Squeezing out power

Ultralow power electronics can serve in a wide array of wireless systems that work in transportation infrastructure, medical devices, tire pressure sensing, industrial sensing, building automation and various other areas. These systems generally spend most of their operational lives asleep in standby mode, consuming only a few microwatts of power. When awakened, a sensor measures parameters such as pressure, temperature or mechanical deflection and then transmits this data wirelessly to a remote control system.

The entire measurement, processing and transmission time usually takes place in only tens of milliseconds but may consume hundreds of milliwatts of power for this brief period. These applications are prime candidates for energy harvesting systems. The duty cycles of these applications are low, so the average power that must be harvested can also be relatively low. Though the power source for such sensors could simply be a battery, the cost of accessing and replacing the battery can be impractically large. This makes a source that captures ambient energy a more attractive alternative.

In the same vein, building automation systems harvesting ambient energy for occupancy sensors, thermostats and light switches can eliminate the need for power or control wiring. And a wireless network utilizing an energy harvesting technique can link any number of sensors together in a building to reduce heating, ventilation and air conditioning; (HVAC) and lighting costs by turning off power to unoccupied areas.

A typical energy harvesting configuration or wireless sensor node is comprised of five blocks: an ambient energy source such as a solar cell; a power conversion component to power the rest of the node; a sensing component that links the node to the physical world; a computing component consisting of a microprocessor or microcontroller that processes measurement data and stores it in memory; and a communication component consisting of a short range radio for communication with neighboring nodes and the outside world.

Examples of ambient energy sources include thermoelectric generators (TEGs) or thermopiles attached to a heat-generating source such as a HVAC duct, or a piezoelectric transducer attached to a vibrating mechanical source such as a windowpane and solar cells. In the case of a heat source, a compact thermoelectric transducer can convert small temperature differences into electrical energy. A piezoelectric device can convert mechanical vibrations or strain into electrical energy. Photovoltaic cells are capable of generating over 50 mW/cm2 of area in peak sunlight and up to 100 µW of electrical power in indoor lighting.

Until recently, tHE development of each circuit block has been impeded by a unique set of constraints that has hampered commercial viability. Although inexpensive low-power sensors and microcontrollers have been available for sometime, only recently have ultralow power transceivers been integrated with microcontrollers to offer extremely low-power wireless connectivity.

Moreover, there have been limitations in implementations of the energy harvester/manger block. Most such functions to date are discrete configurations with a relatively low performance, usually consisting of 30 or more components. These designs have low conversion efficiency, typically 30 to 40%, and high quiescent currents on the order of 50 µA. Both such deficiencies result in the requirement for relatively bulky batteries and solar cells that can prevent these systems from being as compact and self-contained as designers typically desire.

Without these larger storage elements, systems with low conversion efficiency will take longer to power up, which in turn lengthens the time it takes to build up enough power for transmitting sensor data. High quiescent currents in the power conversion circuitry can severely limit the amount of usable harvested energy made available to the application load. Designing circuitry able to work from low quiescent current while giving high power conversion efficiency also requires a high degree of analog switchmode power supply expertise - which is rarely readily available.

The missing link has been a highly integrated dc/dc converter that can harvest and manage surplus energy from extremely low power sources. To see the difference such a component can make, consider an energy- harvesting-based industrial monitoring system on a remote pipeline. Suppose it must constantly monitor the pipeline flow rate, temperature and pressure of every 50-m section. Built into the wall of the pipe at each node are temperature, pressure and flow sensors. Further suppose the system must take measurements and report them every five seconds.

It's clear this reporting task is a good fit for an energy harvesting system. It would be wildly expensive to run power to nodes spaced every 50 m along a pipeline running hundreds of miles. Installing batteries every 50 m would be equally problematic. The task calls for a power source that can continually generate enough power to run the node and which is always readily available. One of the most widely used and readily available sources of harvested energy would be a small solar cell combined with a storage device such as a battery or supercap. The combination could deliver power continually.

Linear Technology recently introduced the LTC3105 - an ultralow voltage step-up converter. It is specifically designed to simplify the task of harvesting and managing energy from low-voltage, high-impedance alternative power sources such as photovoltaic cells, TEGs and fuel cells. Its synchronous step-up design starts up from input voltages as low as 250 mV, so it can harvest energy from even the smallest photovoltaic cells in less than ideal lighting conditions.

The chip has an input voltage range of 0.2 to 5 V, wide enough for an array of applications. An integrated maximum-power-point controller (MPPC) lets the device operate directly from such high-impedance sources as photovoltaic cells by preventing the input power source voltage from collapsing below the user-programmable MPPC. Peak current limits are automatically adjusted to maximize power extraction from the source, while Burst Mode operation reduces quiescent current to only 18 µA.

The accompanying circuit diagram is an example of how the LTC3105 can charge a single-cell Li-ion battery from a single photovoltaic cell. This circuit lets the battery charge when the solar source is available. The battery can power the application, typically a wireless sensor node, from the stored energy when solar power is no longer available.

The LTC3105 provides the capability to start with voltages as low as 250 mV. During start-up, the AUX output initially is charged with the synchronous rectifiers disabled. Once V_AUX has reached approximately 1.4 V, the converter leaves start-up mode and enters normal operation. MPPC is off during start-up. However, the currents are internally limited to levels low enough to allow start-up from weak input sources. While the converter is in start-up mode, the internal switch between AUX and V_OUT remains disabled and the LDO (low drop out regulator) is disabled.

The synchronous rectifier is enabled when the output voltage exceeds both the input voltage and 1.2 V. In this mode, the N-channel MOSFET between SW and Gnd is enabled until the inductor current reaches the peak current limit. At that point, the N-channel MOSFET turns off and the P-channel MOSFET between SW and the driven output is enabled. This switch remains on until the inductor current drops below the valley current limit, and the cycle repeats. When VOUT reaches the regulation point, the N- and P-channel MOSFETs connected to the SW pin are disabled and the converter enters sleep.

To power microcontrollers and external sensors, an integrated LDO provides a regulated 6 mA rail. The integrated MPPC circuit lets the user set the optimal input voltage operating point for whatever serves as the power source. Furthermore, the MPPC circuit dynamically regulates the average inductor current to prevent the input voltage from dropping below the MPPC threshold.

All in all, power management is the key to making remote wireless sensing a practical endeavor. However, it must be implemented right from the concept of the design. The LTC3105 energy harvesting dc/dc converter is specifically designed to dramatically simplify the task of harvesting and managing energy from low-voltage, high-impedance power sources such as photovoltaic cells, TEGs and fuel cells. Onboard maximum-power-point control optimizes the energy extracted from this wide range of sources under greatly varying conditions.