Simultaneous Power Feedback and Maximum Efficiency Point Tracking for Miniaturized RF Wireless Power Transfer Systems

Abstract

Near-field interfaces with miniaturized coil systems and low output power levels, such as applied in biomedical sensor systems, can suffer from severe efficiency degradation due to dynamic impedance mismatches, reducing battery life of the power transmitter unit and requiring to increase the level of electromagnetic emission. Moreover, the stability of weakly-coupled power transfer systems is generally limited by transient changes in coil alignment and load power consumption. Hence, a central research question in the domain of wireless power transfer is how to realize an adaptive impedance matching system under the constraints of a simultaneous power feedback to increase the system’s efficiency and stability, while maintaining circuit characteristics such as small size, low power consumption and fast reaction times. This paper presents a novel approach based on a two-stage control loop implemented in the primary-side reader unit, which uses a digital PI controller to maintain the rectifier output voltage for power feedback and an on-top perturb-and-observe controller configuring the setpoint of the voltage controller to maximize efficiency. The paper mathematically analyzes the AC and DC transfer characteristics of a resonant inductive link to design the reactive AC matching network, the digital voltage controller and ultimately the DC-domain impedance matching algorithm. It was found that static reactive L networks result in suitable efficiency levels for coils with sufficiently high quality factor even without adaptive tuning of operational frequency or reactive components. Furthermore, the regulated output voltage of the rectifier is a direct measure of the DC load impedance when using a regular DC/DC converter to supply the load circuits, so that this quantity can be tuned to maximize efficiency. A prototype implementation demonstrates the algorithms in a 40.68 MHz inductive link with load power levels from 10 to 100 mW and tuning time constants of 300 ms, while allowing for a simplified receiver with a footprint smaller than 200 mm2 and a self-consumption below 1 mW. Hence, the presented concepts enable adaptive impedance matching with favorable characteristics for low-energy sensor systems, i.e., minimized footprint, power level and reaction time.

Predicting Receiver Characteristics without Sensors in an LC–LC Tuned Wireless Power Transfer System Using Machine Learning

“…Research about Rx-side sensorless WPT systems has been undertaken in [ 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. The study in [ 15 ] proposes a machine learning (ML) assisted method that estimates the power delivered to the Rx by using only measurements at the Tx-side.…”

Load Modulation Feedback in Adaptive Matching Networks for Low-Coupling Wireless Power Transfer Systems

“…Conversely, if the power demanded by the capsule increases or the coupling factor is reduced as a consequence of capsule movements, the Tx needs to know when to increase the transmitted power. The load modulation feedback (LMF) technique (also referred to as back telemetry or data telemetry) [15][16][17] is an effective method of transmitting information back from the Rx to the Tx side, without involving additional links (as, for example, in [18]). This is based on the modulation of the load seen by the Rx coil and the inductive coupling with the transmitting coil [19,20].…”

Behavioral Modeling of DC/DC Converters in Self-Powered Sensor Systems with Modelica

“…Maximum power point tracking (MPPT) is often accomplished by the fractional open-circuit voltage method (FOCV) [ 7 ]. For mid-power systems, the focus is on a high conversion efficiency and there is enough energy available for FPGAs implementing sophisticated algorithms such as the perturb and observe method (P&O) [ 8 , 9 ].…”