The IoT is the idea that vehicles,
appliances, civil structures, manufacturing equipment, and even livestock will
soon have sensors that report information directly to networked servers, aiding
with maintenance and the coordination of tasks.

Those sensors will have to operate
at very low powers, in order to extend battery life for months or make do with
energy harvested from the environment. But that means that they’ll need to draw
a wide range of electrical currents. A sensor might, for instance, wake up
every so often, take a measurement, and perform a small calculation to see
whether that measurement crosses some threshold. Those 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.

 

MIT-Power-Harvesting_feb2017

Researchers from
MIT’s Microsystems Technologies Laboratories (MTL) have designed a new power
converter that maintains its efficiency at currents ranging from 100 picoamps
to 1 milliamp, a span that encompasses a millionfold increase in current
levels.

 

 

Generally, power converters take an
input voltage and convert it to a steady output voltage, but they are efficient
only within a narrow range of currents. At the International Solid-State
Circuits Conference, researchers from MIT’s Microsystems Technologies
Laboratories (MTL) presented a new power converter that maintains its
efficiency at currents ranging from 500 pA to 1 mA, a span that encompasses a
200,000-fold increase in current levels.

 

The researchers’ converter is a
step-down converter, meaning that its output voltage is lower than its input
voltage. In particular, it takes input voltages ranging from 1.2 to 3.3 volts
and reduces them to between 0.7 and 0.9 V.

 

The control circuitry for the
switches includes a circuit that measures the output voltage of the converter.
If the output voltage is below some threshold — in this case, 0.9 V — the
controllers throw a switch and release a packet of energy. Then they perform
another measurement and, if necessary, release another packet.

 

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 1 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. 

 

To accommodate that range of
outputs, a typical converter — even a low-power one — will simply perform 1
million voltage measurements a second; on that basis, it will release anywhere
from 1 to 1 million packets. Each measurement consumes energy, but for most
existing applications, the power drain is negligible. For the internet of
things, however, it’s intolerable.

 

Clocking down

 

The converter features a variable
clock that can run the switch controllers at a wide range of rates. However, that
requires more complex control circuits. The circuit that monitors the
converter’s output voltage, for instance, contains an element called a voltage
divider, which siphons off a little current from the output for measurement. In
a typical converter, the voltage divider is just another element in the circuit
path; it is, in effect, always on.

 

But siphoning current lowers the
converter’s efficiency, so in the MIT researchers’ chip, the divider is
surrounded by a block of additional circuit elements, which grant access to the
divider only for the fraction of a second that a measurement requires. The
result is a 50% 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.

 

This opens up new opportunities to
operate these circuits from new types of energy-harvesting sources, such as
body-powered electronics, according to the researchers.

 

“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,” says 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 work was funded by Shell and Texas
Instruments, and the prototype chips were built by the Taiwan Semiconductor
Manufacturing Corporation, through its University Shuttle Program. For more
information: http://news.mit.edu/2017/efficient-power-converter-internet-of-things-0217