Tony Armstrong and Dave Salerno, Power Products at Linear Technology Corporation discuss how energy harvesting from thermoelectric sources is getting a boost from an ultralow voltage converter for wireless sensor devices
The proliferation of ultralow power wireless sensor nodes for measurement and control, combined with new energy harvesting technology, has made it possible to produce completely autonomous systems that are powered by local ambient energy instead of batteries. Powering a wireless sensor node from ambient or ‘free’ energy is attractive because it can supplement or eliminate the need for batteries or wires. This is a benefit when battery replacement or servicing is inconvenient, costly or dangerous.
Many wireless sensor systems consume very low average power, making them prime candidates to be powered by energy harvesting techniques. Many sensor nodes are used to monitor physical quantities that change slowly. Measurements can therefore be taken and transmitted infrequently, resulting in a low duty cycle of operation and a correspondingly low average power requirement.
For example, if a sensor system requires 3.3V at 30mA (100mW) while awake, but is only active for 10ms out of every second, then the average power required is only 1mW, assuming the sensor system current is reduced to microamps during the inactive time between transmit bursts.
If the same wireless sensor only samples and transmits once a minute instead of once a second, the average power plummets under 20µW. This difference is significant, because most forms of energy harvesting offer very little steady-state power; usually no more than a few milliwatts, and in some cases only microwatts. The less average power required by an application, the more likely it can be powered by harvested energy.
A typical wireless sensor system powered by harvested energy can be broken down into five fundamental blocks, as illustrated in Figure 1. With the exception of the power management block, all of these blocks have been commonly available for some time. For example, microprocessors that run on microwatts of power, and small, cost effective RF transmitters and transceivers that also consume very little power are widely available.
An ideal power management solution for energy harvesting should be small, easy to apply and perform well while operating from the exceptionally high or low voltages produced by common energy harvesting sources, ideally providing a good load match to the source impedance for optimal power transfer. The power manager itself must require very little current to manage the accumulated energy and produce regulated output voltages with a minimal number of discretes.
Some applications, such as wireless HVAC sensors or geothermal powered sensors present another unique challenge to an energy harvesting power converter. These applications require that the energy harvesting power manager be able to operate not only from a very low input voltage, but one of either polarity as the polarity of the ?T across the thermoelectric generator (TEG) changes. This is a particularly challenging problem, and at voltages in the tens or hundreds of millivolts, diode bridge rectifiers are not an option.
The LTC3109 – a step-up converter and power manager from Linear Technology, solves the energy harvesting problem for ultralow input voltage sources of either polarity and is available in either a 4mm by 4mm by 0.75mm 20-pin QFN or 20-pin SSOP package.
It is a highly integrated monolithic power management solution for operation from input voltages as low as ±30mV. This capability enables it to power wireless sensors from a thermoelectric generator (TEG), harvesting energy from temperature differentials (?T) as small as 2°C.
Using two small (6mm by 6mm), off-the-shelf step-up transformers and a handful of low cost capacitors, it provides the regulated output voltages necessary for powering today’s wireless sensor electronics.
This converter uses the step-up transformers and internal MOSFETs to form a resonant oscillator capable of operating from very low input voltages. With a transformer ratio of 1:100, the converter can start up with inputs as low as 30mV, regardless of polarity.
The transformer secondary winding feeds a charge pump and rectifier circuit, which is used to power the IC (via the VAUX pin) and charge the output capacitors. The 2.2V LDO output is designed to be in regulation first, to power a low power microprocessor as soon as possible. After that, the main output capacitor is charged to the voltage programmed by the VS1 and VS2 pins (2.35V, 3.3V, 4.1V or 5.0V) for powering sensors, analogue circuitry, RF transceivers or even charging a supercapacitor or battery.
The VOUT reservoir capacitor supplies the burst energy required during the low duty cycle load pulse when the wireless sensor is active and transmitting. A switched output (VOUT2), easily controlled by the host, is also provided for powering circuits that don’t have a shutdown or low power sleep mode. A power good output is included to alert the host that the main output voltage is close to its regulated value. Figure 2 shows the circuit schematic for the device.
TEGs are simply thermoelectric modules that convert a temperature differential across the device, and resulting heat flow through it, into a voltage via the Seebeck effect. The reverse of this phenomenon, known as the Peltier effect, produces a temperature differential by applying a voltage and is familiarly used in thermoelectric coolers (TECs).The polarity of the output voltage is dependent on the polarity of the temperature differential across the TEG. Reverse the hot and cold sides of the TEG and the output voltage changes polarity.
With its unique ability to operate at input voltages as low as ±30mV, this converter provides a simple, effective power management solution that enables thermal energy harvesting for powering wireless sensors and other low power applications from common thermoelectric devices.
The device interfaces seamlessly with existing low power building blocks to support autonomous wireless sensors and extend the battery life in critical battery backup applications.
Linear Technology Corporation
