Power Management Architecture Selection for Energy Harvesting Systems

The ultimate goal of wireless sensor network design is the ability to power a sensor node from energy already in the environment. Installing tens or hundreds of thousands of battery-powered nodes as part of an Internet of Things (IoT) deployment is a big challenge, but regularly replacing the batteries can incur a huge maintenance cost. However, if the energy could be obtained from the local environment, batteries might then last decades or even be eliminated completely.

Several different ways to harvest that energy exist, and each has its own requirements for the power management. The most popular energy harvesting devices are solar panels which allow a node to be charged during daylight hours or even from indoor lighting. But there are other techniques to gather environmental energy that can be used. A temperature differential, for example between the inside and outside of a building, can be harnessed through a thermal energy generator (TEG). Machinery vibration can also be used, via a piezoelectric transducer, which can power a wireless sensor buried deep in the heart of a piece of equipment. You can read more about these methods in the article ‘Power Management Tips for Energy Harvesting Systems’.

With the on-going trend to reduce both sensor and wireless transceiver power consumption, it is now possible to power the sensor nodes from the radio energy that surrounds them. Without the need to access the sensor in order to change the batteries, these nodes can be located in many more positions, providing more accurate data about the activity of the equipment.

Batteries are often used to collect and store the charge from the energy harvesting source, but these also need to be protected from overcharging or undercharging as a result of the poorly regulated energy harvesting sources. The MAX17710 from Maxim Integrated is designed to manage output levels ranging from 1 µW to 100 mW. For example, with a 0.8 V harvesting source and a 4.1 V cell, the device can deliver over 20 mA (80 mW).

To do this, the device incorporates a boost regulator circuit for charging a lithium battery from a source as low as 0.75 V, while an internal regulator protects the cell from overcharging and an internal voltage protection function prevents the cell from over discharging. The selectable output voltages, from 1.8 V through 2.3 V to 3.3 V, are regulated using a low-dropout (LDO) linear regulator.

The MAX17710 evaluation board gives developers the ability to charge a protected lithium battery cell from an energy harvesting source such as a solar cell

With all the different energy harvesting sources that are available, it is also necessary to understand the upper and lower limits of the source. A buck/boost converter will have a lower limit below which the power stage may either shut down or not start, interrupting system operation. This also means the power up sequencing must know the implications of when to power each device along with other devices so that the power drain does not push the power conversion stage below that lower limit.

It is also necessary to be aware of the potential peak power in order to avoid overwhelming the additional energy storage element such as a capacitor or battery.

The peak power on startup is a key factor to look at, since bringing up a microcontroller, power management chip and a wireless transceiver can take a lot more power compared to steady state operation. This is where timers become important. Fortunately, the startup period of a sensor node can be varied to accommodate the peak power capability of the energy harvesting source.

Different ways to handle this are available, each with their own advantages. Using hardware timers and interrupts has the potential to reduce the overall power requirements. However, using a software based approach to manage power, while possibly the higher power option, does permit over-the-air (OTA) upgrades that can help to boost the efficiency. If OTA is not being used, then the hardware approach may be more effective.

Isolating all of the loads in a system and making them switchable gives the power manager more opportunities to avoid problems and optimize performance. Additionally, this helps isolate any devices that are consuming too much power.

When harvesting solar energy, a key algorithm is maximum power point tracking (MPPT). This algorithm tracks the most efficient power production of the solar panel. MPPT can be handled by a microcontroller, although more commonly these algorithms are now being integrated into the power management chips to help minimize the overall power consumption of the node.

For a piezoelectric transducer, a buck/boost converter with a protective shunt at the input, allows the power manager to provide a variety of different piezoelectric elements which can have short-circuit currents of around 10 µA.

A wide range of power management chips and evaluation boards that are suited for the different types of energy harvesting systems are available. From solar cells to RF, vibrational energy to thermal generators, designers can test out the multiple topologies and power management architectures for their specific wireless sensor network implementations.