Low power conversion for energy harvesting

There is plenty of ambient energy in the world around us and the conventional approach for energy harvesting has been through solar panels and wind generators. However, new harvesting tools allow us to produce electrical energy from a wide variety of ambient sources. Furthermore, it is not the energy conversion efficiency of the circuits that is important, but more the amount of ‘average harvested’ energy that is available to power it. For instance, thermoelectric generators convert heat to electricity, Piezo elements convert mechanical vibration, PVs convert sunlight (or any photon source) and galvanism converts energy from moisture. This makes it possible to power remote sensors, or to charge a storage device such as a capacitor or thin film battery, so that a microprocessor or transmitter can be powered from a remote location without a local power source.

It is becoming more common at the lower end of the power spectrum, such as nanopower conversion in Wireless Sensor Networks (WSNs) and other sensors. This is creating a need for power conversion ICs that can work with very low levels of power and current; often just 10s of microwatts and nanoamps of current, respectively. However, the availability of such power conversion products, including battery chargers, operating at sub-1µA of current, are extremely limited.

In general terms, the necessary IC performance characteristics needed for inclusion in these applications include: low standby quiescent currents (typically less than 6µA and as low as 450nA); low start-up voltages (as low as 20mV); high input voltage capability (up to 34V continuous and 40V transients); the ability to handle AC inputs; multiple output capability and autonomous system power management; Maximum Power Point Control (MPPC) for solar inputs and compact solution footprints with minimal external components.

Density dilemma

WSNs are basically a self-contained system consisting of some kind of transducer to convert the ambient energy source into an electrical signal, usually followed by a DC/DC converter and manager to supply the downstream electronics with the right voltage level and current. The downstream electronics consist of an MCU, a sensor and a transceiver.

When trying to implement WSNs, a good question to consider is how much power is needed to operate it. Conceptually this would seem fairly straightforward, however in reality it is a little more difficult due to a number of factors. For instance, how frequently does a reading need to be taken? Or, more importantly, how large will the data packet be and how much power is needed for it to be transmitted? This is due to the transceiver consuming approximately 50% of the energy used by the system for a single sensor reading and transmission. Several factors affect the power consumption characteristics of an energy harvesting system or WSN and they all need to be taken into consideration.

Of course, the energy provided by the energy harvesting source depends on how long the source is available. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. Energy harvesting is generally subject to low, variable and unpredictable levels of available power, so a hybrid structure that interfaces to the harvester and a secondary power reservoir are often used. The harvester, because of its unlimited energy supply and deficiency in power, is the energy source of the system. The secondary power reservoir, either a battery or a capacitor, yields higher output power but stores less energy, supplying power when required but otherwise regularly receiving charge from the harvester. Thus, in situations when there is no ambient energy from which to harvest power, the secondary power reservoir must be used to power the WSN. Of course, from a system designer’s perspective, this adds a further degree of complexity since they must now take into consideration how much energy must be stored in the secondary reservoir to compensate for the lack of an ambient energy source.

It is clear that WSNs must use very low levels of energy when available. This, in turn, means that the components used in the system must be able to deal with these low power levels. While this has already been attained with the transceivers and MCUs, on the power conversion and battery charging side of the equation there has been a void. However, Linear Technology developed its LTC3388-1/-3 and LTC4071 to specifically address these requirements.

The LTC3388-1/-3 is a 20V input capable synchronous buck converter than can deliver up to 50mA of continuous output current from a 3x3mm (or MSOP10-E) package. It operates from an input voltage range of 2.7 to 20V, making it ideal for a wide range of energy harvesting and battery-powered applications including ‘keep-alive’, sensor and industrial control power. The LTC3388-1/-3 utilises hysteretic synchronous rectification to optimise efficiency over a wide range of load currents. It can offer over 90% efficiency for loads ranging from 15μA to 50mA and only requires 400nA of quiescent current, enabling it to provide extended battery life where one is used for auxiliary power.

Figure 1 - LTC3388-1/-3 typical application schematic

The LT3388-1/-3 incorporates an accurate undervoltage lock-out feature to disable the converter when the input voltage drops below 2.3V, reducing quiescent current to only 400nA. Once in regulation (at no load), the LTC3388-1/-3 enters a sleep mode to minimise quiescent current to only 720nA. The buck converter then turns on and off as needed to maintain output regulation. An additional standby mode disables switching while the output is in regulation for short duration loads, such as wireless modems, which require low ripple. This high efficiency, low quiescent current design is ideal for energy harvesting, which requires long charging cycles accompanied by short burst loads for powering sensors and wireless modems.

Even though portable applications and energy harvesting systems have a broad range of power levels for their correct operation, from microwatts to greater than 1W, there are many power conversion ICs available for selection by the system designer. However, it is at the lower end of the power range, where the levels fall into the nanopower level that the choice becomes limited. Fortunately, there are power conversion and battery charging solutions available for the designer to select from with quiescent currents of less than a microamp to prolong battery life for keep-alive circuits in low power sensors and a new generation of WSNs.

Often times, a battery is used as auxiliary back-up power in WSNs; however, the design challenge of how to charge it from low power sources is not a trivial one. Linear’s LTC4071 is a shunt battery charger system that includes integrated battery pack protection and a low battery disconnect feature to protect low capacity batteries from damage due to self-discharge. It is a simple, yet sophisticated charger and protector for Lithium-Ion/Polymer batteries. Its ultra low 550nA operating current enables charging from previously unusable very low current, intermittent or continuous charging sources such as that supplied from energy harvesting applications. An internal thermal battery conditioner reduces the float voltage to protect Li-Ion/Polymer cells, coin cells or thin film batteries at elevated battery temperatures. Housed in a low profile 8-lead 2x3mm DFN package, the LTC4071 provides a complete and ultra-compact charger solution with just a single external resistor required in series with the input voltage.