CBC915 EnerChip Energy Processor

©2012 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com

DS-72-15 Rev F

Page 6 of 16

Figure 2. Current-Voltage Profile of a Constant Impedance Transducer

Most other energy harvesting transducers (e.g., thermoelectric and piezoelectric generators) have constant

output impedance. These constant impedance transducers can be further categorized into subgroups based on

impedance and typical output voltage.

Most - but not all - thermoelectric generators (TEGs) have low impedances (less than 300Ω) and output voltages

that vary linearly with the temperature difference across the generator. The matched impedance output voltage

of a TEG used in an energy harvesting application is typically in the low tens of millivolts to around 1V depending

on the number of elements in the TEG and temperature difference across the TEG. The CBC915 energy processer

is designed to work with TEGs with several hundred ohms of impedance and open circuit output voltages ranging

from 500mV to 2V. Extracting maximum efficiency from a TEG requires careful mechanical design which allows

good thermal conduction from the hot to cold side of the TEG but at the same time insulates any thermal leakage

path around the TEG that can reduce the temperature differential. Piezoelectric generators also have a constant

impedance characteristic, in that changes in input excitation cause a fairly linear change in output voltage.

Piezoelectric generators typically have output impedances in the 10kΩ to 100kΩ range, with output voltage

that changes linearly with input excitation. Most piezoelectric energy harvesters elements resonate at only one

particular frequency with a power bandwidth of only a few Hertz (2-3Hz being typical). The CBC915 Energy

Processer is designed to work with piezoelectric generators having an output voltage - after rectification and

filtering into a matched load - ranging from 4.5V to 20V DC.

The current-voltage (I-V) profile depicted in Figure 2 is indicative of a constant impedance transducer. From the

I-V curve, it is evident that operation at a point away from the peak power point results in a significant reduction

of power available from the transducer and therefore to the load. Consequently, to transfer a useful amount

of power to the load when input power is scarce, it is imperative to match the impedance of the transducer;

moreover efficient power conversion using impedance matching must be done dynamically, as the transducer I-V

profile will often vary in accordance with fluctuations in ambient conditions.

0

10

20

30

40

50

60

70

80

90

100

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

0

10

20

30

40

50

60

70

80

90

100

Current as a percentage of short circuit current

Normalized Power From a Constant Impedance Transducer

Power

Volts