Researchers from MIT’s Microsystems Technologies Laboratories (MTL) have designed a new power converter that maintains efficiency at currents ranging from 100 picoamps to 1 milliamp, a span that encompasses a million-fold increase in current levels.
If we really want to fully develop the Internet of Things (IoT), we will need more efficient power sources to drive the billions of IoT devices that we expect to have in the near future. The IoT is a developing idea where every imaginable appliance, vehicle, civil structure, medical and manufacturing tool, and even pets and domestic animals, is embedded with sensors and actuators that gather information and send it directly to networked servers.
These sensors and actuators must be driven with (1) very low power sources and (2) a precise management of current. Therefore, in order to extend battery life and provide the exact electrical current and voltage to the sensors, energy efficiency is, probably, the most critical concern of designers. Some of these IoT devices will be deployed in hard to reach locations with the expectation that the actuators and sensors will run for years without human intervention.
Voltage converters are devices that convert an input voltage to a steady output voltage, but the existing converters are effective only within a small range of currents. Powering IoT and small battery-operated devices with these converters is challenging.
First, these devices use various power-saving schemes such as entering into hibernation when not active, or waking up to take a measurement and going back to “sleep” mode. These operations require a small amount of current, in the order of microamps. But sometimes a sensor must transmit a message to a distant server or to a radio receiver. This operation requires a large amount of current, in the order of hundreds of milliamps. In both cases, the steady voltage must be the same for efficient operation of the device. This represents a typical ratio of 1,000,000:1 between the minimum and maximum of current levels. With existing converters this is often not possible.
Second, most power converters consume power while in operation, even in the hibernation mode.
The MIT Solution
To overcome these two hurdles — and some others — in the development of IoT systems, researchers from MIT’s Microsystems Technologies Laboratories (MTL) presented a new converter design at the 2017 IEEE International Solid-State Circuit Conference (ISSCC) in San Francisco, California. The new design can maintain voltage efficiency at currents ranging from 500 picoamps to 1 milliamp. This represents a range of 2,000,000:1.
“Typically, converters have a quiescent power, which is the power that they consume even when they’re not providing any current to the load,” said Arun Paidimarri, who was a postdoc at MTL when the work was done and is now at IBM Research. “So, for example, if the quiescent power is a microamp, then even if the load pulls 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.”
The new MIT power converter is a step-down converter that takes input voltages from 1.2 to 3.3 volts and produces outputs between 0.7 and 0.9 volts. In addition, the device uses a novel approach where power is delivered only when needed. Using electronic switches, the device sends energy “packets” when required and in the amount required.
“In the low-power regime, the way these power converters work, it’s not based on a continuous flow of energy,” Paidimarri said. “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. If no device is drawing current from the converter, or if the current is going only to a simple, local circuit, the controllers might 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.”
Paidimarri’s converter uses a variable clock that runs the switch controller at a big range of rates.
“This opens up exciting new opportunities to operate these circuits from new types of energy-harvesting sources, such as body-powered electronics,” said 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.”
The abstract of the article can be found here.