Abstract
This paper proposes a discontinuous charging technique for switched-capacitor converters that improves the power conversion efficiency (PCE) at low power levels and extends the input power harvesting range at which high PCE is achievable. Discontinuous charging delivers current to energy storage only during clock non-overlap time. This enables tuning of the output current to minimize converter losses based on the available input power. Based on this fundamental result, an input power-aware, two-dimensional efficiency tracking technique for Wireless Sensor Network (WSN) nodes is presented. In addition to conventional switching frequency control, clock non-overlap time control is introduced to adaptively optimize PCE according to the sensed ambient power levels. The proposed technique is designed and simulated in 90nm CMOS with post-layout extraction. Under the same input and output conditions, the proposed system maintains at least 45% PCE at 4μW input power, as opposed to a conventional continuous system which requires at least 18.7μW to maintain the same PCE. Therefore, the input power harvesting range is extended by 1.5x. The technique is applied to a wearable WSN implementation utilizing the IEEE 802.15.4-compatible GreenNet communications protocol. This allows the node to meet specifications and achieve energy autonomy when deployed in environments where the input power is 49% lower than what is required for conventional operation.
Highlights
The Internet of Things (IoT) enables intelligent monitoring and management in many applications
SYSTEM SPECIFICATIONS AND TRANSIENT SIMULATIONS The proposed system is designed and simulated in 90nm CMOS technology in order to demonstrate the power conversion efficiency (PCE) improvement that discontinuous charging achieves. While this technique is applicable to any CMOS process, the 90nm process node is selected because it provides lower MOSFET threshold voltages (Vth) and gate capacitances compared to larger nodes such as 0.18μm
The system is designed for an input voltage range from a DC energy harvester, such as mm-scale solar cells commonly used in wireless sensor network (WSN)
Summary
The Internet of Things (IoT) enables intelligent monitoring and management in many applications. A charging technique for SC converters is proposed in order to discontinuously deliver current to energy storage (ES) only during clock non-overlap time (tNOL) [24] The proposed scheme is capable of periodically sensing the available PIN in a given environment and dynamically optimizing PCE based on that information This enables high-efficiency operation at considerably lower PIN and allows the system to achieve a wide harvesting range. This is especially valuable for wearable and implantable WSN nodes and other applications that deploy nodes in harsh environments, where ambient power levels vary considerably and can be very low.
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