Researchers at the Massachusetts Institute of Technology (MIT) have discovered a new way to convert power that could increase the energy efficiency of IoT devices by up to 50 percent.
The scientists, based at MIT’s Microsystems Technology Laboratories (MTL), revealed the research at the International Solid-State Circuits Conference, held two weeks ago in San Francisco.
Their research holds great promise for sensor networks, particularly in environments where sensors need to operate at low power in order to extend battery life or harvest energy from the environment. In these situations, sensors must be able to ‘feed’ off a wide range of electrical currents – some of the operations they perform require very little charge, others far more.
This is where a power converter comes in handy, by taking an input voltage and converting it to a steady output voltage – but the trouble with many converters is that they can only deliver a narrow range of currents.
The new power converter invented at MIT, by contrast, maintains its efficiency at currents ranging from 500 picoamps to 1 milliamp, a span that covers a 2,000,000-fold increase in amperes.
The invention is a step-down converter, meaning that its output voltage is lower than its input voltage. It takes input voltages ranging from 1.2 to 3.3 volts and reduces them to between 0.7 and 0.9 volts.
Arun Paidimarri, a postdoc at MTL when the work was carried out and now employed at IBM Research, said that, typically, converters have a quiescent power, which is the power that they consume even when they’re not providing any current to the load.
“So, for example, if the quiescent power is a microamp, then even if the load pull’s only a nanoamp, it’s still going to consume a microamp of current. My converter is something that can maintain efficiency over a wide range of currents,” he said.
“In the low-power regime, the way these power converters work, it’s not based on a continuous flow of energy,” he continued. “It’s based on these packets of energy. You have these switches, and an inductor, and a capacitor in the power converter, and you basically turn on and off these switches.”
In this case, if there is no active device on the converter, the controllers release between one and a couple hundred packets per second. But, if the converter is feeding power to a radio, it might need to release a million packets a second.
The new converter includes a variable clock, which can run the switch controllers at a wide range of rates, but needs more complex circuits. The circuit that monitors the converter’s output voltage contains an element called a voltage divider, which takes a little current from the output for measurement.
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Low power, big energy efficiency
As siphoning current lowers the converter’s efficiency, the researchers had the voltage divider surrounded by a block of additional circuit elements, which grant access to the divider only for the fraction of a second that taking a small measurement requires. This results in a 50 percent reduction in quiescent power over even the best previously reported experimental low-power, step-down converter and a tenfold expansion of the current-handling range, according to the scientists.
“This opens up exciting new opportunities to operate these circuits from new types of energy-harvesting sources, such as body-powered electronics,” commented Anantha Chandrakasan, the Vannevar Bush Professor of Electrical Engineering and Computer Science at MIT and Paidimarri’s thesis advisor.
“This work pushes the boundaries of the state of the art in low-power DC-DC converters, how low you can go in terms of the quiescent current, and the efficiencies that you can achieve at these low current levels,” said Yogesh Ramadass, the director of power management research at Texas Instruments’ Kilby Labs.
“You don’t want your converter to burn up more than what is being delivered, so it’s essential for the converter to have a very low quiescent power state.”
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