CBC915 EnerChip Energy Processor

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DS-72-15 Rev C

Page 5 of 16

CBC915 Operation

The CBC915 performs the function of efficiently converting energy from an external power transducer to a voltage

and current usable by typical applications such as remote wireless sensors. The CBC915 performs this function

by dynamically matching its input impedance to the output impedance of the transducer. At impedance match,

maximum power will be extracted from the transducer.

Differences Among Power Transducers

There are many different types of power transducers used in energy harvesting applications; they are broadly

divided into two categories. Photovoltaic (PV) cells are unique and consequently in their own category due to

the diode-like current-voltage (IV) characteristics of PV cells. The PV cell impedance changes with changes in

incident light intensity. As the light intensity increases, the PV cell impedance decreases. For example, typical

Therefore, transferring maximum power from the PV cell into CBC915 Energy Processor boost converter requires

the input impedance of the boost converter to change dynamically in response to light intensity (thus PV cell

impedance) fluctuations. Plotting a load line of current vs. voltage on a graph will show a diode-like response

curve, in contrast to a purely resistive source which having a linear load line response. When presented with

a matched impedance, the output voltage of an efficient PV cell is fairly constant over varying incident light

intensity. In contrast, the voltage at the peak power point of a less efficient voltage will change with variations

in light intensity. The CBC915 adjusts its input impedance to match the output characteristics of any type or

quality of PV cell. The CBC915 was designed to work with PV cells arrays of 1-series to 8-series cells, equating to

approximately 0.5V to 4V at matched impedance. In most cases it is most power efficient to use a PV array with

two cells in series. Series-cell configurations with fewer cells have the advantage of not losing as much efficiency

due to shading and have more efficiency per unit area because there are fewer gaps in the array that do not

contribute to energy conversion.

The power curve of Figure 1 is typical of a low power PV cell used in energy harvesting applications. Electrical

impedance of the cell varies strongly as a function of ambient light. As illustrated, the power curve is highly non-

linear, meaning that connecting an electrical load to the PV cell that is not matched to its impedance results in

inefficient power transfer to that load.

Figure 1. Maximum Peak Power Point for Variable Resistance Transducer

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Current as a percentage of short circuit current

Normalized Power From a Photovoltaic Cell

Power

Volts