Thermal Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 101

3.3 Implementation of an Optimal Thermal Energy Harvesting
Wireless Sensor Node
The schematic diagram of a self-autonomous wireless sensor node powered
by the designed TEH system and its ultralow-power and efficient power
management circuit is illustrated in Figure 3.10 . Referring to Figure 3.10 ,
the designed power management circuitry with resistor emulation MPPT
approach can essentially consist of three main building blocks: (1) a buck
converter with MPP tracker and its control and PWM generation circuit that
manipulates the operating point of the TEH to keep harvesting power at the
MPP, (2) an energy storage element (i.e., supercapacitor to buffer the energy
transfer between the source and the load), and (3) a regulating buck converter
to provide constant voltage to the wireless sensor node and other electronic
circuitries.

3.3.1 Buck Converter with Resistor Emulation-Based

Maximum Power Point Tracking

The buck converter in the DCM has been illustrated to perform well as a
near-constant resistance at its input port to manipulate the thermal energy
harvester to transfer maximum power to its output port, which is connected
to a 0.1-F supercapacitor, based on the duty cycle d of the PWM gate signal
applied to the buck converter [70]. Within the operational bandwidth of the
buck converter under different loading conditions, in the range of 10 to 56 k ,

a duty cycle value of 0.16 has been selected to operate the buck converter to its
near MPPs. Substituting the duty cycle value, d = 0.16, into Equation 3.9 , the
emulated resistances, with the connected load of 10 and 56 k

,are calculated

to be 68 and 126 k ,respectively. Referring to Figure 3.3 , it can be observed

that the power losses at these resistance values, which are near to the MPPs,
are less than 5% of the maximum obtainable power at R em =82k

. This power
waste (few tens of microwatts) is much less than the power consumed by the
high-power overhead of complex control circuitry required to perform the
accurate and precise closed-loop MPPT techniques.
The operation of the buck converter as an open-loop resistance emulator
is as follows: A low-frequency PWM control signal, about 100 Hz, of the
desired duty cycle of 0.16 is generated by a Texas Instruments microcontroller
(TI MSP430F2274). An ultralow-power PWM generation circuit is designed to
transform the low-frequency PWM control signal generated from the reduced
clock speed microcontroller to a higher switching frequency of 10 kHz, so
smaller filter components are used in the buck converter to miniaturize the
overall TEH system. The PWM generation circuit is made up of a micro-
power resistor set oscillator (LTC6906) used for sawtooth generation and a
micropower, rail-to-rail complementary metal-oxide semiconductor (CMOS)