P.C. Theron

A series resonant converter for arc striking applications

Initiation of a plasma conduction state requires a relatively large voltage to ionise the gas. A new version of the series resonant power converter is identified which uses the magnetising inductance of the transformer for resonance. This power converter is not suitable for most power supply applications, but the unique load characteristics associated with plasma loads make this type of power converter well suited for arc striking, while allowing safe operation during the plasma state. Both these power converter characteristics are obtained with minimum controller complexity. The series resonant power converter is verified experimentally in a tungsten inert gas welding application.<<ETX>>

Control of nonlinear resonant pole inverters

A version of the resonant pole inverter includes a saturating inductor and a current feedback winding. The standard resonant pole inverter becomes a special case of this inverter and design degrees of freedom increase significantly. The focus is on the control of this type of inverter, and the switching frequency variation and dynamic response are addressed. It is shown that a saturating inductor can improve the stability of the switching frequency while very fast dynamic response is possible in the case of an inductive load. Experimental results are presented.<<ETX>>

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>>

Component loss modelling in hard switched and resonant pole inverters

A three-fold comparison between two types of resonant pole inverter and the hard switched inverter is presented, based on loss modeling in the switching devices. It is shown that the resonant pole inverters can reduce the losses in the switches and diodes, compared to the hard switched inverter, by almost an order of magnitude depending on the type of device. It is pointed out that devices should be designed differently for resonant pole inverters. Since reverse recovery times in diodes and switching times of controllable devices are not directly related to switching losses, devices with less conduction loss can be used. The nonlinear resonant pole inverter displays lower loss, peak current stress, and RMS inductor current than the linear version.<<ETX>>

The partial series resonant converter: a new zero voltage switching converter with good light load efficiency

A novel high-frequency DC-to-DC power converter, suitable for application in the kW range, is presented. The topology is series resonant for a part of each switching cycle and transfers a discrete energy pulse to the load every half cycle, rendering a variable-frequency controller for controlling the output power. The transformer is a pulse transformer which operates only during resonance, i.e., for part of each switching cycle, irrespective of the switching frequency. The converter features zero-voltage turn-on and snubbed turn-off at reduced current for the switching devices, and zero current commutation of the rectifier devices, making it suitable for IGBT (insulated-gate bipolar transistor) switches operating at a switching frequency higher than 20 kHz. Therefore, the efficiency of the converter is very high under full load and low load conditions, while the dynamic output voltage range is two times larger than that of the conventional series resonant converter.<<ETX>>

A new induction heating circuit with clamped capacitor voltage suitable for heating to above Curie temperature

The partial series-resonant power converter, previously only used for DC-DC converters, is applied to induction heating. The circuit is a half-bridge, series-resonant circuit with clamping diodes that constrain the voltage of the resonant capacitors to values between zero and the supply voltage. Three types of control are investigated: constant switching frequency; constant turn-off current; and a combination thereof. The latter seemed to be the best. This circuit is only found suitable for loads for which the Q value is low while the temperature of the workpiece is below the Curie temperature.<<ETX>>

Welding power supplies using the partial series resonant converter

Present technology in high frequency welding power supplies is mainly based on hard switching nonresonant power converters. Their disadvantages are high semiconductor and copper losses, complex short circuit protection, and a complex controller for shaping of the output load line. The partial series-resonant power converter, a new high frequency DC-to-DC power converter is applied to welding applications. It is based on the half-bridge version of the series resonant power converter, but the dynamic output voltage range is twice as large. Discrete energy pulses are transferred to the output every half switching cycle, permitting a simple variable frequency controller for controlling the output power. Therefore, a constant output power load line is simply achieved by keeping the switching frequency constant, which, in turn, provides excellent welding characteristics. The power converter features zero voltage turn-on, and snubbed turn-off at reduced current, making it suitable for IGBT switches operating well above 20 kHz.<<ETX>>

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

The authors introduce a family of topologies utilizing both zero voltage switching and self-oscillation. The Royer oscillator is modified to use MOSFETs as power switches. Thus insulated gate bipolar transistors (IGBTs) can also be used as power switches, enhancing the power capability for higher voltage and current ratings. A further advantage obtained by the use of FET-input switches, compared to the bipolar-transistor-based Royer and Jensen oscillators, is that stable operation can be obtained without the saturation of any core material. This enables very high efficiency of the power converter as well as a very high oscillation frequency. Typical applications of the family of topologies considered include DC-DC converters, high-frequency inverters, and high-frequency link converters. In high-power (>1 kW) applications, the use of IGBTs in the nonlinear resonant pole soft switching self-oscillating inverter will be advantageous to the efficiency, due to the reduced onstate voltage of a minority carrier device.<<ETX>>

Optimized transformer for arc striking applications

An in-line arc striking power converter used in electric welding applications needs to be galvanically isolated with a high-frequency power transformer. A feature of the particular circuit application is that the magnetising inductance of the isolation transformer is used in a resonant circuit to generate 2 kV at 500 kHz. Two loading conditions exist for the transformer, namely a large voltage at the secondary during open circuit prior to arc striking and a secondary current typically larger than 100 A during welding itself. Possible transformer configurations are investigated which include an air core and a number of ferrite cored transformers. Analysis and experimental results of the transformers are presented and an optimum configuration is identified.