Bountiful power harvesting

A wide range of low-power industrial sensors and controllers are turning to alternative sources of energy as their primary or supplemental source of 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 for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are on track to become near “zero” power devices using just harvested energy (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. Energy harvesting sensors for building-automation applications have already been deployed in Europe. Thus the growth stage may already be underway.

Commercial acceptance

Though the concept of energy harvesting has been around for a number of years, the implementation of a system in the real world 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, 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, 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 converted by an energy harvesting circuit 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. 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.

Each circuit system block in this chain, with the possible exception of the energy source itself, has had its own special set of constraints that have impaired its economical viability. The situation has changed recently, however. Low cost and low power sensors and microcontrollers have been available for a couple of years or so; but 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 30 or more discrete components. Such designs have low conversion efficiency and high quiescent currents. Both of these deficiencies impact the end system. With the low conversion efficiency, it takes longer 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 reduces the output of the energy-harvesting source, because it must first supply the current needed for its own operation before it can supply any to external devices.

What has been missing until now has been a highly integrated, high efficiency dc/dc converters that can both harvest and manage the energy from either a thermal or piezoelectric source. In that regard, Linear Technology's 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 from small solar panels. Its step-up topology operates from input voltages as low as 20 mV. This is significant because it lets the LTC3108 harvest energy from a TEG with as little as a 1°C temperature change - something a discrete implementation struggles to do because of its high quiescent current.

In use, a small step-up transformer typically boosts the input voltage source going to a LTC3108 which then manages power for wireless sensing and data acquisition. The chip can harvest small temperature differences and generate system power instead of using traditional battery power.

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 lets it boost input voltages as low as 20 mV enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the transformer secondary winding and is typically in the range of 20 kHz to 200 kHz.

For input voltages as low as 20 mV, a primary-secondary turns ratio of about 1:100 is recommended. Higher input voltages can use a lower turns ratio. These transformers are standard off-the-shelf components and are readily available. A compound depletion mode N-channel MOSFET is what makes 20-mV operation possible.

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 gets boosted and rectified using an external charge pump capacitor and rectifiers internal to the LTC3108.

The device's internal 2.2-V 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 lets it become active as soon as V_AUX has charged to 2.3 V, 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 3 mA.

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 V_S1 and V_S2. The four fixed output voltage are: 2.35 V for supercapacitors, 3.3 V for standard capacitors, 4.1 V for Lithium-Ion battery termination, or 5 V for higher energy storage and a main system rail to power a wireless transmitter or sensors. The 5-V rail thus eliminates the need for multi-meg-Ohm external resistors. As a result, the LTC3108 does not need special PCB coatings to minimize leakage, as with discrete designs where large value resistors are a necessity.

A host microprocessor can control a second output, V_OUT2, using the V_{OUT2_EN} pin. When enabled, V_OUT2 connects to V_OUT through a P-channel MOSFET switch. This output can be used to power external circuits such as sensors or amplifiers that lack low-power sleep or shutdown capability. An example would be powering a MOSFET on and off as part of a sensing circuit within a building thermostat.

The V_STORE capacitor may be a large value (thousands of microfarads or even Farads), to provide holdup if input power is lost. Once Power-up is complete, the Main, Backup and switched outputs are all available. If the input power fails, operation can still continue using the V_STORE capacitor for power. 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 5.3 V. 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 P_GOOD output will go high. If V_OUT drops more than 9% from its regulated voltage, P_GOOD will go low. The P_GOOD output is designed to drive a microprocessor or other chip I/O and is not intended to drive a higher current load such as an LED.

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 an external capacitor. It can also bleed off any excess power via an internal shunt regulator and maintain a regulated output voltage by means of a Nanopower high efficiency buck regulator.

The LTC3588-1's internal full-wave bridge rectifier is accessible via two differential inputs, P_Z1 and P_Z2, that rectify ac inputs. This rectified output is then stored on a capacitor at the V_IN 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 400 mV with typical Piezo-generated currents, which are normally around 10 µA. This bridge is capable of carrying up to 50 mA of current. The buck regulator is enabled once there is sufficient voltage on V_IN to produce a regulated output.

The buck regulator uses a hysteretic voltage algorithm to control the output through internal feedback from the V_OUT 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 260 mA through an internal PMOS switch and then ramping it down to 0 mA 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 100 mA of average load current when it is switching.

With analog switchmode power supply design expertise in short supply around the globe, it has been difficult to design an effective energy harvesting system. However, the introduction of the LTC3108 and LTC3588-1 simplifies the process dramatically. High integration, including power management control and off-the-shelf external components, make them small, simple and easy-to-use to complete the energy harvesting chain.