Finally: The Missing Link for Energy Harvesting

Optimized power acquisition and utilization now a reality

Everywhere you look, engineers are coming up with new and innovative ways to harness non-traditional energy sources to solve real world problems. Increased safety and accessibility, lower maintenance costs, improved energy efficiency and system flexibility are just a few of the benefits attainable with "harvested" energy, wireless sensing and monitoring/control systems. The high cost of energy, new government regulations and environmental concerns have greatly increased demand for more efficient use of power everywhere. Emerging technologies in alternative energy and improvements in power utilization have the potential to enable performance breakthroughs in many diverse markets. Furthermore, new products that can take advantage of these new technologies represent excellent growth opportunities both in short and long term.

By Tony Armstrong, Director of Product Marketing, Power Products, Linear Technology Corporation

A wide range of low-power industrial sensors and controllers are turning to alternative sources of energy as the primary or supplemental means of supplying power. Ideally, such harvested energy will eliminate the need for wired power or batteries altogether. Transducers that create electricity from readily available physical sources such as temperature differentials (thermoelectric generators or thermopiles), mechanical vibration (piezoelectric or electromechanical devices) and light (photovoltaic devices) are becoming viable sources of power for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near "zero" power devices using harvested energy only (commonly referred to as "nanoPower" by some).

Although energy harvesting has been emerging since early 2000 (its embryonic phase), recent technology developments have pushed it to the point of commercial viability. In short, in 2010 we are poised for its "growth" phase. Building automation sensor applications utilizing energy harvesting techniques have already been deployed in Europe, illustrating that the growth stage may have already begun.

Existing Applications Demonstrate Commercial Viability

Even though the concept of energy harvesting has been around for a number of years, the implementation of a system in a real world environment has been cumbersome, complex and costly. Nevertheless, examples of markets where an energy harvesting approach has been used include transportation infrastructure, wireless medical devices, tire pressure sensing, and of course, building automation. In the case of building automation, systems such as occupancy sensors, thermostats and light switches can eliminate the power or control wiring normally required and use a mechanical or energy harvesting system instead. This alternative approach can also mitigate the costs of routine maintenance normally associated with wired systems in addition to eliminating the need for wiring to be installed in the first place, or for regular battery replacement in wireless applications.

Similarly, a wireless network utilizing an energy harvesting technique can link any number of sensors together in a building to reduce heating, ventilation & air conditioning (HVAC) and lighting costs by turning off power to non-essential areas when the building has no occupants. Furthermore, the cost of energy harvesting electronics is often less than running sense wires, so there is clearly economic gain to be had by adopting a harvested power technique.

A typical energy scavenging configuration or system, (represented by the four main circuit system blocks shown in Figure 1 below), usually consists of a free energy source such as a thermoelectric generator (TEG) or thermopile attached to a heat generating source, such as an HVAC duct for instance. These small thermoelectric devices can convert small temperature differences into electrical energy. This electrical energy can then be converted by an energy harvesting circuit (the second block in Figure 1) and modified into a usable form to power downstream circuits. These downstream electronics will usually consist of some kind of sensor, analog-to-digital converter and an ultralow power microcontroller (the third block in Figure 1). These components can take this harvested energy, now in the form of an electric current, and wake up a sensor to take a reading or a measurement then make this data available for transmission via an ultralow power wireless transceiver – represented by the fourth block in the circuit chain shown in Figure 1.

Figure 1: The four main blocks of a typical energy-scavenging system

Each circuit system block in this chain, with the possible exception of the energy source itself has had its own unique set of constraints that have impaired its commercial viability until now. Low cost and low power sensors and microcontrollers have been available for quite sometime; however, it is only within the last couple of years that ultralow power transceivers have become commercially available. Nevertheless, the laggard in this chain has been the energy harvester and power manager.

Existing implementations of the power manager block are a low performance discrete configuration, usually consisting of 35 components or more. Such designs have low conversion efficiency and high quiescent currents. Both of these deficiencies result in performance compromised in an end system. The low conversion efficiency will increase the amount of time required to power up a system, which in turn increases the time interval between taking a sensor reading and transmitting this data. A high quiescent-current limits how low the energy-harvesting source can be since it must first overcome the current level needed for operation before it can use any excess to supply power to the outputs. Finally, it also requires a very high degree of analog switchmode power supply expertise – something that is in short supply around the world!

The "missing link," if you will, has been a highly integrated DC/DC converter that can harvest and manage surplus energy from extremely low input voltage sources. However, that’s all about to change.

The Missing Link

Linear Technology has recently introduced its LTC3108 – an ultralow voltage step-up converter and power manager specifically designed to greatly simplify the task of harvesting and managing surplus energy from extremely low input voltage sources such as thermopiles, thermoelectric generators (TEGs) and even small solar panels. Its step-up topology operates from input voltages as low as 20mV. This is significant since it allows the LTC3108 to harvest energy from a TEG with as little as 1C temperature change – something a discrete implementation struggles to meet due to its high quiescent current.

The circuit shown in Figure 2 uses a small step-up transformer to boost the input voltage source to a LTC3108 which then provides a complete power management solution for wireless sensing and data acquisition. It can harvest small temperature differences and generate system power instead of using traditional battery power.

Figure 2: LTC3108 used in a wireless remote sensor application powered from a TEG (Peltier Cell)

The LTC3108 utilizes a depletion mode N-channel MOSFET switch to form a resonant step-up oscillator using an external step-up transformer and a small coupling capacitor. This allows it to boost input voltages as low as 20mV high enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the transformer’s secondary winding and is typically in the range of 20kHz to 200kHz. For input voltages as low as 20mV, a primary-secondary turns ratio of about 1:100 is recommended. For higher input voltages, a lower turns ratio can be used. These transformers are standard, off-the-shelf components, and are readily available from magnetic suppliers.

Our compound depletion mode N-channel MOSFET is what makes 20mV operation possible. As can be seen in Figure 3, the LTC3108 takes a “systems level” approach to solving a complex problem. It can convert the low voltage source and manage the energy between multiple outputs.The AC voltage produced on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor (from the secondary winding to pin C1) and the rectifiers internal to the LTC3108. This rectifier circuit feeds current into the V_AUX pin, providing charge to the external V_AUX capacitor and then the other outputs. The internal 2.2V LDO can support a low-power processor or other low power ICs. The LDO is powered by the higher value of either V_AUX or V_OUT. This enables it to become active as soon as V_AUX has charged to 2.3V, while the V_OUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main V_OUT capacitor if V_AUX drops below V_OUT. The LDO output can supply up to 3mA.

Figure 3: LTC3108 block diagram

The main output voltage on V_OUT is charged from the V_AUX supply and is user programmable to one of four regulated voltages using the voltage select pins VS1 and VS2. The four fixed output voltage are: 2.35V for supercapacitors, 3.3V for standard capacitors, 4.1V for Lithium-Ion battery termination or 5V for higher energy storage and a main system rail to power a wireless transmitter or sensors – thereby eliminating the need for multi-meg-Ohm external resistors. As a result, the LTC3108 does not require special board coatings to minimize leakage, such as discrete designs where very large value resistors are required.

A second output, V_OUT2, can be turned on and off by the host microprocessor using the V_{OUT2_EN} pin. When enabled, V_OUT2 is connected to Vout through a P-channel MOSFET switch. This output can be used to power external circuits such as sensors or amplifiers that do not have low power sleep or shutdown capability. An example of this would be to power on and off a MOSFET as part of a sensing circuit within a building thermostat.

The V_STORE capacitor may be a very large value (thousands of microfarads or even Farads), to provide holdup at times when the input power may be lost. Once Power-up has been completed, the Main, Backup and switched outputs are all available. If the input power fails, operation can still continue, operating off the V_STORE capacitor. The V_STORE output can be used to charge a large storage capacitor or rechargeable battery after V_OUT has reached regulation. Once V_OUT has reached regulation, the V_STORE output will be allowed to charge up to the V_AUX voltage, which is clamped at 5.3V. Not only can the storage element on V_STORE be used to power the system if the input source is lost but it can also be used to supplement the current demanded by V_OUT, V_OUT2 and the LDO outputs if the input source has insufficient energy.

A power good comparator monitors the V_OUT voltage. Once V_OUT has charged to within 7% of its regulated voltage, the PGOOD output will go high. If V_OUT drops more than 9% from its regulated voltage, PGOOD will go low. The PGOOD output is designed to drive a microprocessor or other chip I/O and is not intended to drive a higher current load such as a LED.

Conclusion

In summary, the LTC3108 thermal energy harvesting, DC-to-DC step-up converter and system manager is a revolutionary device that extracts energy from solar cells, thermo-electric generators or other similar thermal sources. Its unique resonant power converter topology allows it to start up at an extremely low 20mV input voltage. Its high integration, including power management control and off-the-shelf external components, make it the smallest, simplest and easy-to-use solution available to complete the energy harvesting chain.