Lukas Fässler

Flux Balancing of Isolation Transformers and Application of “The Magnetic Ear” for Closed-Loop Volt–Second Compensation

Semiconductor switches possess nonideal behavior which, in case of isolated dc-dc converters, can generate dc-voltage components which are then applied to the isolation transformer. This dc-voltage component is translated into a dc flux density component in the transformer core, increasing the risk of driving the core into saturation. In this paper, a novel noninvasive flux density measurement principle, called “The Magnetic Ear,” based on sharing of magnetic path between the main and an auxiliary core is proposed. The active compensation of the transformers dc magnetization level using this transducer is experimentally verified. Additionally, a classification of the previously reported magnetic flux measurement and balancing concepts is performed.

Analysis of rotary transformer concepts for high-speed applications

In many applications electrical energy has to be transferred to rotating parts. Usually a cylindrical transformer with a rotating and a stationary part is used, which are separated by a small air gap. In order to achieve a high magnetic coupling, on both parts a highly permeable core material is employed. In high-speed applications the diameter of the rotary transformer should be small since the mechanical stresses can be high. Therefore, a high electrical frequency has to be selected. This high switching frequency would result in high core losses; ferrite would be the best suited material. However, ferrite is brittle and has a limited mechanical strength. Therefore, in this paper two concepts of rotary transformers are analyzed, where no core material is used on the rotating part. The major advantage of these concepts is a simple and mechanically robust design with a lightweight construction resulting in very small unbalanced mass.

Comprehensive analysis and comparative evaluation of the isolated true bridgeless Cuk single-phase PFC rectifier system

In the public low voltage mains the stress caused by harmonic currents is strongly increased due to the higher demand of electronic equipment in everyday use. Therefore, standards were created in order to limit the harmonic currents injected into the network, thus maintaining a high voltage quality of these networks. Hence, converter systems with active PFC are indispensable, where in general the compliance of the created directives for lower harmonics can be easily fulfilled. Typically, in single-phase systems, boost PFC converter topologies are used, which offer a wide input voltage range and a controlled output voltage. Depending on the underlying applications, a subsequent reduction and/or isolation of the output voltage based on a DC/DC-converter is needed. The major disadvantages of this converter cascading are the reduction of the overall efficiency, the large component count and the increasing control complexity. In comparison to the conventional two-stage PFC converter system, the true bridgeless Cuk PFC rectifier system can perform the PFC functionality and the galvanic isolation in a single-stage, thus eliminating the mentioned disadvantages. In this paper a comprehensive analysis of the operation, design and limits of the recently proposed single-stage Cuk concept is investigated. In addition, a comparative evaluation concerning efficiency and power density against a conventional two-stage approach based on bridgeless PFC rectifier and a subsequent LLC-resonant DC/DC-converter is presented.

Application of the magnetic ear for flux balancing of a 160kW/20kHz DC-DC converter transformer

The non-ideal behavior of semiconductor devices used in power electronic circuits exciting a transformer can cause DC voltage components applied to the transformer terminals. This DC voltage in turn generates a DC current and/or DC flux density component only limited by the parasitic resistance of the windings and semiconductors. The biased flux density operation deteriorates the performance of the converter, since the core can be driven into saturation, generating higher currents and hence higher temperatures in the circuit. In order to overcome this problem, the previously reported “Magnetic Ear”, a non-invasive flux density transducer concept, is used in this paper. The design, placement and all implementation issues for this flux density transducer are described. For verifying the theoretical considerations, the transducer is used to perform a closed loop control over the DC component of the flux density in the 166kW/20kHz transformer, therefore ensuring its unbiased magnetic flux density operation.

Detailed analysis and design of a three-phase phase-modular isolated matrix-type PFC rectifier

Phase-modular isolated PFC rectifiers are an interesting alternative to phase-integrated three-phase rectifiers. The use of a matrix-type converter allows to achieve isolation with a single-stage energy conversion. This paper presents a phase-modular isolated matrix-type rectifier, which can be connected to the mains either in star (Y) or delta (Δ), enabling a wide input voltage range. A detailed analysis of the operating principles and switching behavior of the converter is presented. Then, the design of the active and passive components is discussed considering a 7.5kW, 400V output voltage system. Different implementation alternatives are evaluated regarding efficiency and realization effort.