Regan Zane

Recycling ambient microwave energy with broad-band rectenna arrays

This paper presents a study of reception and rectification of broad-band statistically time-varying low-power-density microwave radiation. The applications are in wireless powering of industrial sensors and recycling of ambient RF energy. A 64-element dual-circularly-polarized spiral rectenna array is designed and characterized over a frequency range of 2-18 GHz with single-tone and multitone incident waves. The integrated design of the antenna and rectifier, using a combination of full-wave electromagnetic field analysis and harmonic balance nonlinear circuit analysis, eliminates matching and filtering circuits, allowing for a compact element design. The rectified dc power and efficiency is characterized as a function of dc load and dc circuit topology, RF frequency, polarization, and incidence angle for power densities between 10/sup -5/-10/sup -1/ mW/cm/sup 2/. In addition, the increase in rectenna efficiency for multitone input waves is presented.

Resistor Emulation Approach to Low-Power RF Energy Harvesting

This paper presents an approach and associated circuitry for harvesting near maximum output power from electromagnetic waves in the RF/microwave region of the spectrum with variable incident power densities in the range of tens of muW/cm2. It is shown that open loop resistor emulation at the input port of a power converter is a suitable solution for tracking the peak power point of a low-power rectifying antenna source over a wide range of incident RF power densities. A boost converter with a simple low-power control approach for resistor emulation is presented. A hardware design example with detailed efficiency analysis is given using commercially available discrete circuitry. Experimental results are presented for a system harvesting 420 muW to 8 muW from a 6 cm times 6 cm rectifying antenna with incident RF power ranging from 70 muW/cm2 to 30 muW/cm2, respectively. The results demonstrate that resistor emulation is a simple and practical approach to energy harvesting with variable low-power radiative RF sources.

Small-Signal Discrete-Time Modeling of Digitally Controlled PWM Converters

The letter presents an exact small-signal discrete-time model for digitally controlled pulsewidth modulated (PWM) dc-dc converters operating in constant frequency continuous conduction mode (CCM) with a single effective A/D sampling instant per switching period. The model, which is based on well-known approaches to discrete-time modeling and the standard Z-transform, takes into account sampling, modulator effects and delays in the control loop, and is well suited for direct digital design of digital compensators. The letter presents general results valid for any CCM converter with leading or trailing edge PWM. Specific examples, including approximate closed-form expressions for control-to-output transfer functions are given for buck and boost converters. The model is verified in simulation using an independent system identification approach.

System identification of power converters with digital control through cross-correlation methods

For digitally controlled switching power converters, on-line system identification can be used to assess the system dynamic responses and stability margins. This paper presents a modified correlation method for system identification of power converters with digital control. By injecting a multiperiod pseudo random binary signal (PRBS) to the control input of a power converter, the system frequency response can be derived by cross-correlation of the input signal and the sensed output signal. Compared to the conventional cross-correlation method, averaging the cross-correlation over multiple periods of the injected PRBS can significantly improve the identification results in the presence of PRBS-induced artifacts, switching and quantization noises. An experimental digitally controlled forward converter with an FPGA-based controller is used to demonstrate accurate and effective identification of the converter control-to-output response.

Power Management System for Online Low Power RF Energy Harvesting Optimization

For many years, wireless RF power transmission has been investigated as a viable method of power delivery in a wide array of applications, from high-power space solar power satellites to low-power wireless sensors. However, until recently, efficient application at the low sub-milliwatt power levels has not been realized due to limitations in available control circuitry. This paper presents a “smart” microcontroller-based power management system with online power stage efficiency optimization and maximum power point tracking (MPPT). The system is experimentally evaluated using a new, more accurate four-quadrant rectenna model and circuit realization that enables rigorous testing of the power management system for a wide range of rectenna arrays and power characteristics. Hardware results are presented with online optimization over a converter input power range from 10 μW to 1 mW. Results are also shown based on the application of harvesting RF power from a nearby cellular tower, where the power management system collects up to seven times more energy when compared to a direct battery connection.

Low-Power Far-Field Wireless Powering for Wireless Sensors

This paper discusses far-field wireless powering for low-power wireless sensors, with applications to sensing in environments where it is difficult or impossible to change batteries and where the exact position of the sensors might not be known. With expected radio-frequency (RF) power densities in the 20-200- μW/cm2 range, and desired small sensor overall size, low-power nondirective wireless powering is appropriate for sensors that transmit data at low duty cycles. The sensor platform is powered through an antenna which receives incident electromagnetic waves in the gigahertz frequency range, couples the energy to a rectifier circuit which charges a storage device (e.g., thin-film battery) through an efficient power management circuit, and the entire platform, including sensors and a low-power wireless transmitter, and is controlled through a low-power microcontroller. For low incident power density levels, codesign of the RF powering and the power management circuits is required for optimal performance. Results for hybrid and monolithic implementations of the power management circuitry are presented with integrated antenna rectifiers operating in the 1.96-GHz cellular and in 2.4-GHz industrial-scientific-medical (ISM) bands.

Nonlinear-carrier control for high-power-factor rectifiers based on up-down switching converters

In this paper, nonlinear-carrier (NLC) control is proposed for high-power-factor rectifiers based on flyback, Cuk, Sepic, and other up-down power converters operated in the continuous conduction mode (CCM). In the NLC controller, the switch duty ratio is determined by comparing a signal proportional to the integral of the switch current with a periodic nonlinear-carrier waveform. The shape of the NLC waveform is determined so that the resulting input-line current follows the input-line voltage, as required for unity power factor rectification. A simple exponential carrier waveform generator is described. Using the NLC controller, input-line voltage sensing, error amplifier in the current-shaping loop, and multiplier/divider circuitry in the voltage feedback loop are eliminated. The simple high-performance controller is well suited for integrated-circuit implementation, Results of experimental verification on a 150 W flyback rectifier are presented.

LED Driver Circuit with Series-Input-Connected Converter Cells Operating in Continuous Conduction Mode

This paper introduces an LED driver circuit implemented by series-input-connected converter cells with a common duty cycle control approach operating from a dc voltage bus. With this structure, low-voltage high-frequency ICs and low-profile components can be applied in high-voltage applications. Flexibility is provided for the cells to work under different voltage supply conditions by simply changing the number of series converters. With the converters operating in continuous conduction mode, the common duty cycle control approach enables automatic line voltage sharing and output current copying. The approach results in a control-to-output transfer function of the system close to that of a single converter for ease of feedback loop design. The modular approach also allows continued operation in the presence of open-circuit LED failures. Design considerations and experimental results are presented for a 25-W three-cell system with 9 Luxeon K2 high-brightness LEDs, demonstrating line voltage sharing, output current copying, and LED failure response.

Small-signal Discrete-time Modeling of Digitally Controlled DC-DC Converters

The paper presents an exact small-signal discrete-time model for digitally controlled DC-DC converters. The model, which is based on well-known approaches to discrete-time modeling and the standard Z-transform, takes into account modulator effects and delays in the control loop. The model is well suited for direct digital design of digital compensators

Design and Implementation of an Adaptive Tuning System Based on Desired Phase Margin for Digitally Controlled DC–DC Converters

This letter presents an online adaptive tuning technique for digitally controlled switched-mode power supplies (SMPS). The approach is based on continuous monitoring of the system crossover frequency and phase margin, followed by a multi-input-multi-output (MIMO) control loop that continuously and concurrently tunes the compensator parameters to meet crossover frequency and phase margin targets. Continuous stability margin monitoring is achieved by injecting a small digital square-wave signal between the digital compensator and the digital pulsewidth modulator. The MIMO loop adaptively adjusts the compensator parameters to minimize the error between the desired and measured crossover frequency and phase margin. Small-signal models are derived, and the MIMO control loop is designed to achieve stability and performance over a wide range of operating conditions. Using modest hardware resources, the proposed approach enables adaptive tuning during normal SMPS operation. Experimental results demonstrating system functionality are presented for a synchronous buck SMPS.