Low Power Conversion for Energy Harvesting

However, it is at the “low” end of the power spectrum (where nanopower conversion in WSNs and sensors is becoming increasingly common) that the need for power conversion ICs that can use very low power and currents is real. These are often tens of microwatts (μW) of power and tens of nanoamps (nA) of current, respectively. However, the supply of such power conversion products (including battery chargers) that operate at less than 1μA is extremely limited.

Generally speaking, the performance characteristics of ICs that must be accepted and adopted for energy harvesting applications include:

l Low standby quiescent current - typically less than 6μA and can be as low as 450nA

l Low start-up voltage - as low as 20mV

l High input voltage capability - up to 34V (continuous) and 40V (transient)

l Capable of handling AC input

l Multiple output capability and autonomous system power management

l Maximum Power Point Control (MPPC) for solar input

Compact solution footprint and minimal external components

A wireless sensor node (WSN) is basically a standalone system consisting of a number of transducers that convert ambient energy sources into electrical signals, followed by a DC/DC converter and manager to power downstream electronics with appropriate voltage levels and currents. Downstream electronics include microcontrollers, sensors, and transceivers.

When trying to implement a WSN, a good question to consider is: How much power does it take to run this WSN? Conceptually, this seems like a fairly simple question; however, in reality, it is a bit difficult to answer due to several factors. For example: How often will readings need to be taken? Or, more importantly, how large will the data packet be and how much power will it take to transmit it? This is because approximately 50% of the energy used by the system to obtain and transmit a sensor reading is consumed by the transceiver. There are many factors that affect the power consumption characteristics of an energy harvesting system or WSN, and they all need to be considered.

Of course, the amount of energy provided by the energy harvesting source depends on how long the source is operational. Therefore, the primary metric for comparing energy harvesting sources is power density, not energy density. The available power in energy harvesting systems is generally low, variable, and unpredictable, so a hybrid architecture is typically used that couples an energy harvester to an auxiliary energy reservoir. The harvester (due to its unlimited energy supply and insufficient power) is the energy source for the system. The auxiliary energy reservoir (a battery or a capacitor) produces a higher output power but stores less energy, providing power when needed and otherwise receiving charge periodically from the harvester. Therefore, the auxiliary energy reservoir must be used to power the WSN when there is no ambient energy available for harvesting power. Of course, this adds further complexity from the perspective of the system designer, as they now have to consider how much energy must be stored in the auxiliary energy reservoir to compensate for the lack of ambient energy source.

Obviously, 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 handle these low power levels. While transceivers and microcontrollers have addressed this issue, there is still a gap in the associated power conversion and battery charging. However, Linear Technology has developed the LTC3388-1 / LTC3388-3 and LTC4071 to specifically address these requirements.

The LTC3388-1 / LTC3388-3 is a synchronous step-down converter that accepts a 20V input and delivers up to 50mA of continuous output current in a 3mm x 3mm (or MSOP10-E) package, see Figure 1. The device operates over an input voltage range of 2.7V to 20V, making it ideal for a wide variety of energy harvesting and battery-powered applications, including “keep-alive” sensors and industrial control supplies.

Figure 1: LTC3388-1 / LTC3388-3 Typical Application Schematic

The LTC3388-1 / LTC3388-3 uses a hysteretic synchronous rectification method to optimize efficiency over a wide load current range. It can provide over 90% efficiency in the 15μA to 50mA load range and requires only 400nA of quiescent current, enabling it to extend battery life when using a battery as an auxiliary power source.

The LTC3388-1 / LTC3388-3 features accurate undervoltage lockout (UVLO) protection to disable the converter when the input voltage drops below 2.3V, reducing quiescent current to only 400nA. Once in regulation (no load), the LTC3388-1 / LTC3388-3 enters sleep mode to minimize quiescent current to only 720nA. The buck regulator then switches on and off as needed to keep the output regulated. Another standby mode disables switching when 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 applications that require long charging cycles while powering sensors and wireless modems with short burst loads.

Batteries are often used as auxiliary backup power sources in WSNs; however, the design challenge of how to charge them from low-power sources is not trivial! Linear Technology's LTC4071 is a shunt battery charger system with integrated battery pack protection and a low battery disconnect function to prevent low-capacity batteries from being damaged due to self-discharge. It is a simple but very sophisticated charger and protector for lithium-ion/lithium-polymer batteries. Its ultra-low 550nA operating current enables charging with very low current, intermittent or continuous charging power sources that were previously unavailable (for example: charging power sources that rely on energy harvesting applications to provide power). An internal battery thermal conditioner reduces the floating voltage to protect lithium-ion/lithium-polymer batteries, button cells or thin-film batteries when the battery temperature rises. The LTC4071 uses a flat 8-pin 2mm x 3mm DFN package, which provides a complete and ultra-compact charger solution with only a single external resistor in series with the input voltage.

Although portable applications and energy harvesting systems operate at widely varying power levels (from microwatts to over 1W), there are many power conversion ICs available to system designers. However, in lower power situations where nanoamps of current need to be converted, the choices become limited.

Fortunately, designers now have a choice of power conversion and battery charging solutions with sub-1μA quiescent currents that extend battery life for low-power sensors and “keep-alive” circuits in next-generation WSNs.