J.J. Schoeman

A simplified maximal power controller for terrestrial photovoltaic panel arrays

Perusal of the V-I characteristics of commercial solar panels with insolation, ambient temperature and production spreads as parameter, indicate that the maximum power is obtained from such a panel when it is loaded to a working voltage that is a fixed percentage of its open circuit voltage within (+2)%. This contribution describes a system that uses this characteristic to achieve maximum power control by determining the open circuit voltage and automatically loading the panel to the maximum power point as applied to a battery charging installation. The description includes the application of a novel modified darlington circuit to boost the efficiency of a pulse width controlled switch mode type regulator by reducing the darlington saturation voltage by a compensating voltage. Advanced switching technology is applied to reduce switching losses, and maximise efficiency.

Relaxation effects in high-voltage barium titanate nonlinear ceramic disk capacitors

Room-temperature capacitance-voltage-frequency measurements are reported for an 85-nF barium titanate high-voltage ceramic-disk nonlinear capacitor, intended for use in a power electronics turnoff snubber circuit. Bias-voltage excursions are from 0 to 1500 V DC, and the frequency responses are measured from quasi-DC to 1000 Hz. The observed C-V-frequency responses are modeled in terms of series-capacitance contributions from ferroelectric grains and p-n junction grain boundaries, involving 16 parameter variables. The ferroelectric capacitance terms are given by a modified Langevin function, and the grain-boundary capacitances are modeled by back-to-back p-n junction diodes on either side on an insulator boundary. The observed frequency dependence of the C-V response is attributed here to a Debye-type relaxation of the compensation regions at the grain boundaries, with time constant 15 ms. Good agreement between theory and experiment is obtained over the 0-1500-V bias range. >

Aspects of modeling of high voltage ferroelectric nonlinear ceramic capacitors

Experimental and theoretical studies are reported on the room temperature capacitance-voltage (CV) and (tan delta )-V response of a high-voltage barium titanate ceramic disc capacitor with prescribed nonlinear CV response, as fabricated for an intended power electronics application. The CV response was modeled in terms of a voltage-dependent series equivalent circuit incorporating grain-boundary junction capacitances and grain ferroelectric capacitances. A modified Langevin function was used to relate intergrain ferroelectric polarization, which yielded good agreement between theory and experiment for a 3-to-1 capacitance variation over the DC bias range up to 1000 V. >

High field capacitance-temperature behavior of BaTiO/sub 3/ ceramic disc capacitors

Experimental and theoretical studies are reported on the capacitance-temperature (CT) characteristics of an undoped BaTiO/sub 3/ high-voltage (1000 V) nonlinear ceramic disc capacitor with a 3.6:1 capacitance ratio over the DC bias voltage range, which has application to power electronics snubbers. CT responses for both the zero-bias and 1000-V-bias conditions are modeled in terms of the polarization of permanent dipole moments. High field modeling employed a modified Langevin function, incorporating an empirical domain sensitivity parameter together with a smeared Curie temperature. >

A high performance gate/base drive using a current source

A gate drive that uses forced commutation to switch on the main device is described. The fundamental limits of the drive are pointed out, including minimum off-time and maximum on-time. The results of a loss analysis performed on the drive are presented. Experimental results show that the efficiency of the drive is exceptionally high when compared to conventional gate drives, and measured turned-on times of a zero turn-off thyristor are extremely low.<<ETX>>

Transformer-coupled direct base drive technology for high-power, high-voltage, bipolar transistor PWM converters

The drive requirements of large high-voltage bipolar power transistors as applied in pulsewidth modulation (PWM) converters and the fundamentally different topologies for efficient high-power transformer-coupled base drives delivering up to 100 A are discussed. Possibilities for high-power base drives with a single transformer were examined, and the limitations of high-efficiency pulsed direct base drives are discussed. The main disadvantage of all pulsed transformer-coupled drives is the absence of continuous negative base-emitter voltage. This is essential in PWM applications in high-voltage converters, and transformer-coupled drives require carrier wave systems. The high-frequency carrier wave system as optimal topology for high-power transformer-coupled direct base drives is analyzed. The different base drive topologies are evaluated in terms of capability, losses, component count, and physical size. Some aspects of the design of a 1 MHz carrier wave transformer-coupled base drive are discussed and experimental results, including efficiency, are shown. >

Soft switching self-oscillating IGBT-based DC-DC converters

The characteristics and advantages of soft switching self-oscillating insulated-gate bipolar transistor (IGBT)-based DC-DC converters are described. The main features offered by this family of self-oscillating inverters include: a full-bridge self-oscillating inverter having a minimum component count; extremely high efficiency, even at ultrasonic switching frequencies; low electromagnetic interference due to zero voltage switching; automatic protection against short-circuit output; automatic protection to all other switches during failure of one switch due to overheating; automatic protection against spurious simultaneous turn-on of two switches in the same inverter leg; negligible overrating of the switches for current and voltage; and negligible VA rating of the saturating transformer compared to the power transformer.<<ETX>>

An evaluation of resonant snubbers applied to GTO converters

Aspects of the application of resonant snubber technology to gate-turn-off (GTO) converters are investigated. GTO power switches require snubbing at turn-off when applied in converters. This leads to the extensive use of regenerative snubber circuits in GTO converters due to the difficulties involved in using dissipative snubbers. At present, it is accepted that the repetition switching frequency of GTO pulse-width modulated (PWM) circuits is largely determined by the resettling time of regenerative snubbers. The resonant DC link may be seen as a nondissipative snubber and holds the promise of a faster resettling time. The behavior of GTO devices is investigated under the conditions applied to them by this kind of snubbing and its application in GTO converters. The concept has been demonstrated with 90 A 1200 V GTOs. >

Low-voltage, high-efficiency switch-mode high-power inverters for AC link converter applications

A 30 kVA high-frequency link converter, which consists of 6*5 kVA center-tap power block topologies operating in parallel, is considered for battery-fed DC-AC converter applications. Practical solutions for minimizing currents circulating between the different power blocks, avoiding transformer saturation in forced commutated center-tap topologies, and minimizing transistor-on losses are incorporated and illustrated by means of practical measurements and results. A simple single-capacitor snubber network, operating in parallel with a second-stage capacitor snubber, is used for these converter types. Typical applications include mobile and telecommunications uninterruptible power supply systems; high AC-voltage loads fed from photovoltaic or hybrid energy systems; and battery-fed, mobile, variable-speed AC and DC drives. >

A high density three phase high frequency link system for variable frequency output

A three phase converter was implemented using a self-oscillating inverter as the primary inverter and a direct converter as the secondary converter. The different structures that can be used for the secondary converter are investigated systematically with a view to reduction of component count, stresses, low weight, and high packing density. The problem of synchronization between the two converters was solved by using a simple magnetic circuit. The primary converter had to be overdimensioned by a factor 1.732 due to the reactive power which had to be supplied by the primary converter. Fundamental problems in the implementation, however, were not found. Experimental results are given.<<ETX>>