We propose planar coils with bistable buckling (snapping) of wire segments providing different magnetic field patterns and thus reconfigurable induced electric field distributions in transcranial magnetic stimulation. We demonstrate a tunable magnetic field focality by 60% at distance of 25 mm to the coil surface. Furthermore, for the given exemplary single layer geometry, simulations of the induced electric field on an adult head model offers adjustment of the maximal electric field by 64% and modification of field focality by 28%.

We propose a group theory based mathematical model for unambiguous inductance states of a reconfigurable air coil inductor utilizing elastic snapping of wire segments. Vertical (axial) and horizontal winding symmetries under group actions reveal distinguishable tuning regions and individual variation states therein. The model is applied to coils of prismatic (cylinder like) and truncated pyramidal (conical like) shape with edges forming a regular polygon as basis, and verified by numerical magnet field simulation.

We propose near-field inductive power transfer with three coils in the frequency range of a few MHz. The relay coil is part of a non-linear resonator having a diode connected in parallel to the resonant capacitor. We achieve power transfer at multiple frequencies which can be tuned by the input power level and mutual coupling between the coils. Higher harmonic generation is found dominant at lower input power, whereas subharmonic generation (period doubling and period quadrupling) is observed at high levels. Power transfer at subharmonics reveals robustness against level perturbations due to pronounced amplitude hysteresis effect. A circuit simulation based on standard non-linear diode SPICE models confirms the experimental results which are obtained on a demonstration prototype.

We propose to introduce environmental sustainability in a radio frequency (RF) and electromagnetic compatibility (EMC) laboratory course for an electrical engineering bachelor's program utilizing reusable, easily recyclable and ecologically harmless (biodegradable) circuits. Our concept is based on the replacement of state-of-the-art printed circuit boards by substrates with mechanically structured adhesive Cu and Zinc films. Furthermore, organic based circuit carriers can be substituted by biodegradable paper substrates. As an example for the RF lab, we discuss the design, simulation and experimental characterization of a microwave low-pass filter with 1.2 GHz cut-off frequency. Secondly, for an EMC lab we suggest a prototype for power integrity (PI) studies based on the current imbalance method for reducing common mode radiated emission. As a novel feature during the lab course, the instructor in addition gives a lecture highlighting the electronic waste challenge, which is accompanied by group discussions on sustainability and recycling. As a final lab course task students are asked to disassemble the boards or prepare them for reuse, followed by material sorting and submission to recycling institutions.

A mechanically tunable planar single-turn inductor in air is proposed. The bistable flexible deflection of axially compressed wire segments is used for variation of the loop area of regular polygonal loop inductors. We present analytical expressions for the loop inductance and confirm the theoretical findings by magneto-quasi-static field simulation on exemplary polygonal circuits.

By axially loading of a cylindrical wire we utilize the first order buckling mode to form the shape of a segment of a wire loop inductor. The shape of the deformed wire is expressed in analytical form and provides the basis for a semi-empirical formulation of the loop inductance. Combining several buckling segments and taking advantage of the snap-through behavior of such a segment, the shape of the wire loop in air and hence the loop inductance can be varied mechanically on purpose. The analytical model for the tunable inductance enables a design methodology for bistable compliant inductors. We present two designs of mechanically tuned inductors in the range from 20nH to 50nH. Furthermore the electromagnetic properties of the proposed innovative components are confirmed by method of moments numerical modeling based on quasi-static magnetic fields and by experiment. We finally show that the design process is applicable to regular plane polygonal inductors representing multi-stable compliant stages.

Enhanced inductive power transmission (IPT) between coaxially arranged circular coils is introduced by timemodulation of the mutual inductance. This modulation is accomplished by coherent switching of the short circuit current induced in a third loop. An analytical circuit model supporting the design for future time-modulated IPT systems is derived. We demonstrate an increase in load rms-voltage by 13% (28% gain in power) compared to an unmodulated transfer on an experimental system operating at fundamental frequencies ranging from 40 kHz to 500 kHz.

A Magneto-Inductive Wave (MIW) structure for wireless power transfer (WPT) is proposed with switchable mutual inductance between neighbored coupling coils. Orthogonally placed switchable short circuit loops are added in order to modulate the wave propagation properties of the MIW structure without changing the resonance frequency. We derive an analytical model based on lumped circuit analysis for the MIW waveguide and successfully evaluate the theoretical findings by circuit modeling and field simulation. We demonstrate switchable, i.e. addressable, guided wireless power transfer along an experimental MIW structure at a resonance frequency of 20.5 MHz.

We present inductive power transfer (IPT) with a half-bridge converter based on differential relaxation self-oscillation. The oscillation dynamics of the converter automatically adapts to variation in the inductive coupling link and changes in the load, respectively. Design equations based on theoretical circuit analysis reveals a high power transfer efficiency (> 90%) for alteration of coupling distance in the strong coupling regime. A prototype system employing GaN-HEMTs confirms the theoretically predicted characteristics of the proposed circuit.

We present a fast and accurate measurement technique for quasi-static magnetic fields by employing a progressive sampling method in an unconfined input space. The proposed machine learning algorithm is tested against uniform sampling on printed circuit board test structures and a buck converter. We prove the allocation of multiple, separated regions with predefined lateral field limits at megahertz frequencies. The feasibility of equivalent magnetic dipole source modeling based on a small number of samples is demonstrated. Compared to uniform testing, progressive expansion sampling identifies contours of given field limits in less than 3% of the reference measurement time.

We present a capacitive power transfer system with self-adapting capability to multiple, variable load receivers. The proposed self-oscillating GaN-based half-bridge converter with load current feedback automatically adapts to the coupling path. We show design equations based on network theory and demonstrate experimentally more than 100W power transfer at 92% efficiency.

In this research we propose a novel common mode (CM) EMI filter without discrete components for power electronic converters which is realized on a thin film standard printed circuit (PCB) substrate. The EMI low-pass filter features an integrated shunt capacitor, a defective ground structure (DGS) and provides EMI noise reduction greater than 10dB from 1MHz to 1GHz. We present a filter prototype and demonstrate its performance on a DC/DC buck converter with 650V rated GaN HEMTs which operate with a dv/dt=100V/ns.

This paper introduces a common mode EMI filter based on defected ground structures (DGS), which is integrated into a half bridge switching cell with Gallium Nitride (GaN) power semiconductors. A four-layer printed circuit board with a low-inductance vertical commutation loop was used for the switching cell design. The performance of the integrated DGS filter was studied by a single-phase DC/AC power conversion to an ohmic-inductive load. Measurements showed critical CM noise reduction of more than 17 dB for frequencies above the cut-off frequency of 34MHz compared to a switching cell without DGS filter.

We present a fast and accurate measurement technique for magnetic near-fields by employing an adaptive sampling method with unbounded input space. The proposed machine learning algorithm is tested against uniform sampling on a printed circuit board test structure and a buck converter. We demonstrate allocation of multiple, separated regions with predefined lateral field limits at MHz frequencies. Compared to uniform sampling, active expansion sampling identifies contours of given field limits in less than 3% of the reference measurement time.

We present a novel differential relaxation oscillator for inductive power transfer. The proposed oscillator offers an automatic adaption of the self-oscillation period to a change in the power link impedance, which ensures a high efficient operation for variable loads and coupling factors. A detailed circuit model of the self-oscillation system is provided with analytical design equations. We find a large self-oscillation range for variable coupling factors and loads. The theoretical results are confirmed by circuit simulation and measurements carried out on a low-power demonstrator system. We demonstrate non-resonant and robust self-adaption of the relaxation oscillator supplying an inductive power link with coupling factors ranging from k=0.6-0.9.

In this research we present a short distance capacitively coupled wireless power transfer (WPT) system with a self-oscillating half-bridge converter, which uses a positive feedback signal from the WPT system. By this novel implementation we find additional features of the system compared to linear amplifier based systems. With the DC bias voltage supplying the power converter, we achieve a controllable output power, tunable self-oscillation frequency and extension of the self-oscillation range. We present the analytical expressions for the converter waveforms. The theoretical analysis is confirmed by circuit simulation results and measurements on a prototype featuring a half-bridge converter with Gallium-Nitide (GaN) HEMTs. We achieve a system efficiency of 93% at a load power of 83W and 150kHz switching frequency.

We present the design of a half-bridge converter for a capacitively coupled wireless power transfer system. The load signal acts as a positive feedback signal for a self-oscillation circuit which generates the driving gate signals for the converter. Our research reveals a robust wireless operation for a large range of load resistors and coupling capacitors. Based on an electric network analysis we obtain design criteria for self-oscillation and for the power transfer. The theoretical predictions are confirmed by circuit simulation and measurement results on a converter prototype with GaN-HEMTs. We report on a transfer efficiency of 92% for a power greater than 50W at a switching frequency of 300kHz and 200V DC link voltage.

This work shows a numerical and experimental analysis of a Neutral-Point-Clamp (NPC) three level inverter featuring an ultra low inductance printed circuit board (PCB) design in consideration of the mutual inductive and capacitive couplings. The commutation loops in this design are found to be strongly dependent on the vertical thickness of the used prepregs and the core. For vertical thicknesses & 100 mu m capacitive coupling must be taken into account in the switching cell design. Experimental measurements of a test set-up with a total PCB thickness of 400 mu m results in commutation loop inductances from 1.4nH up to 3.1 nH. In this set-up, switching tests without external gate resistor showed only a maximum voltage overshoot of 7% at 800V. Based on a numerical analysis of the NPC cell we propose a further switching performance improvement with significant smaller parasitic inductance due to the application of novel printed circuit technologies such as the integration of bare dies into the printed circuit board or polyimide as an interlayer dielectric material.

We present a numerical and experimental design study for reducing conducted common mode noise for frequencies below 10MHz generated in switching cells comprising sub-nH commutation loops. Compared to conventional methods of adding external gate resistors our approach does not degrade efficiency. A parallel plate loop design (characterized by a minimum loop inductance and small board area usage) is found suboptimal in terms of conducted EMI compared to a micro-strip line loop geometry. The net ground impedance rather than the net ground inductance is found to mainly contribute to the observed EMI noise.

This paper proposes an ultra-low inductance half-bridge switching cell with substrate integrated 650V GaN bare dies. A vertical parallel-plate waveguide structure with 100 μm layer thickness results in a commutation loop inductance of 0.5 nH resulting in a negligible drain-source voltage overshoot in the inductive load standard pulse test. On the other hand reliable circuit operation requires an assessment of the isolation strength of the thin dielectric layer in the main commutation loop, because critical high local electric fields might occur between the pads. Measurements of the dielectric breakdown voltage followed by a statistical failure analysis provide a characteristic life of 14.7 kV and a 10% quantile of 13.5kV in the Weibull fitted data. This characteristic life depends strongly on the ambient temperature and drops to 4.1kV at 125°C. Additionally, ageing tests show an increasing in dielectric breakdown voltage after 500h, 1000h and 2000h at 125°C high-temperature storage due to resin densification processes.