Power Converter Design for Nanopower Applications

A typical energy-scavenging sensor node consists of a en-ergy producer or scavenger, a storage element, and a load. The energy source can be a variety of things, for example, a solar cell, piezoelectric bender or thermoelectric genera-tor. The storage element is typically either a battery (large capacity over a nearly-fixed voltage range) or a capacitor or ultra-capacitor (less capacity over a varying voltage). The electronic load will be a sensor or processor IC, running at a low, fixed voltage level. 1 Converter Structure For this project, I will neglect the energy scavenging source and use a lithium-type battery ranging from 3 to 4 volts de-pending on state-of-charge (SOC). The load will consist of IC’s running at 1V and 2V. As a baseline circuit, I will use a 3-stage ladder circuit[1], as shown in figure 1. This circuit has the advantage over other topologies that all capacitors and switches are used to their full voltage rating. In this circuit, each capacitor and (off) transistor has 1 volt across it. This converter can be im-plemented ideally in a triple-well 0.13 µm CMOS process (such as the ST microelectronics process). 1.1 Output Impedance A two-port converter can be modeled as an ideal trans-former (with a conversion ratio defined by the converter structure) and a output impedance. For the time being, I will neglect the affects of the third port. The output impedance (assuming the capacitors are the dominant factor) is given by [2]