Wolfgang Heering

Switching speed-control of an optimized capacitor-clamped normally-on Silicon Carbide JFET cascode

This paper presents the results of controlling the switching speed (dv/dt) of an optimized version of the capacitor-clamped normally-on Silicon Carbide (SiC) JFET cascode. The cascode circuit is an easy way to employ the normally-on SiC JFET in a normally-off configuration using a low-voltage Silicon (Si) MOSFET in series connection. The traditional cascode circuit exhibits some disadvantages, namely limited control of the switching speed and high frequency oscillations on the drain-source-voltage of the low-voltage Si MOSFET. The optimized capacitor-clamped cascode reduces the oscillations and enables control of the voltage and current slopes as well as implements a protection of the JFET gate. The theoretical background of the capacitor-clamped cascode and the control of voltage slope are explained and investigated experimentally.

Universal resonant topology for high frequency pulsed operation of Dielectric Barrier Discharge light sources

This paper reports about emerging techniques for the efficient drive of Dielectric Barrier Discharge (DBD) plasma light sources in pulsed operation mode. Besides a classification of pulse topologies, the novel Universal Sinusoidal Pulse topology, that provides high voltage amplification and high frequency operation, is presented. Transformer equipped and transformerless variations of the topology are presented. Experiments prove electrical efficiencies of more than 80%, while power measurements are cross-checked by thermal measurements. Additionally, frequency domain characteristics of the used DBD-lamp were analyzed and magnetic storage devices with distributed center gap are presented.

Reflective optics for cosine-corrected irradiance measurements

Most of the detectors commonly used for radiometric measurements have a more or less poor cosine response. By means of a reflective entrance optics in front of the detection system, we have reduced relative cosine errors below 5% up to zenith angles of 75°. Cosine correction is performed by a roughened polytetrafluoroethylene (PTFE) plate which diffusely reflects parts of the incident radiation into a light guide with the detector at its end. It provides for a neglectible wavelength cosine error dependence and a significantly better radiant efficiency. Originally designed for irradiance measurements of solar global radiation and its photobiological effective irradiance, the entrance optics may also be used for determination of radiant flux from lamps by integrating the lamp irradiance over an enveloping surface as it is done in spectrogoniometric measurements.

Modelling of indium(I) iodide-argon low pressure plasma

A new collisional-radiative model for a mercury-free low pressure plasma based on an indium(I) iodide-argon system is presented. The electron impact cross sections and rate coefficients for ionization, excitation and dissociation, as well as de-excitation, three-body recombination and dissociative recombination, of studied fillings have been calculated. Additionally, the coefficients for free and ambipolar diffusion were determined. The rate balance equations for individual generation and loss processes have been created. Densities of ions, electrons and neutral particles (ground or metastable state) are presented as a function of electron temperature for varied lamp parameters, such as argon buffer gas pressure and cold spot temperature (coldest point of discharge vessel). With the help of the presented model, the line emission coefficients of essential emission lines of indium for given electron temperatures and densities can be predicted.

Improved power converter for pulsed operation of DBD

In this paper an electronic ballast for pulsed operation of dielectric barrier discharges (DBD) is presented. The converter is designed as a transformer coupled square wave power source optimized for capacitive loads like DBD. The special features are the bipolar trapezoid waveform with variable slew rate (dU/dt), duty cycle, frequency and amplitude of the output voltage, which is balanced to ground. The power stage is designed in zero voltage switching technology. The ballast is primarily designed to investigate the discharge characteristics of DBD in dependence on the waveform parameters. A simple electrical DBD model is presented, which allows to predict the external DBD voltage and the discharge power in dependence of slew rate and duty cycle. The paper is closed with some experimental results of pulse XeCl*-excimer lamps.

High-Intensity Discharge Lamps With High Color Rendering Indices Based on Molecular Radiation of Indium Iodide

This paper deals with mercury-free high-intensity discharges based on indium iodide, that are driven by guided microwaves at 2.45 GHz. The influences of different additives to the lamp on the luminous efficacy as well as on the color rendering index will be shown. Furthermore, the spectral output of pure indium iodide lamps, using argon as auxiliary gas, will be explained by a combination of indium line radiation and molecular emission of the indium iodide molecules. A theoretical model for the molecular emission is shown, which uses the semiclassical description of molecular radiation.

Threshold current density for homogeneous excitation of pulsed xenon excimer DBD

Xe excimer DBD operate in a filamented or homogeneous mode depending on several parameters, e.g. the electrical excitation. The need of a pulsed excitation for homogenous discharge is well known from literature. However, the role of the electrical excitation to homogenize the DBD is not complete understood. In this work the transfer from filamented to homogenous mode is investigated for sub-microsecond pulses. Measurements of the gap voltage and plasma current show the influence of reignition for a homogeneous operation. The correlation between short time NIR images and current measurements point out that filamentation is depending on current density during ignition. It is shown that the filaments wide up with increasing current, like in an arc discharges. The filament expands with increasing ignition current and form a 2-dimensional pattern which goes finally over to a complete homogeneous discharge. It can be shown, that the current density through the ignited area is constant and a threshold current density for homogenous discharges can be defined depending on the strength of reignition and Xe pressure. The constant current density in the ignited area explains the transition from filamented to homogenous mode. This shows that current during ignition is the critical electrical parameter and not the rise time of the lamp voltage. To display the dependency of threshold current density and reignition, sub-microsecond pulses with a negative lamp voltage undershoot are shown. The threshold current lowers with increasing reignition. This behavior could be explained with accumulated remaining charges on the barriers from the last excitation pulse. Based on the need of a high current during ignition a novel DBD pulse gear with enforced reignition is presented with good system efficiency.

Integrated PFC and series resonant frequency converter analysis and control

A control algorithm for an integrated power factor correction (PFC) and series resonant converter (SRC) using only two switches is presented, allowing to open loop control the AC input current and the DC output current independently. The theory for this approach is analyzed in time domain and a novel approach for open loop regulation is presented and verified. The software defined power (SDF) control algorithms determines the switching frequency and duty cycle of the converter.

Investigations of SMPD SiC-MOSFET phase-leg modules for MHz inverters

In this work we present results of investigations on the usability of ISOPLUS SMPD SiC-MOSFET phase-leg modules for medium-frequency resonant inverters. These modules combine excellent thermal performance, very low parasitic inductances and good switching performance with the benefits of surface-mountable packages. In the first step, a SPICE and a thermal simulation of a half-bridge inverter using the SMPD module and one using discrete SiC-MOSFETs in TO-247 package was performed. Here, the semiconductor losses of the discrete devices were 2.5 times higher than the one of the SMPD module. In the second step, a modular prototype of a resonant MHz half-bridge inverter with the SMPD module was built. A short circuit test and efficiency measurements with an ohmic load showed an excellent switching performance of the SMPD SiC-MOSFET phase-leg module in the MHZ range. With the inverter prototype, an efficiency of > 95% was measured, at a switching frequency of 2.01 MHz and an output power of 5.6 kW delivered to the load resistor.

Diagnostics of surface wave driven low pressure plasmas based on indium monoiodide-argon system

Indium monoiodide is proposed as a suitable alternative to hazardous mercury, i.e. the emitting component inside the compact fluorescent lamps (CFL), with comparable luminous efficacy. Indium monoiodide-argon low pressure lamps are electrodelessly driven with surface waves, which are launched and coupled into the lamp by the ‘surfatron’, a microwave coupler optimized for an efficient operation at a frequency of 2.45 GHz. A non intrusive diagnostic method based on spatially resolved optical emission spectroscopy is employed to characterize the plasma parameters. The line emission coefficients of the plasma are derived by means of Abel’s inversion from the measured spectral radiance data. The characteristic plasma parameters, e.g. electron temperature and density are determined by comparing the experimentally obtained line emission coefficients with simulated ones from a collisional-radiative model. Additionally, a method to determine the absolute plasma efficiency via irradiance measurements without any goniometric setup is presented. In this way, the relationship between the plasma efficiency and the plasma parameters can be investigated systematically for different operating configurations, e.g. electrical input power, buffer gas pressure and cold spot temperature. The performance of indium monoiodide-argon plasma is compared with that of conventional CFLs.