Green energy boosted by analog circuitry

PORTLAND, Ore. — Energy harvesting could get a boost from an analog circuit that harnesses the lowest power outputs from individual photovoltaic cells, thermoelectric generators and electromagnetic sources, according to its developer.

Advanced Linear Devices Inc. (ALD, Sunnyvale, Calif.) said it increased the output of green power sources with an array of zero-threshold transistors called a super voltage booster. The energy-harvesting modules can now harness even the trace output power from sources under 1 volt.

"We developed a new circuit that takes advantage of our zero-threshold MOSFET's," said Bob Chao, CEO of ALD. "This analog circuit can take advantage of ultra-low power supply voltages in the neighborhood of 100 millivolts [0.1 volts] and boosts it to 4 volts so that it can charge energy-harvesting modules."

Green power supplies based on photovoltaic cells, thermoelectric generators and electromagnetic sources lack sufficient power to directly charge energy-harvesting modules. The sub-1-volt outputs from individual solar cells, for instance, force designers to gang together at least 10 cells in order to boost voltage output to drive conventional CMOS circuitry.

Likewise, thermoelectric generators convert heat to electrical energy using an array of thermocouples made from two dissimilar metals that are fused to harness a potential temperature difference. However, that scheme generates only a few millivolts.

ALD claims its zero-threshold MOSFET array can boost the output from individual solar cells and thermocouples so that they can drive an energy-harvesting module. By boosting the trace power outputs from individual power sources, their energy can be stored as an accumulating charge on a capacitor. When energy builds up, the capacitor's output is then switched on to drive intermittent duty-cycle devices such as remote sensors.

"The challenge was how to make a circuit work with energy sources that only supply 100 millivolts output" Chao said, and then " boosting them by 40 times to 4 volts. Now we can harness the trace outputs from sources that were not useful before."

Conventional transistors require at least 0.7 volts to turn on, meaning they are of little use when trying to harvest energy from solar cells and other sources. ALD said it solved the problem by biasing MOSFET input with buried, floating gates with a charge of -0.7 volts. This enabled the transistors to turn on for trace voltages since the bias voltage on the floating gate adds to trace inputs, making them appear to be above 0.7 volts.

ALD claims its booster circuit will enable a new generation of power sources for implantable medical devices, remote and photo sensor arrays, wireless transmitters, RFID tags and other embedded applications. The circuit is now in prototype stage, and will be available to customers by the third quarter of 2009.