The design of the circuits between the PE's sine-wave output and the final 3.6-Vdc delivered output is where the second part of the Thief's intellectual property crown jewels resides. At this point, the circuit design is patent pending, so the company is still tight-lipped on the specifics, aside from indicating that a very-low-loss MOSFET for switching is at the heart of the converter's design, along with other low-loss components and design techniques.
The current module uses capacitive storage but Lithium-ion battery versions are also available using a 40-mA/hour coin cell. "But those take longer to charge: We use capacitors for demonstrations as they have a faster charge/discharge cycle," said Tanner.
Next-generations of Joule-Thief will further decrease the device size, but will also enhance the broadband tuning capability to let designers tune its frequency response rather than rely on it to resonate with the vibration source. The current design has a 1/2-power bandwidth of ±3 Hz. "But the effective bandwidth is a lot wider than the 1/2-power bandwidth," said Tanner, adding that the company plans to take better advantage of that.
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Also on the agenda is improved battery technology, including thin-film Lithium batteries with solid-state electrolytes. "Then we can truly claim we have a solid-state replacement, versus Lithium-ion," said Tanner. Those chemistries have higher energy-collection efficiencies, simpler circuitry, and behave more like capacitors. Sources for the thin-film batteries include Infinite Power Solutions and Cymbet.
The module is currently priced at $499 and the kit at $699, but high-volume production prices can rival the approximately $10 cost of Lithion-ion batteries, so designers benefit once they get past the evaluation stage, Tanner said.