Revolutionary Solutions for Energy Harvesting

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 or strain (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 at an inflection point and are poised for the commencement of the “growth” phase. Building automation sensor applications utilizing energy harvesting techniques have already been deployed in Europe, illustrating that the growth stage may already be underway.


Existing Deployments Demonstrate Commercial Acceptance
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 the largest so far, 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.

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 lower than running supply wires, so there is clearly an 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), usually consists of a free energy source. Examples of such sources include a thermoelectric generator (TEG) or thermopile 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. In the case of a heat source, then a compact thermoelectric device can convert small temperature differences into electrical energy. And in the case where vibration or strain is available, a piezoelectric device can also convert these small vibrations or strain differences into electrical energy. In either case, the electrical energy produced can then be convertered 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 economical viability until now. Low cost and low power sensors and microcontrollers have been available for a couple of years or so; however, it is only recently that ultralow power transceivers have become commercially available. Nevertheless, the laggard in this chain has been the energy harvester.

Existing implementations of the energy harvester block typically consist of low performing discrete configurations, usually comprising of 30 components or more. Such designs have low conversion efficiency and high quiescent currents. Both of these deficiencies result in compromised performance 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 output of the energy-harvesting source can be, since it must first overcome the current level needed for its own operation before it can supply any excess power to the output.


Revolutionary Solutions
What has been missing until now has been a highly integrated, high efficiency DC/DC converter solutions that can both harvest and manage the energy from either a thermal or piezoelectric source. However, Linear Technology’s revolutionary LTC3108 and LTC3588-1 will greatly simplify the task of harvesting surplus energy from a variety of sources.

The recently introduced LTC3108 is 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. The 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 VAUX pin, providing charge to the external VAUX 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 VAUX or VOUT. This enables it to become active as soon as VAUX has charged to 2.3V, while the VOUT storage capacitor is still charging. In the event of a step load on the LDO output, current can come from the main VOUT capacitor if VAUX drops below VOUT. The LDO output can supply up to 3mA.



Figure 3. The LTC3108 Block Diagram.


The main output voltage on VOUT is charged from the VAUX 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, VOUT2, can be turned on and off by the host microprocessor using the VOUT2_EN pin. When enabled, VOUT2 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 VSTORE 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 VSTORE capacitor. The VSTORE output can be used to charge a large storage capacitor or rechargeable battery after VOUT has reached regulation. Once VOUT has reached regulation, the VSTORE output will be allowed to charge up to the VAUX voltage, which is clamped at 5.3V. Not only can the storage element on VSTORE be used to power the system if the input source is lost but it can also be used to supplement the current demanded by VOUT, VOUT2 and the LDO outputs if the input source has insufficient energy.

A power good comparator monitors the VOUT voltage. Once VOUT has charged to within 7% of its regulated voltage, the PGOOD output will go high. If VOUT 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.

The circuit shown in Figure 4 utilizes a small piezoelectric transducer to convert mechanical vibration into an AC voltage source that is fed into the LTC3588’s internal bridge rectifier. It can harvest small vibration sources and generate system power instead of using traditional battery power.



Figure 4. The LTC3588-1 circuit schematic converters a vibration or strain source into electric current


The LTC3588-1 is an ultralow quiescent current power supply designed specifically for energy harvesting and/or low current step-down applications. It can interface directly to a piezoelectric or alternative AC power source, rectify the voltage waveform and store harvested energy on to an external capacitor, bleed off any excess power via an internal shunt regulator and maintain a regulated output voltage be means of a Nanopower high efficiency buck regulator.

The LTC3588-1’s internal full-wave bridge rectifier is accessible via two differential inputs, PZ1 and PZ2 that rectifies AC inputs. This rectified output is then stored on a capacitor at the VIN pin and can be used as an energy reservoir for the buck converter. The low-loss bridge rectifier has a total voltage drop of about 400mV with typical Piezo generate currents, which are normally around 10A. This bridge is capable of carrying up to 50mA of current. The buck regulator is enabled once there is sufficient voltage on VIN to produce a regulated output.

The buck regulator uses a hysteretic voltage algorithm to control the output through internal feedback from the VOUT sense pin. The buck converter charges an output capacitor through an inductor to a value slightly higher than the regulation point. It does this by ramping the inductor current to 260mA through an internal PMOS switch and then ramping it down to 0mA through an internal NMOS switch, thereby efficiently delivering energy to the output capacitor. Its hysteretic method of providing a regulated output reduces losses associated with FET switching and maintains an output at light loads. The buck converter delivers a minimum of 100mA of average load current when it is switching.

Conclusion
With analog switchmode power supply design expertise in short supply around the globe, it has been difficult to design an effective energy harvesting system as illustrated in Figure 1. However, with the introduction of the LTC3108 and LTC3588-1 that’s all about to change. These revolutionary devices can extract energy from almost any source of heat or mechanical vibration. Furthermore, with their comprehensive feature set and ease of design, they greatly simplify the hard-to-do power conversion design aspects of an energy harvesting chain. This is good news for the system designer because high integration, including power management control and off-the-shelf external components, make them the smallest, simplest and easy-to-use solutions available to complete the energy harvesting chain

Author Information
Tony Armstrong is the Director of Product Marketing, Power Products, for Linear Technology Corp.