These sensors will need to operate at very low power levels to extend battery life for months or run on energy harvested from the environment. But that means they will need to draw a wide range of electrical currents. For example, a sensor might wake up periodically, take a measurement, and perform a simple calculation to see if that measurement crosses a certain threshold. These operations require relatively little current, but occasionally, the sensor might need to transmit an alert to a distant radio receiver. That requires much larger currents.
In general, power converters, which take an input voltage and convert it into a constant output voltage, are efficient only within a narrow range of currents. But at the International Solid-State Circuits Conference a few days ago, researchers from the Microsystems Technology Laboratory at MIT presented a new power converter that maintains its efficiency at currents ranging from 500 picoamps to 1 milliamp, a span that encompasses a 200,000-fold increase in current levels.
"Typically, converters have a quiescent power, which is the energy they consume even when they're not supplying any current to the load," says Arun Paidimarri, a postdoctoral fellow at MTL when the work was done and now at IBM Research. "So, for example, if the quiescent power is one microamp, then even if the load only draws one nanoamp, it's still going to consume one microamp of current. My converter is something that can maintain efficiency over a wide range of currents.".
Paidimarri, who also holds doctoral and master's degrees from MIT, is the first author of the conference paper. He is joined by his dissertation advisor, Anantha Chandrakasan, the Vannevar Bush Professor of Electrical Engineering and Computer Science at MIT.
Package Perspective:
The researchers' converter is a buck converter, meaning its output voltage is lower than its input voltage. Specifically, it takes input voltages ranging from 1.2 to 3.3 volts and reduces them to between 0.7 and 0.9 volts.
"In the low-power regime, the way these power converters operate is not based on a continuous flow of energy," says Paidimarri. "It's based on these energy packets. You have some switches and an inductor, and a capacitor in the power converter, and you're basically turning these switches on and off."
The control circuit for the switches includes a circuit that measures the converter's output voltage. If the output voltage is below a certain threshold—in this case, 0.9 volts—the controllers activate a switch and release an energy packet. They then take another measurement and, if necessary, release another packet.
If no device is receiving power from the converter, or if the power is going only to a simple local circuit, the controllers might release between 1 and a couple of hundred packets per second. But if the converter is powering a radio, it might need to release a million packets per second.
To accommodate that range of outputs, a typical converter, even a low-power one, will simply perform a million voltage measurements per second; on that basis, it will release between 1 and 1 million packets. Each measurement consumes power, but for most existing applications, the power consumption is negligible. For the Internet of Things, however, it is unacceptable.
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an element called a voltage divider, which siphons a small output current for measurement. In a typical converter, the voltage divider is simply another element in the circuit path; in effect, it is always on.
But the siphoning current reduces the converter's efficiency, so in the MIT researchers' chip, the divider is surrounded by a block of additional circuit elements that allow access to the divider only for the fraction of a second required for a measurement. The result is a 50 percent reduction in quiescent power, compared to the best experimental low-power buck converter, and a tenfold expansion of the current handling range.
"This opens up exciting new opportunities to operate these circuits using new types of energy harvesting sources, such as body-powered electronics," says Chandrakasan.
This work has been funded by Shell and Texas Instruments, and the prototype chip was built by the Taiwan Semiconductor Manufacturing Corporation, through its university satellite program.
Author: Larry Hardesty, MIT News Office
