Background Having started an initial adoption phase in the market place, energy harvesting ICs can convert an appropriate transducer’s output into an electric current for a battery charger device. Recent technology developments have pushed energy harvesting to the point of commercial viability, even though it has been emerging since early 2000. The opportunities in energy harvesting applications are widespread and include the following:
• Replace or recharge battery powered systems in situations where battery replacement is inconvenient, impractical or dangerous • Eliminate the need for wires to carry power or to transmit data • Smart wireless sensor networks to monitor and optimize complex industrial processes, remote field installations and building heating & cooling systems • Harvesting otherwise wasted heat from industrial processes, solar panels, internal combustion engines • Various consumer electronic accessory chargers
Many of these applications inherently have intermittent or low-power sources. And, so many implementations will want to charge a battery for a backup power source.
Shunt voltage references are simple to use; they have been around for many years and are in a myriad of products. However, they cannot effectively charge a battery. To configure one to do such a task is extremely cumbersome. Moreover, the ability to accurately and safely charge a lithium-ion/polymer, coin cell or a thin-film battery from a low-current source or an intermittent harvested energy source has been difficult to attain.
On the battery side, although technology has improved, portable electronic device battery cells or battery packs still require protection and conditioning to keep them running optimally. Lithium-ion/polymer batteries are a mature technology and a popular choice to power many electronic devices due to their high energy density, low self-discharge, low maintenance, and wide voltage range, among other features. Coin cells offer high energy density, stable discharge characteristics and low weight in a small form factor. Thin-film batteries are an emerging technology with such benefits as a high number of charge cycles and physical flexibility, i.e. they may be formed to fit in almost any shape depending on the end application. However, some potential detrimental effects on all these battery types exist if not properly charged and conditioned.
Design challenges for low power consumption chargers An adjustable shunt reference can be programmed for an appropriate battery float voltage, but it will lack the NTC function of a battery charger. More importantly, the required operating current is so high that battery charging from low power or intermittent sources is not practical. Alternatively, a discrete shunt reference can be built from a Zener diode, resistors, an NPN transistor and comparators for the NTC function. However, it will still suffer from the same limitations outlined above. Additionally, it will be cumbersome to implement and will consume much valuable PCB area by comparison.
Typical battery charger ICs require a constant DC input voltage and cannot handle bursts of energy. However, intermittent energy harvesting sources such as indoor photovoltaic arrays or piezoelectric transducers provide bursts of power. A unique IC with sub-1uA quiescent operating current is necessary to charge a battery from this type of energy source.
Lithium-ion/polymer chemistry batteries provide the high performance features necessary for portable electronic devices but must be treated with care. For example, Lithium-ion/polymer cells can become unstable if charged over 100mV beyond their recommended float voltage. Further, simultaneous instances of high voltage and high temperatures adversely affect battery life, and in extreme cases can lead to their self-destruction. In addition to potential adverse effects of simultaneous high temperature & high voltage, coin cells and thin-film batteries have capacity issues due to their small form factor.
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