Harvester Circuit for Small Variable Temperature Gradients

Recent developments in the field of wireless sensor technology and in low power electronics, driven by internet of things and industry 4.0 applications, require further solutions of harvesting technologies for autonomous sensor nodes, to overcome the problem of implementing batteries, if dealing in harsh environments.
Especially, if very small and variable thermal gradients with just very little energy must be harvested, sophisticated circuits based on an intelligent sequencing of the load devices are necessary. The general valid block diagram, as it is mainly described in the literature and shown in Figure 1, just consists of a sensor or detector, which coverts the physical quantity into an electrical signal and the converter block containing a step-up DC-DC converter to boost the small input voltage to an useful level, needed to supply any electronics. Depending on the application, additionally an energy storage module follows, enabling to store surplus energy, which is currently not needed, followed by the load block represented by the sensor electronic circuit. This simple block diagram can be used as basic principle for harvesting circuits dealing with constant input measures. If the input energy is very little, first it must be harvested over a certain time before activating the circuit. To maximize the extracted power, also perfect matching between the detector, formed by the thermo-electric generator in this specific case and the input impedance of the converter unit is necessary. Therefore, this simple block diagram of a general harvesting unit must be modified, to consider the previously mentioned items. The most important modification concerns the implementation of a load management block, which just activates the electronic circuit, if enough energy can be provided, either to start-up the components or to supply the microcontroller circuit for the necessary time span. Commercially available integrated circuits, as it is given for example by the LTC3108 from Linear Technology Inc., provides already all the necessary blocks inside the chip, controlled by a fixed internal sequence, which is just suitable if dealing with constant and stable input levels. In the case of using thermal gradients, on one hand the energy is limited and on the other hand also the slope and levels are small and may change. Therefore, this sequencing must be modified, which is just possible by applying external components to this converter chip. Figure 2 illustrates the developed circuit, based on the LTC3108. The main circuit modifications are realized by just connecting a storage capacitor to the output VOUT, the possibility to activate the
circuit via VOUT2 and the discrete built-up OR-gate to overcome a ringing phenomenon, appearing, if the set level of VOUT is reached and the voltage drops again below the internal level, defined by the hysteresis, due to the start-up current of the microcontroller. This OR-gate configuration represents a self-locking circuit formed by an output of the microcontroller, which is set via the software algorithm right after the configuration of the microcontroller in the program.
The presented circuit ensures the proper function over a certain time of a self-sustaining sensor node, if being used in harsh environments and dealing with small and variable temperature gradients, which contain just very little energy.