Harnessing ambient power to create battery-free wireless sensors

Advances in low-power technology have made it easier to build wireless sensor networks for applications such as remote sampling, HVAC monitoring, asset tracking, and industrial automation. The problem is that even wireless sensors require batteries that need to be replaced periodically, which is an expensive and complex maintenance task. A better wireless power solution is to harvest mechanical, thermal, or electromagnetic energy from the environment where the sensor is located.

Typically, the amount of ambient energy that can be harvested is in the tens of milliwatts, so energy harvesting requires careful power management to successfully capture several milliwatts of ambient energy and store it in a usable energy reservoir. A common source of ambient energy is mechanical vibration energy, which is generated by engines running in factories, airflow on fan blades, and even moving vehicles. Piezoelectric transducers can be used to convert this vibration energy into electrical energy, which is then used to power circuits.

To manage energy harvesting and energy release to the system, the LTC3588-1 piezoelectric energy harvesting power supply (Figure 1) integrates a low-loss internal bridge rectifier and a synchronous step-down DC/DC converter. The device uses an efficient energy harvesting algorithm to collect and store energy from high-impedance piezoelectric elements, which may have short-circuit currents of about tens of mA.

Energy harvesting systems often must support much higher peak load currents than a piezoelectric device can produce, so the LTC3588-1 can accumulate energy and then release it to the load in short bursts of energy. Of course, for continuous operation, these bursts of energy must be provided at a low duty cycle so that the total output energy during the burst does not exceed the average source energy accumulated during the energy accumulation period. A sensor system that takes measurements at fixed intervals, sends data and powers down between measurements is the preferred energy harvesting solution.

The key to energy harvesting is low quiescent current

The energy harvesting process relies on a low quiescent current energy accumulation phase. The LTC3588-1 achieves this through an undervoltage lockout (UVLO) mode with a wide hysteresis window that draws less than 1mA of quiescent current. The UVLO mode allows charge to accumulate on an input capacitor until the internal buck converter can efficiently transfer a portion of the stored charge to the output.

Figure 2 shows the UVLO quiescent current curve, which varies monotonically with VIN, so a current source as low as 700nA can charge the input capacitor to the rising threshold of UVLO, resulting in a regulated output. Once regulation is achieved, the LTC3588-1 enters a sleep state where both input and output quiescent currents are minimal. For example, at VIN = 4.5V, the quiescent current is only 950nA when the output is in regulation. The buck converter then switches on and off as needed to maintain regulation. In both sleep and UVLO modes, the low quiescent current allows as much energy as possible to accumulate in the input capacitor bank, even when the available supply current is very low.

When VIN reaches the UVLO rising threshold, the integrated high efficiency synchronous buck converter turns on and starts transferring energy from the input capacitor to the output capacitor. The buck regulator uses a hysteresis voltage algorithm to control the output through internal feedback from the VOUT sense pin. The buck regulator charges the output capacitor to a value slightly above the regulation point through an inductor by ramping up the inductor current up to 250mA through an internal PMOS switch and then ramping down to zero through an internal NMOS switch. This provides energy to the output capacitor with high efficiency.

If the input voltage drops below the UVLO falling threshold before the output voltage reaches regulation, the buck converter shuts down and does not turn back on until the input voltage rises above the UVLO rising threshold. During this time, the leakage on the VOUT sense pin is no more than 90nA, and the output voltage remains close to the value it reached when the buck regulator switched. Figure 3 shows a typical startup waveform of the LTC3588-1 charged by a 2mA current source. While the synchronous buck regulator brings the output voltage into regulation, the converter enters a low quiescent current sleep state that monitors the output voltage with a sleep comparator. In this mode of operation, the load current is provided by the output capacitor of the buck converter. When the output voltage drops below the regulation point, the buck regulator wakes up and the cycle repeats. This hysteretic method of providing a regulated output minimizes the losses associated with FET switching and enables high efficiency regulation at very light loads.

The buck converter provides up to 100mA of average load current when switching. Four output voltages, 1.8V, 2.5V, 3.3V, and 3.6V, are pin-selectable to facilitate powering microprocessors, sensors, and wireless transmitters. Figure 4 shows the extremely low quiescent current when in regulation and sleep, which allows high efficiency operation at light loads. Although the buck regulator's quiescent current when switching is much higher than the sleep quiescent current, it is still a small percentage of the load current, allowing high efficiency over a wide range of load conditions (Figure 5).

The buck converter operates only when sufficient energy has been accumulated in the input capacitor, transferring that energy to the output in short bursts that take much less time than it takes to accumulate the energy. When the quiescent current of the buck converter is averaged over the entire accumulation/burst cycle, the average quiescent current is very low, making it very easy to provide a power supply that harvests small amounts of ambient energy. The very low quiescent current in steady state allows the LTC3588-1 to achieve high efficiency at loads less than 100mA.

Get vibration energy

Piezoelectric elements convert mechanical energy (usually vibration energy) into electrical energy. Piezoelectric elements can be made of PZT (lead zirconate titanate) ceramics, PVDF (polyvinylidene fluoride) or other composite materials. Ceramic piezoelectric elements exhibit a piezoelectric effect when the crystal structure of the ceramic is compressed, and the internal dipole movement will generate a voltage. Polymer elements composed of long chains of molecules will generate a voltage when the molecules repel each other and bend. Ceramics are often used under direct pressure, while polymers are more easily bent.

There are many types of piezoelectric devices available, which produce a variety of open circuit voltages and short circuit currents. The open circuit voltage and short circuit current form a "load line" for the piezoelectric device, which rises as the available vibration energy increases, as shown in Figure 6. The LTC3588-1 can handle input voltages up to 20V, at which point a protective shunt circuit protects the device from overvoltage conditions on VIN. If sufficient ambient vibration causes the piezoelectric device to produce more energy than the LTC3588-1 needs, the shunt circuit dissipates the excess energy, effectively clamping the piezoelectric device on its load line.

The LTC3588-1 interfaces to the piezoelectric device via an internal low-loss bridge rectifier, which can be connected via the PZ1 and PZ2 pins. The rectified output is stored in the VIN capacitor. At a typical piezoelectric current of 10uA, the voltage drop associated with the bridge rectifier is on the order of 400mV. The bridge rectifier has a reverse leakage current of less than 1nA at 125°C, a bandwidth of more than 1MHz, and can carry 50mA current, making it suitable for a variety of other input power supplies.

The characteristics of the ambient vibration can be determined to select a piezoelectric device with the best characteristics. The vibration frequency and force, as well as the time interval using the LTC3588-1 output capacitor bank and the energy required for each burst, help determine the best piezoelectric device. The system can be designed in this way so that it performs tasks as often as the available energy allows. In some cases, as long as the energy can be harvested, no matter how much, it is not necessary to optimize the piezoelectric device.

Alternative energy storage methods

The harvested energy can be stored on either the input capacitor or the output capacitor. The wide input range takes advantage of the fact that the energy stored on the capacitor is proportional to the square of the capacitor voltage. Once the output voltage is regulated, any excess energy is stored on the input capacitor, and the voltage on the input capacitor rises. When a load is applied to the output, the buck regulator is able to efficiently transfer the energy stored in the form of a high voltage to the regulated output. Although the energy storage at the input utilizes the high voltage at the input, the load current is limited to the 100mA that the buck regulator can provide. If a larger transient load needs to be serviced, the output capacitor can be sized to support a larger current during the transient.

The PGOOD output aids in power management. When the output first reaches regulation, PGOOD goes high (with respect to VOUT) and remains high until the output drops to 92% of the regulation point. PGOOD can be used to trigger a system load. For example, a current burst can begin when PGOOD goes high and continue to drain the output capacitor until PGOOD goes low. In some cases, it is important to use every last joule, and if the output is still within 92% of the regulation point, the PGOOD pin will remain high, even if the input drops below the lower UVLO threshold (as might happen if vibration stops). LTC3588-1 Extends Battery Life

Energy harvesting systems can not only eliminate the need for batteries, but can also complement battery solutions. Energy harvesting systems can be configured to unload the battery when ambient energy is available, but enable the battery as a backup power source when ambient power is gone. This approach not only improves reliability, but also produces a more responsive system. For example: an energy harvesting sensor node deployed on a transport equipment (such as a tractor trailer) can harvest energy while the trailer is in motion. When the truck is parked and there are no vibrations, a backup battery will still provide the energy needed for the transport equipment query.

The battery backup circuit in Figure 8 shows a 9V battery and a blocking diode connected in series to VIN. The piezoelectric device charges VIN through an internal bridge rectifier, while the blocking diode prevents reverse current from flowing into the battery. A 9V battery is shown, as long as the battery stack voltage does not exceed 18V, which is the maximum voltage that can be applied to VIN through an external low impedance source. When designing a battery backup system, the piezoelectric transducer and battery should be selected so that the peak piezoelectric voltage exceeds the battery voltage. This allows the piezoelectric device to "take over" and power the LTC3588-1.

A wide range of alternative energy solutions

In addition to the ambient vibration energy available from the piezoelectric device, the LTC3588-1 can also harvest energy from other sources. The integrated bridge rectifier allows many AC power sources to power the LTC3588-1. The fluorescent lamp energy harvester shown in Figure 9 capacitively harvests the alternating electric field energy radiated by an AC-powered fluorescent lamp tube. Copper plates can be placed above the lamp tube on the lamp rack to harness the electric field energy generated by the lamp tube and feed that energy to the LTC3588-1 and the integrated bridge rectifier. Such a harvester can be used throughout a building to power HVAC sensor nodes.

Another useful application of the LTC3588-1 is to power the IC from the AC line voltage with a current limiting resistor, as shown in Figure 10. This provides a low cost, transformerless solution for simple plug-in applications. When designing a circuit that connects directly to the line voltage, some proper UL guidelines should be followed.

The LTC3588-1 is not limited to AC power supplies, DC power supplies such as solar panels and thermocouples can also be used, as shown in Figure 11. Such supplies can be connected to one of the PZ1/PZ2 inputs to take advantage of the reverse current protection provided by the bridge. They can also be connected together with external diodes and connected to the VIN pin. This facilitates the use of multiple solar panels with different orientations to capture sunlight at different times of the day. Multiple Output Rails Sharing a Single Piezoelectric Power Supply

Many systems require multiple supply rails to power different components. A microprocessor may use 1.8V, but a wireless transmitter may require 3.6V. Two LTC3588-1 devices can be connected to a piezo and power each output simultaneously, as shown in Figure 12. This configuration features automatic power sequencing, with the LTC3588-1 with the lower voltage output (i.e., lower UVLO rising threshold) operating first. As the piezo provides input power, both VIN rails initially operate simultaneously, but when one output begins to consume power, only its corresponding VIN drops because of the isolation provided by each LTC3588-1’s bridge. The input piezo energy is then transferred to this lower voltage capacitor until the two VIN rails are equal again. This configuration can be extended to multiple LTC3588 devices powered by a single piezo, as long as the piezo can support the total quiescent current of all LTC3588-1s combined.