DC/DC converter design principles for wireless sensor node applications
With the advancement of energy harvesting elements, energy can be found and captured in almost any application, allowing for a wide variety of systems to be powered remotely. Applications such as “smart homes” and “mesh sensor networks” take advantage of the harvesting elements in order to power back-end electronics (microcontrollers, radios, and so on) in remote wireless sensor nodes (WSN). These systems can be completely self-powered by the harvesting element or assisted by a battery. In both cases, the primary power source is variable in nature and must be regulated for the back-end electronics via power conversion stages (linear or switch-mode).
The size of the energy source (harvester/battery)is primarily dependent on two factors: 1) system load demand, and 2) power conversion efficiency. A common way to reduce system load demand is to reduce the duty cycle (percentage of time the system is active) of the system load. While aggressive duty cycling and the ensuing long OFF times help reduce the average power consumed by the sensor node, they place stringent requirements on the quiescent current of the DC/DC converter. Further, the DC/DC converter needs to respond fast to the WSN modules waking up. The efficiency of the power converter plays a direct role in determining the current drawn from the storage element and, hence, the size and cost of the wireless sensor node system.
Switch-mode DC/DC converters are the preferred choice to interface to the energy storage elements owing to their superior efficiency compared to linear regulators. Specifically, in order to support a wide variety of analog, digital and radio loads, these switch-mode converters need control techniques that can maintain high efficiencies at load current levels from 10µA up to 50mA.Traditional pulse-width modulation (PWM) control techniques, which work well at currents above 100mA, suffer from poor efficiency at the load current levels encountered in WSNs. Further, to obtain high efficiency at low current levels and consume ultra-low quiescent current, the converter needs to enable charge transfer on an as needed basis. This precludes using circuit blocks that are a constant power draw or control techniques that require running high-frequency clocks.
Download a datasheet for the TPS62736: www.ti.com/tps62736-ca .