FEATURE POWER ELECTRONICS
LOW POWER DESIGN - HOW LOW IS ENOUGH?
Tony Armstrong, marketing director, power products at Analog Devices Inc. presents a new solution T
he power architecture of many handheld devices mirrors that of a
cell phone. Typically, a 3.7V Li-Ion battery is used as the primary power source due to its high gravimetric (Wh/kg) and volumetric (Wh/m3
) energy
density. In the past, many high- powered devices used a 7.4V Li-ion battery to reduce current requirements, but the availability of inexpensive 5V power management ICs has pushed more and more handhelds to the lower voltage architecture. Nevertheless, high charge currents do not prevent consumers from wanting to charge their high-powered devices from a USB port if a high current wall adapter is not available. To satisfy these requirements, a battery charger must be able to charge at a high current (>2A) when a wall adapter is available, but still efficiently make use of the 2.5W to 4.5W available from USB.
At the other end of the power spectrum are the nanopower conversion requirements of energy harvesting systems such as those commonly found in WSNs that necessitate the use of power conversion ICs, which deal in very low levels of power and current.
AN ENERGY HARVESTING WSN New energy harvesting tools allow us to produce electrical energy from a wide variety of ambient sources. 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 and photovoltaics convert sunlight (or any photon source). 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. When trying to implement WSNs, a good question to consider is: How much power do I need to operate it?
30 DECEMBER/JANUARY 2019 | ELECTRONICS
Conceptually this would seem straight forward; 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 far does it need to be transmitted? This is due to the transceiver consuming approximately 50% of the energy used by the system for a single sensor reading. Of course, the energy provided by an 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. 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 is often used. Figure 1 illustrates that the type of energy harvesting IC to be used in any WSN system must be closely matched, from an electrical perspective, to the type of ambient energy that is going to be scavenged. The harvester, because of its potentially unlimited energy supply and deficiency in power, will be the energy source of the system. Moreover, a secondary power reservoir, either a battery or a capacitor, yields higher output power but stores less energy,
Figure 1:
Energy harvesting ICs are designed to work with the type of ambient energy harvested
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. This adds a further degree of complexity since now you must take into consideration how much energy should be stored in the secondary reservoir to compensate for the lack of an ambient energy source. Ambient energy sources include light, heat differentials, vibrating beams, transmitted RF signals, or just about any other source that can produce an electrical charge through a transducer. It is clear that WSNs have very low
Figure :
LTC3388-1/-3 typical application schematic
levels of energy 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 microcontrollers, on the power conversion side of the equation, there has been a void. However, Analog Devices Inc. introduced its LTC3388-1/- 3 to specifically address this need. This device is a 20V input capable synchronous buck converter that can deliver up to 50mA of continuous output current from a 3mm x 3mm (or MSOP10-E) package – see Figure 2 schematic. It operates from an input voltage range of 2.7V to 20V, making it ideal for a wide range of energy harvesting and battery-powered applications including “keep-alive” and industrial control power. 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 nanoamps of currents need to be converted where the choice becomes limited. This monolithic buck converter is ideal for low power applications.
Analog Devices Ltd.
www.analog.com e:
uksales@linear.com
/ ELECTRONICS
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