Researchers from the Microsystems Technology Laboratories (MTL) at MIT have built an ultra-efficient power converter for IoT applications that consumes half of the power when compared to typical converters. Generally, power converters, take an input voltage and convert it to a steady output voltage. They are efficient only within a narrow range of currents. This new design from the researchers at MIT maintains its efficiency at currents ranging from 500 picoamps to 1 milliamp, a span that encompasses a 2,000,000-fold increase.
In the next few years vehicles, appliances, civil structures, manufacturing equipment, and even livestock will have sensors that report information directly to networked servers, aiding with maintenance and the coordination of tasks. The sensors will have to operate at very low powers, in order to extend battery life for months. This means that they’ll need to draw a wide range of electrical currents. Broadly sensors perform two types of tasks, one is where it has to wake up every so often, take a measurement, and perform a small calculation to see whether that measurement crosses some threshold. This type of task requires very low power, the other case would be where it might need to transmit an alert to a distant radio receiver. That requires much higher power level.
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 pulls a nanoamp, it is still going to consume a microamp of current. This converter is something that can maintain efficiency over a wide range of currents. The new 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 volts.
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 volts - 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. However this for IoT applications this is significant.
The converter thus features a variable clock, which can run the switch controllers at a wide range of rates. This 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 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.
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. 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.
