A load-modulated rectifier for RF micropower harvesting with start-up strategies

In this paper, we introduce a new load-modulated two-branch rectifier, designed to dynamically cooperate with an ultra-low power management unit (PMU), interposed between the rectenna and application circuits. The design targets batteryless RF energy harvesting applications with typical input power ranging from 10 to 100um. Energy is stored in a low leakage capacitor. In order to allow activation in discharged states, the PMU implements a low-voltage start-up stage, whose current consumption is specifically optimized for biasing the rectifier accordingly. When a sufficient voltage is reached, the PMU activates a more efficient boost converter stage with maximum power point tracking capabilities and micro-power consumption. Such two circuits are designed to provide two very different loading conditions to the rectifier. A joint design of the nonlinear rectifier paths and of the two PMU subsystems based on two specific optimizations of the matching networks is proposed, along with a circuit solution for automatically switching between the start-up stage and the boost converter. In order to validate the concept, a microstrip prototype operating at 900 MHz with a discrete components PMU is characterized, although the proposed idea is fully technology independent. With respect to a conventional rectifier, the proposed design allows the system to operate with significantly lower input power, while preserving efficiency during steady-state power conversion.