Abstract

This is a proposal for IEEE PICO design contest. A Switched capacitor DC-DC boost converter for low power energy harvester applications. The novel architecture of the recursive output connections in each SC stage significantly reduces the conduction loss, which is the dominant loss in cascaded SC converters for low conversion ratios. Four configurable-ratio SC stages are connected through configurable series/parallel stage interconnect, which provides a wide range of harmonic conversion ratios ranging from 3x to 31x. The design is an improvement on a SC multistage design published in CICC [3].

Motivation

A number of energy harvesters (EH), ranging from bio-potentials, thermoelectric, photovoltaic energy harvesters, have been reported in recent years which are being used in portable electronics, implanted medical devices or IoT systems [1]. Thermoelectric harvesters provide output voltage is typically in the 10s of milli volts range, and can widely vary with harvesting conditions [2]. Whereas, energy storage element, which are typically battery have voltage rating of a few volts. Thus, boost converter is inevitable to transfer energy harvested from low voltage source to battery.  As the harvester voltage range is wide, the boost convert must be efficient over a wide voltage range (3x to 30x) to ensure efficient energy transfer under all operating conditions. [3]. Fig. 1 shows the overall architecture of ambient energy scavenging from energy harvesting source, and storing it in battery using switched capacitor (SC) based boost converter. A novel multi-stage multi-output SC approach inspired from [3] is proposed here for efficient conversion over wide voltage range.SC topology preferred over inductive topology, as it eliminates the need of bulky inductor, high energy density of on-chip capacitor, which translates into less area and control power overhead. Some of the key features of the proposed design are:

1. Inductor less design
2. Low Silicon Area (Cost Effective)
3. Low Power overhead (Longer Battery Life)
4. Multistage Output
5. High Efficiency

Fig. 1 Block level diagram of energy harvesting and storage system

Proposed Circuit

A fully integrated SC-based harvester capable of efficient energy harvesting is presented. In the conventional cascaded SC structure, current harvested from an energy source is only injected to the very first stage. This means all current has to flow through entire chain of SC stages, incurring large conduction loss. In the proposed recursive current injection (RCI) topology, efficient energy harvesting is enabled by:

• Reducing conduction loss by recursively injecting harvested current to each stage and
• Reducing switching loss by reducing number of required stages for obtaining given voltage conversion ratio. This conversion can be done using external switches.
• The ½ RVB stage acts as a half booster for conversion in fractions because major losses occur at peak transition i.e., 3x->4x, but this ½ RVB stage will produce transitions from 3.5x-4.5x, hence increasing the overall performance and reducing switching losses occurring during peaks [3].

Fig.2. Conventional voltage doubler boost converter

Fig. 3: Proposed SC DC-DC boost converter

Working

Fig.2 shows the structure of the proposed harvester with Recursive Current Injection (RCI) [4] topology based on recursive voltage boosters (RVBs) compared with a conventional cascaded SC structure, both with two flying capacitors (CA, CB) in each stage . In the conventional approach, voltage doublers (VD) are cascaded, where the output voltage (VOUT) is 2× of the input voltage (VIN), i.e., VOUT of the previous stage. In the proposed scheme, each RVB receives two input voltages – VIN and VHRV. VIN is from the previous stages, just like VD, whereas VHRV is an additional input from the harvesting source. During the boost phase (ΦB), the bottom node of series connected flying capacitors is connected to VHRV instead of ground, providing an additional current injection path from the harvesting source and additional boosted voltage of VHRV compared with VD. Therefore, while VD provides an output voltage of VOUT = 2VIN, the RVB generates an output voltage of VOUT = 2VIN + VHRV[3].

Combining with ½ RVB stage it reduces the peak switching losses occurring at decimal shifting whereas transitioning from 3.5x-4.5x is bypassing the transition of 4x where the peak switching occurs and hence boosting the voltage as well. This Design greatly enhances the boosting while reducing the losses exponentially. Different configurations can be set from and automated clock monitoring module or with some external switches to find the multiples of VHRV to produce the desired boost from input. One major advantage of this design is all of the capacitors are on chip whereas in few different techniques, mostly large buffer capacitors are used externally for charge storage.

Table.1.  Boost converter target specifications

References

[1]         M. Prauzek, J. Konecny, M. Borova, K. Janosova, J. Hlavica, and P. Musilek, “Energy Harvesting Sources, Storage Devices and System Topologies for Environmental Wireless Sensor Networks: A Review,” Sensors 2018, Vol. 18, Page 2446, vol. 18, no. 8, p. 2446, Jul. 2018, doi: 10.3390/S18082446.

[2]         D. Rozgic and D. Markovic, “A Miniaturized 0.78-mW/cm2 Autonomous Thermoelectric Energy-Harvesting Platform for Biomedical Sensors,” IEEE Transactions on Biomedical Circuits and Systems, vol. 11, no. 4, pp. 773–783, Aug. 2017, doi: 10.1109/TBCAS.2017.2684818.

[3]         Institute of Electrical and Electronics Engineers, IEEE Electron Devices Society, IEEE Solid-State Circuits Society, and G. European Solid State Device Research Conference (48th : 2017 : Dresden, ESSCIRC 2018 - IEEE 44th European Solid State Circuits Conference (ESSCIRC) : 3-6 Sept. 2018.

[4]         S. Sankar, P. H. Chen, and M. S. Baghini, “An Efficient Inductive Rectifier Based Piezo-Energy Harvesting Using Recursive Pre-Charge and Accumulation Operation,” IEEE Journal of Solid-State Circuits, 2022, doi: 10.1109/JSSC.2022.3153590.