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 ~ 100 μW. 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.