See also: Our roundup of the latest news 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.