Tony Armstrong, Linear Technology
The proliferation of wireless sensors supporting the Internet of Things (IoT) has increased the demand for small, compact and efficient power converters tailored to un-tethered lower power devices. One of the more recent emerging market segments covered under the IoT that is particularly interesting from an energy harvesting perspective is the wearable electronics category.
In addition, wearable technology is not just for humans, there are many applications for animals, too. Recent examples include ultrasound-delivering treatment patches and electronic saddle optimization for horses to collars on other animals that variously track, identify, diagnose and so on. Nevertheless, regardless of the end application, most of these devices require a battery as the main power source even if it will be augmented with an ambient energy source provided one is available.
However, for human-based applications, it looks like there will soon be wearable fabrics that can generate electricity from different forms of ambient energy that might only require a small primary battery as a back-up source. These free energy sources include body temperature generation, photovoltaic sources such as indoor lighting or even just plain old daylight, as well as kinetic energy sourced from regular body movements.
A fitting term might be to call them “power suits!” One company at the forefront of such research is the European Union funded project Dephotex, which has developed methods to make photovoltaic material light (as in weight) and flexible enough to be worn. Naturally, the material will convert photons into electrical energy, which in-turn can be used to power various electronic devices worn by the user, or to charge their primary batteries, or even a combination of both.
Similarly, at the low end of the power spectrum there are nanopower conversion requirements for energy harvesting systems such as those commonly found in IoT equipment (think Google Glass) which necessitate the use of power conversion ICs that deal in very low levels of power and current. These can be 10s of microwatts and nanoamps of current, respectively.
State-of-the-art and off-the-shelf Energy Harvesting (EH) technologies, for example in vibration energy harvesting and indoor or wearable photovoltaic cells, yield power levels in the order of milliwatts under typical operating conditions. While such power levels may appear restrictive, the operation of harvesting elements over a number of years can mean that the technologies are broadly comparable to long-life primary batteries, both in terms of energy provision and the cost per energy unit provided.
Moreover, systems incorporating EH will typically be capable of recharging after depletion, something that systems powered only by primary battery cannot do. Nevertheless, most implementations will use an ambient energy source as the primary power source, but will supplement it with a primary battery that can be switched in if the ambient energy source goes away or is disrupted.
Of course, the energy provided by the energy harvesting source depends on how long the source is in operation. Therefore, the primary metric for comparison of scavenged sources is power density, not energy density. EH 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 source is often used. The secondary source could be a re-chargeable battery or a storage capacitor (maybe even supercapacitors).
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 down-stream electronic systems. For example, Linear has introduced a number of power conversion ICs which have the necessary features and performance characteristics to enable such low levels of harvested power to be used in IoT.
The LTC3331 is a complete regulating EH solution that delivers up to 50mA of continuous output current to extend battery life when harvestable energy is available (see Figure 1). It requires no supply current from the battery when providing regulated power to the load from harvested energy and only 950nA operating when powered from the battery under no-load conditions. The LTC3331 integrates a high voltage EH power supply, plus a synchronous buck-boost DC/DC converter powered from a rechargeable primary cell battery to create a single non-interruptible output for energy harvesting applications such as IoT devices, wearables and wireless sensor nodes (WSNs).
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Figure 1. The LTC3331 converts multiple energy sources and can use a primary rechargeable battery
The LTC3331’s EH power supply, consisting of a full-wave bridge rectifier accommodating AC or DC inputs and a high efficiency synchronous buck converter, harvests energy from piezoelectric (AC), solar (DC) or magnetic (AC) sources. A 10mA shunt enables simple charging of the battery with harvested energy while a low battery disconnect function protects the battery from deep discharge. The rechargeable battery powers a synchronous buck-boost converter that operates from 1.8V to 5.5V at its input and is used when harvested energy is not available to regulate the output whether the input is above, below or equal to the output.
The LTC3331 battery charger has a very important power management feature that cannot be overlooked when dealing with micropower sources. The LTC3331 incorporates logical control of the battery charger such that it will only charge the battery when the energy harvested supply has excess energy. Without this logical function the energy harvested source would get stuck at startup at some non-optimal operating point and not be able to power the intended application through its startup. The LTC3331 automatically transitions to the battery when the harvesting source is no longer available. This has the added benefit of allowing the battery operated WSN to extend its operating life from 10 years to over 20 years if a suitable EH power source is available at least half of the time, and even longer if the EH source is more prevalent. A supercapacitor balancer is also integrated allowing for increased output storage.
Since harvested energy from wearables is very low, meaning nanoamps to milliamps, it is imperative that any DC/DC conversion uses as little power as possible in order to ensure optimum energy transfer. Thus, to attain such a strict objective, the DC/DC converter itself must consume current in the order of nanoamps. It was because of this that Linear Technology introduced the LTC3335 – a nanopower buck-boost DC/DC converter with an integrated coulomb counter aimed at WSN’s IoT products, wearables and general purpose energy harvesting application (see Figure 2).
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Figure 2. Typical application schematic for the LTC3335 nanopower buck-boost converter
The LTC3335 is a high efficiency, low quiescent current (680nA) converter. Its integrated coulomb counter monitors accumulated battery discharge in long life battery-powered applications. This counter stores the accumulated battery discharge in an internal register accessible via an I2C interface. The buck-boost converter can operate down to 1.8V on its input and provides eight pin-selectable output voltages with up to 50mA of output current. To accommodate a wide range of battery types and sizes, the peak input current can be selected from as low as 5mA to as high as 250mA and the full-scale coulomb counter has a programmable range of 32,768:1.
Its integrated precision coulomb counter monitors the accumulated charge that is transferred from a battery whenever the buck-boost converter is delivering current to the load. The buck-boost converter operates as an H-Bridge for all battery and output voltage conditions when not in sleep mode (see Figure 3).
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Figure 3. Timing diagram of the LTC3335 operating in H-Bridge mode
Switch A and C turn on a the beginning of each burst cycle. The inductor current ramps to Ipeak and then switches A and C turn off. Switches B and D then turn on unitl the inductor current ramps to zero. The cycle repeats until Vout reaches the sleep threshold. If Ipeak and the switch AC(ON) time (tAC) are both known, then the BAT discharge coulombs (shaded area in Figure 3) can be calculated by counting the number of AC(ON) cycles and multiplying by the charge per AC(ON) given in the formula below:
q AC(ON) = (Ipeak * tAC)/2
When the buck-boost is operating, the LTC3335 measures the actual AC(ON) time relative to the full scale ON time (tFS, approximately 11.74µs) which is internally adjusted to compensate for errors in the actual selected Ipeak value due to supply, temperature and process variations. This results in a very accurate “measurement” of the charge transferred ofrm the battery during each AC(ON) cycle.
It is clear that there will be numerous WSNs, wearable and IoT products which will need nanopower DC/DC conversion and coulomb counting to assure their optium performance and longevity. However, it is only recently that such products have become avaible on the market. Thanks to suppliers such as Linear Technology, there will be lots of options open to the designers of nanopower devices to choose from.