Ultralow-voltage energy harvester powers wireless sensors from waste heat (Part 2 of 2)

(Part 1 looked at duty cycles and the basics of thermoelectric generators, there is a link at the upper right corner of this page)

Pulsed-load application design example
A typical wireless sensor application powered by a TEG is shown in Figure 3.

Figure 3: Wireless sensor application example.
(Click on image to enlarge)

In this example, a temperature differential of at least 4°C is available across the TEG, so a 1:50 transformer ratio was chosen for the highest output power.

The LTC3108 provides the multiple outputs necessary for a typical wireless sensor. The 2.2V LDO output powers the microprocessor, while VOUT has been programmed to 3.3V, using the VS1 and VS2 pins, to power the RF transmitter. The switched VOUT (VOUT2) is controlled by the microprocessor to power 3.3V sensors only when needed. The PGOOD output Indicates to the microprocessor when VOUT has reached 93% of its regulated value.

To maintain operation in the absence of an input voltage, a 0.1F storage capacitor is charged in background from the VSTORE pin. This capacitor can charge up to the 5.25V clamp voltage of the VAUX shunt regulator. If the input voltage source is lost, energy is automatically supplied by the storage capacitor to power the IC and maintain regulation of VLDO and VOUT .

The COUT reservoir capacitor has been sized to support a total load pulse of 15mA for a duration of 10ms, allowing for a 0.33V drop in VOUT during the load pulse, according to the formula below. Note that IPULSE includes loads on VLDO and VOUT2 as well as VOUT, and that charging current has not been included, as it may be very small compared to the load.

Given these requirements, COUT must be at least 454µF, so a 470µF capacitor was selected.

With the TEG shown (and a properly sized heat sink), operating at a ∆T of 5°K, the average charge current available from the LTC3108 at 3.3V is about 560µA. With this information, we can calculate how long it takes to charge the VOUT reservoir cap the first time, and how frequently the circuit can transmit a pulse. Assuming the load on VLDO and VOUT is very small during the charging phase, the initial charge time for VOUT is:

Assuming that the load current between transmit pulses is very small, a simple way to estimate the maximum transmit rate is to divide the average output power available from the LTC3108, in this case 3.3V • 560µA = 1.85mW, by the power required during a pulse, in this case 3.3V • 15mA = 49.5mW. The maximum duty cycle that the harvester can support is:

 1.85mW/49.5mW = 0.037 or 3.7%. T

herefore the maximum transmit burst rate is:

 0.01/0.037 = 0.27 seconds or about 3.7Hz.

Note that if the average load current (as determined by the transmit rate) is the highest that the harvester can support, there will be no harvested energy remaining to charge the storage capacitor. Therefore, in this example the transmit rate is set to 2Hz, leaving almost half of the available energy to charge the storage capacitor. The storage time provided by the VSTORE capacitor is calculated using the following formula:

This calculation includes the 6µA quiescent current required by the LTC3108, and assumes that the loading between transmit pulses is extremely small. Once the storage capacitor reaches full charge, it can support the load for 637 seconds at a transmit rate of 2Hz, or a total of 1274 transmit bursts.

Thermal-harvesting applications requiring autopolarity
Some thermal harvesting applications, such as wireless HVAC sensors or geothermal powered sensors, require the power manager to operate not only from a very low input voltage, but from one of either polarity as the polarity of the ∆T across the TEG can change.

The LTC3109 is well suited to this challenge. Using two transformers with a step-up ratio of 1:100, the LTC3109 can operate from input voltages as low as ±30mV. The LTC3109 offers the same feature set as the LTC3108, including an LDO, a digitally programmable output voltage, a power good output, a switched output and an energy storage output. Figure 4 shows a typical example of the LTC3109 in an autopolarity application.

Figure 4: Autopolarity application example.
(Click on image to enlarge)

The output current vs VIN curves for the converter, shown in Figure 5, illustrate the device's ability to function equally well from input voltages of either polarity.

Figure 5: Output current vs Vin for the converter in Figure 4.
(Click on image to enlarge)

Conclusion
With their unique ability to operate at input voltages as low as 20mV, or from very low voltages of either polarity, the LTC3108 and LTC3109 provide simple, effective power management solutions that enable thermal energy harvesting for powering wireless sensors and other low power applications from common thermoelectric devices. Available in either a 12-pin DFN or 16-pin SSOP package (LTC3108 and LTC3108-1), and 20-pin QFN or SSOP packages (LTC3109), these products offer unprecedented low voltage capabilities and a high level of integration to minimize the solution footprint. The LTC3108, LTC3108-1 and LTC3109 provide all the necessary outputs to interface seamlessly with existing low power building blocks to support autonomous wireless sensors.

About the author
David Salerno is a Design Section Leader with Linear Technology Corporation.