LINEAR Tiny 2-Cell Solar Panel Charging Battery Design Note 491 - Specifications, User Guide

SO LA PA NE OU TP UT UR RE NT (m A)

Tiny 2-Cell Solar Panel Charges Batteries in Compact, Off-Grid Devices Design Note 491 Fran Hoffart

Introduction beyond the power peak, the power curve quickly drops Advances in low power electronics now allow placement to zero (far left). Likewise, light loads push power toward of battery-powered sensors and other devices in locations zero (far right), but this tends to be less of an issue.

far from the power grid. Ideally, for true grid independence,

Of course, panel illumination affects available power—

the batteries should not need replacement, but instead be

less light means lower power output; more light, more

SOLAR PANEL OUTPUT POW ER (m

recharged using locally available renewable energy, such

power. Although illumination directly affects the value

as solar power. This Design Note shows how to produce

of peak power output, it does not do much to affect the

a compact battery charger that operates from a small peak’s location on the voltage scale. That is, regardless 2-cell solar panel. A unique feature of this design is that of illumination, the panel output voltage at which peak the DC/DC converter uses power point control to extract power occurs remains relatively constant. Thus, it makes maximum power from the solar panel. sense to moderate the output current so that the solar

panel voltage remains at or above this peak power voltage,

The Importance of Maximum Power Point Control

in this case 750m V. Doing so is called maximum power

Although solar cells or solar panels are rated by power

point control (MPPC).

output, a panel’s available power is hardly constant. Its

output power depends heavily on illumination, temperature Figure 2 shows the effects of varying sunlight on the and on the load current drawn from the panel. To illustrate charge current, with maximum power point control and this, Figure 1 shows the V-I characteristic of a 2-cell solar without. The simulated sunlight is varied from 100% down to approximately 20%, then back up to 100%. Note that as

panel at a constant illumination. The I-vs-V curve features

the sunlight intensity drops about 20%, the solar panel’s

a relatively constant-current characteristic from short-

output voltage and current also drop, but the LTC3105

circuit (at the far left) to around 550m A load current, at

maximum power point control prevents the panel’s

which point it bends to a constant-voltage characteristic

output voltage from dropping below the programmed

at lower currents, approaching maximum voltage at open

750m V. It accomplishes this by reducing the LTC3105

circuit (far right). The panel’s power output curve shows

output charge current to prevent the solar panel from

a clear peak in power output around 750m V/530m A, at

collapsing to near zero volts, as is shown in the plot on

the knee of the I-vs-V curve. If the load current increases

the right side of Figure 2. Without power point control, a small reduction in sunlight can completely stop charge

SHORT-CIRCUIT

current from flowing.

CURRENT

MAXIMUM

SOLAR PANEL

POWER LTC3105 Boost Converter with Input Power Control

OUTPUT

POINT

CURRENT

The LTC3105 is a synchronous step-up DC/DC converter SOLAR PANEL designed primarily to convert power from ambient energy

OUTPUT POWER

sources, such as low voltage solar cells and thermoelectric generators, to battery charging power. The LTC3105 uses

OPEN-CIRCUIT

VOLTAGE MPPC to deliver maximum available power from the source. It accomplishes this by reducing the LTC3105 output cur-

2 SERIES POLYCRYSTALLINE CELLS

rent to prevent the solar panel from collapsing to near zero

SOLAR PANEL OUTPUT VOLTAGE (m V) DN F01

L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of

Figure 1. Solar Panel Output Voltage, Current and Power

their respective owners.