A 1.58-nA CEPE-Based Hill-Climbing MPPT Technique with Compensated Ton Achieving 67.3% Efficiency at 10-nA Lload and > 97% MPPT Efficiency at VCR from 2 to 6

Energy harvesting techniques with low quiescent current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(lQ)</tex> , high maximum power-point tracking (MPPT) efficiency become crucial for the Internet of Things (loT) nodes. Meanwhile, the applicability to multiple energy sources is favorable, demanding a wide voltage conversion ratio VCR range. The MPPT is usually achieved by tuning the input impedance of the cascading boost DC-DC converter. The fractional open-circuit voltage (FOCV) [1] is the one of simplest MPPT methods, but the MPPT efficiency is good only when we obtain the precise fraction constant. Hill-climbing (HC, as shown in Fig. 1) [2], [3] is another widely used MPPT scheme that achieves high MPPT efficiency, and is applicable to all energy sources. However, it senses the input voltage and current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VIN</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">llN</tex> for the input power <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PIN</tex> , usually in an analog way and hence demands a high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lQ</tex> . Work from [4] proposed an MPPT method based on the Constant-Energy-Packet-Extraction (CEPE), as shown in Fig. 1. Eq. (1) presents the energy obtained in each switching cycle. Using the peak inductor current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(lPEAK)</tex> and discontinuous-conduction mode (DCM) control, and assuming <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tON > > tOFF</tex> (when VCR is high), each packet energy <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ECEPE</tex> is almost constant (eq.2). [4] utilizes CEPE to achieve MPPT by finding out the switching frequency (fsw) where duty cycle varies greatly (large <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"></tex> ). This may consume a high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lQ</tex> , and is only applicable to thermoelectric generators (TEG). Besides, its MPPT efficiency decreases if <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tON</tex> is not <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">> > tOFF</tex> (small VCR cases). As given in Fig. 1, the calculated relative error increases when VCR decreases.