V20W Datasheet, PDF (15/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

APPLICATIONS INFORMATION - SWITCHED CAPACITOR BOOST CIRCUIT
Goals
Allow intermittently operating circuits to be
powered from extremely weak vibration sources,
or moderate vibration sources at frequencies sig-
nificantly different from the energy harvester's res-
onant frequency (Figure 3)
Physically disconnect the load during periods of
insufficient voltage to avoid "over-the-hump" prob-
lems of cold circuit start-up from harvested power
Maximize chances of success where vibration
source's characteristics (amplitude and frequency
content) cannot be known in advance.
Simple “One-Shot” Application
A simple usage scenario is an embedded sensor with
data storage/transmission capability, which takes one
set of measurements each time it is powered up
(relying on the loss and subsequent re-application of
power to start the next measurement). In this case, the
measurement frequency is variable and depends on the
vibration amplitude. To operate the sensor directly from
the boost circuit requires:
Estimation (or measurement of) the run-time and
power consumption of your application within its
voltage limits
Sizing the CP_out according to worst-case usage,
allowing some headroom
For such one-shot sensors, it is recommended to
create a large load (e.g. drive an LED or GPIO pin tied
to ground) after completing the task in order to ensure
the power output cycles in high-vibration conditions.
A typical microcontroller sensor application’s load
profile will be “bursty”, complicating the task of
estimating the required value of CP_out. However, if the
load can be approximated in terms of a resistive load,
the following equations can be used to estimate the
required capacitance, available runtime, energy per
discharge or power stored.
1
C
1 _{R ln} V_o
T_d
V
Equation 1: Capacitance needed for
a given runtime (F)
T_d
R C ln ^Vo
V
Equation 2: Runtime for a given
capacitance (sec.)
W
1C
2
V₀₂
V²
Equation 3: Energy per discharge
(Joules or watt-seconds)
PW
T_c T_d
Equation 4: Average Power (Watts)
In the equations above, T_d is the runtime or discharge
time in seconds, T_c is the charge time in seconds, R is
the equivalent load resistance in ohms, V₀ is the
starting output voltage (2.4), V is the final output
voltage (1.8V or the minimum operating voltage of the
sensor, whichever is greater), and C is the capacitance
in Farads. Likewise, the output voltage can be modeled
as a simple RC time constant, V = V₀e^-T/RC.
Continuously-Powered Application with
Input-Dependent Triggering
Sometimes it may be advantageous to incorporate
vibration-powered battery maintenance and/or
triggering into a continuously-running circuit. For
example, a sensor may require low-level continuous
power to maintain a realtime clock, but measurement
tasks are only needed when a piece of machinery is
known to be operating. Alternately, it may be desired to
dynamically adjust the measurement rate based on the
incoming power to maintain a given power budget. In
REVISION N0. 002 REVISION DATE: 01-23-2013
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