Oscar Garcia

For decades now, conventional sinusoidal signals have been exclusively used in magnetic hyperthermia as the only alternating magnetic field waveform to excite magnetic nanoparticles. However, there are no theoretical nor experimental reasons that prevent the use of different waveforms. The only justifiable motive behind using the sinusoidal signal is its availability and the facility to produce it. Following the development of a configurable alternating magnetic field generator, we aim to study the effect of various waveforms on the heat production effectiveness of magnetic nanoparticles, seeking to prove that signals with more significant slope values, such as the trapezoidal and almost-square signals, allow the nanoparticles to reach higher efficiency in heat generation. Furthermore, we seek to point out that the nanoparticle power dissipation is dependent on the waveform’s slope and not only the frequency, magnetic field intensity and the nanoparticle size. The experimental results showed a remarkably higher heat production performance of the nanoparticles when exposed to trapezoidal and almost-square signals than conventional sinusoidal signals. We conclude that the nanoparticles respond better to the trapezoidal and almost-square signals. On the other hand, the experimental results were used to calculate the normalized power dissipation value and prove its dependency on the slope. However, adjustments are necessary to the coil before proceeding with in vitro and in vivo studies to handle the magnetic fields required.

This article shows the use of a high-frequency Full-Bridge inverter to design an alternating magnetic field generator for experimental studies on anti-cancer treatment with magnetic nanoparticles. In magnetic hyperthermia, nanoparticles are used to raise the pathological cancerous cell's temperature high enough to induce their death by apoptosis, but not as high as to destroy them by thermal ablation of the whole tissue volume, consequently leaving healthy cells alive. Conventionally, sinusoidal alternating magnetic fields are used to heat the nanoparticles, which can be more easily produced than other waveforms. However, there are no theoretical nor experimental reasons to choose the sinusoidal waveform. This work aims to develop an improved power system to study the effect of different waveforms of the magnetic field on heat production when exciting magnetic nanoparticles, aiming at demonstrating that other waveforms can be much more efficient in producing heat than conventional sinusoids. To prove our hypothesis, we designed an inverter able to generate four waveforms at high frequencies and a fifth sinusoidal signal derived by a resonant capacitor, in the range of 100 kHz to 1 MHz and up to 10 mT of peak intensity. Also, we used SiC devices to process high currents at high switching frequencies efficiently. Additionally, to enhance the system efficiency, Zero-Voltage Switching is used to reduce switching losses and minimize electromagnetic noise and interference. The experimental results obtained with non-sinusoidal waveforms have shown a remarkable performance improvement compared to classical sine wave excitation. The nanoparticles' heat dissipation depends on the applied alternating magnetic field's signal slope, signal frequency, and peak field intensity. We conclude that further work deserves to be done to find the optimum work conditions in function of the used particle and biological environment to test if this type of magnetic field generator could overcome the conventional system's performance.

In this paper, analysis and design of a 3.3kW isolated single-stage three-phase buck-type rectifier for aircraft applications is presented. The operating principle and modulation method of the proposed rectifier are introduced. The advantageous Zero-Voltage Switching (ZVS) feature in all of the switching transitions is analyzed. A comparison with the VIENNA Rectifier III is done, being different both in the phase-leg implementation and the modulation sequence. A detailed rectifier design guideline is discussed and losses estimations are provided aiming at reaching 93.7% overall efficiency including the EMI filter. Simulation waveforms are presented. Finally, experimental results obtained with the designed 3.3kW hardware demonstrator are provided. The results verify achievement of ZVS in all of the switching transitions from full load down to 50% of nominal load, while exhibiting 1.9% THDi and 0.9996 PF at nominal power, ultimately showing very good agreement to the simulation results.

In modern telecommunications systems, the requirements for the amount and speed of transmitted data are increasing rapidly. For better spectral efficiency complex type modulations (QAM, OFDM) are used, resulting in high PAPR signals. As a consequence, when conventional linear amplifiers (classes A, AB) are used in transmitters, both prominent linearity and poor efficiency are assured. The efficiency of the transmitters can be significantly improved using some of the popular techniques: Kahn EER principle, ET technique, Doherty amplifier, and Chireix’s amplifier. In this paper, two different approaches in supplying a 2.4 GHz class-E power amplifier (S-band) based on the EER technique are analyzed and compared. The consistent part of the EER transmitters is a switched-capacitor based voltage divider combined with a switching structure -analog multiplexer, in the envelope amplifiers and a class-E power amplifier. The analog multiplexer is assisted with a series linear regulator in the first system and a fourth order LC filter in the second system, providing supply modulation for the power amplifier. The systems are compared in terms of efficiency and system linearity using the standard metrics (EVM, ACPR). Special consideration is devoted to characterization of how the envelope amplifiers affect the quality of the RF output signal. It is shown that the tested EER transmitters can reproduce 16–QAM and 64–QAM signals up to 2.5 MHz, manifesting an average efficiency of about 32% with the first transmitter, and 16–QAM signals up to 660 kHz RF bandwidth and an average efficiency of 44% with the second transmitter. A software tool that accurately estimates the EA linearity is utilized and successfully tested on both EER transmitters.

A power electronics circuit allows the generation of high-frequency magnetic field that can be used to increase the temperature of cancer cells previously invaded with the magnetic nanoparticles. The circuit designed for this purpose is a high-frequency phase-shift inverter implemented with SiC devices and natural zero voltage switching. The inductive load has been optimized to increase as much as possible the magnetic field at the center of it considering the physical restrictions. Into this inductor, an adiabatic probe filled with nanoparticles is placed being the main objective to increase its temperature. The control of the inverter has been designed in such a way that it is easy to try waveforms different from the classical sine waves to see its effect on the temperature of the sample. Although the research is in one of the early stages, the first conclusions about the optimal frequency and field have been obtained showing that this technique could be a real option in the future.

Conventional three-phase rectifiers are controlled to achieve good power factor and low total harmonic distortion in the input. In the case of pulsating power loads, the fast dynamic response implies that the load pulses are reflected in the generator. These pulsating loads affect the lifetime operation of the generator, especially when it is not oversized (as in the case of aircraft applications). In order to smooth the power demanded from the generator, it is preferable to reduce the bandwidth of the output voltage controller for the rectifier but it affects its stability due to the fact that the right-half-plane (RHP) pole given by the negative impedance of a constant power load requires high-bandwidth control loop to compensate it. In this paper, an energy control method is proposed that utilizes the energy stored in the output capacitor of the rectifier to control the output voltage of the rectifier. In such a way, the minimum bandwidth restriction imposed by the RHP pole of the power load is eliminated and the bandwidth of the voltage loop can be defined low enough to demand smooth power from the generator.

In this work it is presented a design tool to estimate the figures of merit of a radiofrequency power amplifier (RFPA) based on the envelope amplifier (EA) design for an EER (Envelope Elimination and Restoration) application. Using as the input data the transfer function of a buck derived converter for an envelope amplifier application, the main figures of merit of the complete amplifier can be obtained. The input data can be defined either analytically, by measurements or by simulation. Different control strategies have been analyzed and compared at the simulation level. Additionally, the non-linearities of the envelope amplifier can be analyzed and quantified. The proposed design tool has been validated experimentally for an OFDM, QAM-based modulations and for a two tone test signal.

Optimization of the radio frequency (RF) transmitter performance in envelope tracking (ET) and envelope elimination and restoration (EER) techniques directly implies optimization of the envelope amplifier (EA) design. Since EA should have fast dynamic response together with high efficiency, synchronous buck is a suitable topology for this kind of application. The key issues for optimization of the converter design are the characteristics of the switching devices, the LC filter design, and the driving circuit for the switches. In this paper, design optimization of the LC filter for sinusoidal output voltage together with new technological solution for switching devices based on gallium nitride is proposed, while the applied drivers are commercially available ones, suitable for aforementioned GaN switches. Design of the output filter in synchronous buck converter for high-frequency application is important from two points of view: the first one is sufficient attenuation of the unwanted harmonics of the switching frequency so that envelope reference has minimum level of distortion and the second one is the overall impact of the filter design on the converter's efficiency. In order to increase the efficiency, the LC filter should be designed in a way to obtain compromise between the conduction and switching losses of the buck converter. In the case of sinusoidal output voltage, filter design parameter has been defined as the ratio between the maximum values of the inductor current and load current and power losses dependence on this parameter has been obtained. Theoretically obtained optimum design has been confirmed through experimental measurements, where four different designs were built and tested. On the other hand, application of new switching devices based on gallium nitride provided significant efficiency enhancement in comparison to Si MOSFETs and laterally diffused MOS transistors, due to the lower values of the gate charge and on-resistance. Tests with 64QAM and wideband code division multiple access (WCDMA) RF signals showed that these devices are the best choice for this kind of application, where the main challenge is to increase the efficiency of the signal transmission simultaneously with the bandwidth.

Recently an important increase of the electrical equipment in modern aircrafts is leading to an increase in the demand for electrical power. The usual electrical power distribution in aircraft applications is done via a three-phase 115Vac grid. A new trend of DC distribution is emerging employing a 270 Vdc grid. This yields the need for high-efficiency and high power-density AC-DC converters, connecting the two grids while providing galvanic isolation. Traditionally 12-pulse autotransformer passive rectifiers are used in aircraft applications. This converter is robust and highly reliable, however it is a non-controlled topology and it is relatively heavy because of the low frequency power transformer. Two-stage rectifier system approaches, employing three-phase PWM rectifiers or active filters and isolated DC-DC converters, are good alternatives to be applied in aircraft applications, to reduce the weight by using high frequency transformers and inductors. However, two-stage topologies process the energy twice leading to lower efficiency, power density. The reliability is also not very high due to a high semiconductor component count. This paper presents a new isolated single-stage PWM rectifier system, based on the recently presented non-isolated Swiss rectifier topology, called the Swiss-Forward rectifier. The principle of operation of this converter topology is presented together with detailed design guidelines and experimental validation on a 3.3kW 115Vac to 270Vdc prototype.

This paper presents a primary-parallel secondary-series multicore forward micro-inverter for photovoltaic AC-module application. The proposed solution changes the number of active phases depending on the grid voltage, thus enabling the usage of low-profile unitary turns ratio transformers. Therefore, the transformers are well coupled and the overall performance of the inverter is improved. Due to the multiphase solution the number of devices increases but, the current stress and losses per device are reduced contributing to an easier thermal management. Furthermore, the decoupling capacitor is split between the phases, contributing to a low-profile solution without electrolytic capacitors suitable to be mounted in the frame of a PV module.