V20W Datasheet, PDF (18/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

APPLICATIONS INFORMATION - LOAD ISOLATION EXAMPLE
Load Isolation Example for Maximum
Efficiency with Low-Impedance Loads
A cantilevered-beam piezoelectric energy harvester is a
complex electromechanical system in which the
electrical and mechanical loading of the beam are inter-
related. Understanding of this relationship is critical to
getting the most out of the system.
A properly tuned switched boost-buck circuit will
always outperform any circuit in which you do not
tune to the characteristics of its application. This
application note demonstrates a high-efficiency
switched step-down (buck) converter and
optimizations for maximum performance in a real-
world application. This circuit isolates the end-
application’s electrical load from the piezo beam,
providing proper impedance-matching of the circuit’s
“virtual load” to the beam, as well as minimizing
mechanical loading effects. Such a circuit is ideal for
low-impedance loads such as rechargeable batteries,
“bursty” loads such as intermittently-operating
sensors/transmitters, and applications where the
electrical load cannot be known in advance.
Basic Piezoelectric Beam Model
Before mechanical loading effects are taken into
account, each piezo beam can be thought of as a small
current source in parallel with a capacitor and parasitic
resistance, as shown in Figure 1. Typical values for this
parallel capacitance and resistance are on the order of
10nF and >40M, respectively. This parallel resistance
is insignificant for our purposes and may be ignored.
The current flow is equal to the derivative of the strain-
induced charge, or dQ/dt. The voltage transfer function
of each beam therefore is V(s)/q(s) = sR/(1+sRC),
where R is the parallel resistance and C is the parallel
capacitance of the beam.
In its simplest form, power is drawn from a small
capacitor which is constantly recharged by its
R
leak
R
leak
Figure 1: Piezo parallel and series equivalent circuits
environment. Care must be taken to choose when this
power is drawn. To maximize transfer efficiency, the
load must be matched to the piezo’s equivalent
impedance. In practice, the piezo impedance at a given
amplitude and frequency, as well as the load
impedance, can be thought of as a pair of simple (but
unknown) resistances which make up a resistor
divider. The power transfer between the two is
optimized when their values match. This corresponds
to the point at which the piezo’s loaded voltage is equal
to half its open-circuit voltage. Thus the impedance
match can be optimized without formally measuring or
knowing the impedance of the piezo source or load in-
situ.
Implementation
The circuit of Figure 2 provides a simple but effective
approach to meeting these goals.
The main components are a bridge rectifier formed by
D1-D4, low-power comparator (U1) and buck
converter (U2). During vibration, main storage
capacitor C1 slowly charges until its voltage reaches
the operating point (V_oc/2) set by R1 and R2. Buck
converter U2 is enabled once the stored voltage
exceeds this value plus a small hysteresis. At typical
loads, the buck converter operation into the load will
REVISION N0. 002 REVISION DATE: 01-23-2013
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