Plamen Proynov

Maximum Power Transfer Tracking for Ultralow-Power Electromagnetic Energy Harvesters

This paper describes the design and operation of power conditioning system with maximum power transfer tracking (MPTT) for low-power electromagnetic energy harvesters. The system is fully autonomous, starts up from zero stored energy, and actively rectifies and boosts the harvester voltage. The power conditioning system is able to operate the harvester at the maximum power point against varying excitation and load conditions, resulting in significantly increased power generation when the load current waveform has a high peak-to-mean ratio. First, the paper sets out the argument for MPTT, alongside the discussion on the dynamic effects of varying electrical damping on the mechanical structure. With sources featuring stored energy, such as a resonant harvester, maximum power point control can become unstable in certain conditions, and thus, a method to determine the maximum rate of change of electrical damping is presented. The complete power conditioning circuit is tested with an electromagnetic energy harvester that generates 600 mV rms ac output at 870 μW under optimum load conditions, at 3.75 m·s-2 excitation. The digital MPTT control circuit is shown to successfully track the optimum operating conditions, responding to changes in both excitation and the load conditions. At 2 V dc output, the total current consumption of the combined ancillary and control circuits is just 22 μA. The power conditioning system is capable of transferring up to 70% of the potentially extractable power to the energy storage.

Energy harvesting study on single and multilayer ferroelectret foams under compressive force

Cellular polypropylene (PP) ferro electret is a thin and flexible cellular polymer foam that generates electrical power under mechanical force. This work investigates single and multilayer ferro electret PP foams and their potential to supply energy for human-body-worn sensors. Human foot-fall is emulated using an electrodynamic instrument, allowing applied compressive force and momentum to be correlated with energy output. Peak power, output pulse duration, and energy per strike is derived experimentally as a function of force and momentum, and shown to be a strong function of external load resistance, thus providing a clear maximum energy point. The possibility of increasing pulse time and reducing voltage to CMOS compatible levels at some expense of peak power is shown. To further increase the output power, multilayer ferro electret is presented. The synchronized power generation of each layer is studied and illustrated using simulation, and results are supported by experiments. Finally, the energy output of single-layer and multi-layer ferro electrets are compared by charging a capacitor via a rectifier. A ten-layer ferro electret is shown to have charging ability 29.1 times better than that of the single-layer ferro electret. It demonstrates energy output that is capable of powering the start-up and transmission of a typical low-power wireless sensor chipset.

Experimental investigation of inductorless, single-stage boost rectification for sub-mW electromagnetic energy harvesters

This paper demonstrates single-stage boost rectification for electromagnetic energy harvesters down to approximately 100 μW using practical low-power techniques. The circuits exploit the inductance of the generator, and operate without a discrete inductor, which facilitates integration. Experimental results demonstrate the importance of switching device selection, and the compound effect of the duty ratio on energy harvester output power and converter efficiency, as a function of load current. The circuits demonstrate up to 84.1% harvester utilization at the maximum extractable harvester power of 141 μW, and conversion efficiencies of 73.3% and 59.4% for half- and full-wave operation respectively, neglecting gate drive losses.

Experimental study of RF energy transfer system in indoor environment

This paper presents a multi-transmitter, 2.43 GHz Radio-Frequency (RF) wireless power transfer (WPT) system for powering on-body devices. It is shown that under typical indoor conditions, the received power range spans several orders of magnitude from microwatts to milliwatts. A body-worn dual-polarised rectenna (rectifying antenna) is presented, designed for situations where the dominant polarization is unpredictable, as is the case for the on-body sensors. Power management circuitry is demonstrated that optimally loads the rectenna even under highly intermittent conditions, and boosts the voltage to charge an on-board storage capacitor.

Resistive matching with a feed-forward controlled non-synchronous boost rectifier for electromagnetic energy harvesting

Impedance matching techniques have been shown to extract close to the maximum theoretical power from kinetic energy harvesters. The output impedance of electromagnetic energy harvesters is frequency-dependent, which must be compensated for by the interfacing power electronics. Switched mode power converters are used to synthesise optimum, matched load impedance, controlled typically by varying duty ratio pulse width modulated gate signals. The emulated input impedance of the power converter is affected by factors such as the input and output voltage levels. In this paper, the non-synchronous boost rectifier, operated entirely in discontinuous current conduction mode, is controlled dynamically to ensure that the emulated input resistance remains at a set value under varying input and output conditions. This is achieved by employing a feed-forward control scheme that is based on calculating the time-varying optimum duty ratio as a function of the excitation frequency, the generated voltage and the converter output voltage, following the analytically derived equations. Experimental results are obtained using the proposed feed-forward control method implemented on a real-time platform. The results demonstrate the effectiveness of the presented control in terms of emulating a set resistance, and the average power that can be extracted using optimum resistance matching over a range of excitation frequencies.

Comparison of low-power single-stage boost rectifiers for sub-milliwatt electromagnetic energy harvesters

Energy harvesting could provide power-autonomy to many important embedded sensing application areas. However, the available envelope often limits the power output, and also voltage levels. This paper presents the implementation of an enabling technology for space-restricted energy harvesting: Four highly efficient and fully autonomous power conditioning circuits are presented that are able to operate at deep-sub-milliwatt input power at less than 1 Vpk AC input, and provide a regulated output voltage. The four complete systems, implemented using discrete components, include the power converters, the corresponding ancillary circuits with sub-10 μW consumption, start-up circuit, and an ultra-lowpower shunt regulator with under-voltage lockout for the management of the accumulated energy. The systems differ in their power converter topology; all are boost rectifier variants that rectify and boost the generator’s output in a single stage, that are selected to enable direct comparison between polarity–dependent and –independent, as well as between full-wave and half-wave power converter systems. Experimental results are derived over a range of 200–1200 μW harvester output power, the system being powered solely by the harvester. Experimental results show overall conversion efficiency, accounting for the quiescent power consumption, as high as 82% at 650 μW input, which remains in the 65–70% range even at 200 μW input for the half-wave variant. Harvester utilisation of over 90% is demonstrated in the sub-milliwatt range using full-wave topologies. For the evaluated generator, the full-wave, polarity-dependent boost rectifier offers the best overall system effectiveness, achieving up to 73% of the maximum extractable power.

Achieving Efficiencies Exceeding 99% in a Super-Junction 5-kW DC–DC Converter Power Stage Through the Use of an Energy Recovery Snubber and Dead-Time Optimization

A highly efficient 5-kW bidirectional dc–dc converter power stage operating from a 400-V supply implementing super-junction (SJ) metal–oxide–semiconductor field-effect transistor (MOSFETs) is presented. SJ MOSFETs have low on-state resistances and low switching losses. However, their application in voltage-source converters can be compromised by the reverse recovery behavior of their intrinsic diodes and their highly nonlinear output capacitances. A series switching-aid circuit is used to control the output capacitance charging current. The dead times between switching transitions are assessed and optimized in order to deactivate the intrinsic diodes. The combination of these techniques enables very high efficiencies to be attained. Calorimetric measurements indicate a full-load efficiency of 99.1% for the prototype 5-kW dc–dc converter power stage. A loss reduction of approximately 50% is achieved with the prototype converter power stage when compared to an equivalent insulated-gate bipolar transistor (IGBT)-based power stage. Lastly, a loss versus duty cycle function is experimentally determined, which can be used to inform the design of a maximum efficiency point tracking system.

A Flexible 2.45-GHz Power Harvesting Wristband With Net System Output From −24.3 dBm of RF Power

This paper presents a flexible 2.45-GHz wireless power harvesting wristband that generates a net dc output from a −24.3-dBm RF input. This is the lowest reported system sensitivity for systems comprising a rectenna and impedance-matching power management. A complete system has been implemented comprising: a fabric antenna, a rectifier on rigid substrate, a contactless electrical connection between rigid and flexible subsystems, and power electronics impedance matching. Various fabric and flexible materials are electrically characterized at 2.45 GHz using the two-line and the T-resonator methods. Selected materials are used to design an all-textile antenna, which demonstrates a radiation efficiency above 62% on a phantom irrespective of location, and a stable radiation pattern. The rectifier, designed on a rigid substrate, shows a best-in-class efficiency of 33.6% at −20 dBm. A reliable, efficient, and wideband contactless connection between the fabric antenna and the rectifier is created using broadside-coupled microstrip lines, with an insertion loss below 1 dB from 1.8 to over 10 GHz. A self-powered boost converter with a quiescent current of 150 nA matches the rectenna output with a matching efficiency above 95%. The maximum end-to-end efficiency is 28.7% at −7 dBm. The wristband harvester demonstrates net positive energy harvesting from −24.3 dBm, a 7.3-dB improvement on the state of the art.

Low-Power Methods of Power Sensing and Frequency Detection for Wideband Vibration Energy Harvesting

Power maximisation techniques in wideband vibration energy harvesting typically require the periodic sensing of input power or excitation frequency. This paper presents low- power circuits and sensing methods to obtain this information. First, an excitation frequency measurement circuit is presented that permits a reduced timer run-time compared to reported methods. Second, a power sensing method is presented, which extends the measurement range of reported techniques by adapting to the levels of the available power. Experimental results for the frequency measurement circuit tested in the range 35-51 Hz show a power consumption of 3.7 μW. The power-sensing technique is experimentally validated over a power range of 370690 μW, and its power consumption is 7.5 μW.