CBC915 EnerChip Energy Processor
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.
Normalized Power From a Constant Impedance Transducer
2500
2250
2000
1750
1500
1250
1000
750
100
90
80
70
60
50
40
30
20
10
0
Power
Volts
500
250
0
0
10
20
30
40
50
60
70
80
90
100
Current as a percentage of short circuit current
Figure 2. Current-Voltage Profile of a Constant Impedance Transducer
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DS-72-15 Rev F
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