Components for Solar Charging Portable Electronics

Consumers continue to drive demand for more powerful and longer-lasting portable gadgets. We love to stay connected, hate to be tethered and don’t want to slow down to recharge. As a result, IC designers and process engineers need new ways to reduce a gadget’s power consumption, giving consumers the ability to work and play for longer, between charging cycles.

Options for increasing the charge time of portable electronics include everything from single use alkaline batteries to hand-crank chargers. However, solar cells provide an additional option that allow designers and process engineers to avoid the complexity of alkaline batteries and hand-cranked chargers.

When we think of solar energy, we generally imagine large rooftop panels composed of arrays of solar cells cut from standard-sized crystalline semiconductor wafers, which are each roughly six inches wide. Don’t worry, though. There is no need to start cleaving or cracking these semiconductor wafers to fit them into a portable charger. There are many types of small solar cells that can be integrated directly into a cell phone or iPod case to supplement battery power with solar power and help increase charge time.

For a cell phone application, form factor will determine which type of solar cell is best. Sanyo Energy devices are available in small sizes of approximately 40 mm by 33 mm (the 869-1012-ND). Pairing two 869-1014-ND cells offers nearly 50 mA of current and will fit relatively easily on the back of most popular smart phones. See Table 1 below for all the Sanyo Energy solar cell options.

DigiKey Part Number	Open-Circuit Voltage	Short-Circuit Current	Physical Dimensions
869-1011-ND	7.7 V	30 mA	150.0 (L) x 37.5 (W) x 2.3 (H) mm
869-1014-ND	6 V	23.9 mA	57.7 (L) x 55.1 (W) x 1.3 (H) mm
869-1010-ND	6 V	22.6 mA	70.0 (L) x 50.0 (W) x 2.3 (H) mm
869-1013-ND	7.7 V	22.3 mA	75.0 (L) x 55.0 (W) x 2.3 (H) mm
869-1008-ND	5.1 V	17.8 mA	60.1 (L) x 41.3 (W) x 2.3 ( H) mm
869-1015-ND	6 V	17.7 mA	57.7 (L) x 41.3 (W) x 1.3 (H) mm
869-1017-ND	6.8 V	16.3 mA	48.1 (L) x 55.1 (W) x 1.3 (H) mm
869-1012-ND	7.7 V	4.8 mA	40.1 (L) x 33.1 (W) x 2.3 (H) mm

For larger gadgets, such as tablet devices, designers might consider the Sanyo Energy solar cell 869-1011-ND at only 150 x 37.5 mm. One strip of 869-1011-ND provides 30 mA of peak current, and a typical tablet device can hold up to four strips. This allows a light scavenging case to potentially provide more than 100 mA of charging current, depending on the ambient environment.

iPod case designers will want to fit a case as closely as possible to the dimensions of the iPod. Although most touch-screen phones today tend to have comparable form factors, we are bound to see both large and small units. A solar cell should completely fill the available area in order to maximize the energy-harvesting capability of a solar scavenging case. Therefore; a designer should create a platform with the flexibility to adapt to various shapes and sizes of individual devices and evolving industry trends in the design of portable gadgets.

Amorphous silicon cells can be used to coat active solar material onto a starting substrate so that it can be almost any two-dimensional shape. Consequently, amorphous silicon cells make an excellent choice for a solar trickle charger accessory. DigiKey stocks a variety of amorphous silicon cells.

Naturally, some regulation is required to get the charge captured by the solar cells safely and efficiently into the rechargeable battery that ultimately powers the portable device. This is true of any power source, but it is especially important for energy harvesting because the available power varies widely. A solar scavenging cell phone case will likely be kept in a variety of environments, from a completely dark pants pocket to outside in direct sunlight.

An example of a power management IC from Linear Technology that is designed specifically for solar charging applications is the LTC3652. This power management IC offers peak power tracking to maximize the charge extracted from the solar cell. Input voltage can be as low as 4.95 V, making the IC a good choice for many of the amorphous cells discussed earlier. For the solar charging accessory, the external component count can often be reduced because a blocking diode is not required for battery voltages up to 4.2 V.

The LT3652 is available in either TSSOP or WFDFN 12-pin packages. These small form factors allow the full solar charging solution to fit into a small accessory case. The DigiKey part number for the TSSOP package is LT3652EMSE#PBF-ND.

Portable electronic gadgets are exposed to various forms of light, such as fluorescent light inside or sun outside. So instead of running to the USB, wall or car charger, IC designers can give potable device users another option. I think you’ll find that harvesting solar energy to extend the battery life of portable electronics is dramatically simplified with the many silicon solar cell technologies available for portable power applications.