A Compact DC-DC Converter With Pulse-Counting MPPT and Fast One-Path Self-Startup for Thermal Energy Harvesting

This paper presents a compact DC-DC converter for energy harvesting from thermoelectric generators (TEGs). Three key features of the proposed approach are 1) a simple and efficient one-path self-startup circuit, 2) a pulse-counting fractional open-circuit voltage (FOCV)-based maximum power point tracking (MPPT) controller, and 3) a fixed-duty pulse frequency modulation (PFM) controller. The one-path self-startup achieves fast startup by simplifying the switch and control circuit for inductor sharing while incurring negligible loss during the main converter operation. The MPPT controller is based mainly on digital implementation, making it suitable for nanometer-scale CMOS design. This approach allows a significant size reduction of the sampling capacitor ( <tex-math notation="LaTeX">~</tex-math> 50 fF). The PFM controller uses a fixed-duty approach, suitable for tracking the source resistance ( <tex-math notation="LaTeX">RS)</tex-math> range of TEGs in wearable applications. To achieve converter operation using a small source voltage ( <tex-math notation="LaTeX">VS)</tex-math> , the converter is self-sustained through three modes (self-startup, auxiliary boost, and main boost modes). The converter system is fabricated using a 28-nm CMOS process supporting transistors with four threshold voltages. Measured results show fast self-startup ( <tex-math notation="LaTeX">$<$</tex-math> 50 ms) at a relatively low input ( <tex-math notation="LaTeX">VIN</tex-math> <tex-math notation="LaTeX">$=$</tex-math> 200 mV). A conversion efficiency of 88.9% is achieved when delivering an output power of 582 <tex-math notation="LaTeX">μ</tex-math> W using <tex-math notation="LaTeX">VS</tex-math> <tex-math notation="LaTeX">$=$</tex-math> 200 mV. The tracking efficiency is higher than 90% for <tex-math notation="LaTeX">VS</tex-math> values ranging from 50 mV to 200 mV. The peak efficiency of 99.2% is achieved using <tex-math notation="LaTeX">VS</tex-math> <tex-math notation="LaTeX">$=$</tex-math> 200 mV and <tex-math notation="LaTeX">RS</tex-math> <tex-math notation="LaTeX">$=$</tex-math> 2 <tex-math notation="LaTeX">Ω</tex-math> , confirming the accurate operation of the MPPT controller. An end-to-end efficiency of 87.6% is achieved using <tex-math notation="LaTeX">VS</tex-math> <tex-math notation="LaTeX">$=$</tex-math> 200 mV. All the functions of the IC are realized with only 818 nW.