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from the IEEE ISSCC (International Solid
State Circuits Conference), San Francisco
MIT has
pulled out all the stops in an energy scavenging circuit for
piezoelectric power sources.
The traditional way to extract power from these is simply to
connect a bridge rectifier to the piezo generator, followed by a
reservoir capacitor.
However, a piezo generator is effectively an AC current source
in parallel with a capacitor, so on every half cycle the current
source wastes energy discharging the internal capacitance, then
charging it to the opposite polarity.
Described at
ISSCC
(International Solid-State Circuits Conference) in San Francisco
this week, MIT retains the rectifier and reservoir, but switches an
inductor across the piezoelectric generator output.
The inductor is chosen to resonate with the internal capacitance
at a frequency far higher than the generator operating frequency,
and is switched in just before zero crossing of the internal
generator current and out again just afterwards.
The switch is on for one half cycle of the resonant frequency,
allowing energy in the capacitor to be completely transferred to
the inductor, then completely transferred back to the
capacitor.
As it is a resonant half cycle, the voltage on the capacitance
is now reversed - completely removing the need for the current
source to do the job.
MIT calls this arrangement a bias-flip rectifier, and it
increased extracted power by four times.
If the arrangement was not clever enough already, away from
zero-crossings the inductor is completely switched out of the
rectifier and is used as the main inductor in the DC-DC converter
than follows the reservoir capacitor.
32.5µW is available for system use.