Comparator Trip Voltage (MSP430): 3.2V
Primary Storage Capacitor: 330uF
The Bill Of Material is ~$6 for a 1k price break at DigiKey.
The complete list including Digi Key part numbers can be found here:
Solar Harvester BOM (CSV / 1k units / DigiKey)
Sanyo Amorphous Silicon Solar Cell Catalog
This Solar Harvester Booster Pack has been developed for the FR57xx FraunchPad way back in 2010. Pauline Lemore had a major role in providing the PCB and layout for the demo sample shown in the photos.