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esigning a powermanagement
system for a source that
harvests energy from the environment
can be challenging. These sources,
from solar cells to vibrational energy,
and even power from thermal
differences, are all small amounts of
power that vary unpredictably. This
creates a significant challenge for
a power management system that
has to run efficiently and provide a
steady output. The voltage and power
requirements of the sensors and
processors in the node being powered
have dropped, so using an energy
harvesting source has become more
practical; but there are still different
ways to manage these power sub-
systems.
It is obviously necessary to optimize
the design for the low average power
in the system, but it is also necessary
to understand the lower and upper
limits of the energy harvesting source.
A buck/boost converter will have a
lower limit below which the power
stage may either shut down or not
start, interrupting the operation of the
system. This also means the power
up sequencing must understand
implications of when to power each
device along with other devices so
that the power drain does not push
the power conversion stage below
that lower limit.
However, it is also necessary to be
aware of the potential peak power
to avoid overwhelming the additional
energy storage element such as a
capacitor or battery.
Using hardware timers and interrupts
rather than software reduces the
overall power requirement, and
having status indications and alerts
implemented across the systems are
essential so that power management
choices can be made with the right
information.
Isolating all the loads in the system
and making them switchable gives the
power manager more opportunities
to avoid problems and optimize the
performance. This also helps isolate
any devices that are consuming too
much power.
A buck/boost converter is a suitable
architecture for harvesting energy
from movement or vibration via a
piezoelectric transducer. A protective
shunt at the input allows the power
manager to accommodate a variety
of different piezoelectric elements,
which can have short-circuit currents
around 10µA.
An example of a typical power
manager for a piezoelectric source is
the LTC3588 from Linear Technology.
This is designed to interface directly
to a piezoelectric or alternative power
source, rectify a voltage waveform
and store the harvested energy on
an external capacitor, as well as bleed
off any excess power via an internal
shunt regulator.
It integrates a low-loss full-wave
D
Power Management Tips for Energy
Harvesting Systems
European Editors
New-Tech Magazine Europe l 66