Josef Deuringer

New x-ray tube performance in computed tomography by introducing the rotating envelope tube technology.

The future demands of computed tomography imaging regarding the x-ray source can be summarized with higher scan power, shorter rotation times, shorter cool down times and smaller focal spots. We report on a new tube technology satisfying all these demands by making use of a novel cooling principle on one hand and of a novel beam control system on the other hand. Nowadays tubes use a rotating anode disk mainly cooled via radiation. The Straton x-ray tube is the first tube available for clinical routine utilizing convective cooling exclusively. It is demonstrated that this cooling principle makes large heat storage capacities of the anode disk obsolete. The unprecedented cooling rate of 4.8 MHU/min eliminates the need for waiting times due to anode cooling in clinical workflow. Moreover, an electronic beam deflection system for focal spot position and size control opens the door to advanced applications. The physical backgrounds are discussed and the technical realization is presented. From this discussion the superior suitability of this tube to withstand g-forces well above 20 g created by fast rotating gantries will become evident. Experience from a large clinical trial is reported and possible ways for future developments are discussed.

New Current Control Scheme for the Vienna Rectifier in Discontinuous Conduction Mode

The Vienna rectifier (VR) is used in applications that require unidirectional, non-isolated, three-phase AC to DC conversion with constant output voltage and sinusoidal input currents. However, because of the unidirectional topology, the input currents become discontinuous at small output power values. As a consequence, the relationship between rectifier input voltage and duty cycle changes compared to continuous conduction mode. Therefore, if no additional measures are taken, the rectifier input currents will be distorted. This work describes a new control scheme that allows operation of the VR with sinusoidal input currents in discontinuous conduction mode (DCM). The limits of operation are described, concerning maximum mains voltage, maximum midpoint current and minimum resistance to the mains in DCM. Further, the noise emission in DCM is compared to continuous conduction mode (CCM) operation. Finally, the proposed scheme is experimentally verified on a hardware prototype.

New boundary mode sinusoidal input current control of the VIENNA rectifier

In hard switching boost-type or buck-type three-phase power factor corrected rectifier systems the turn-on losses in continuous conduction mode (CCM) are usually higher than the turn-off losses. This is mainly caused by the reverse recovery effect of the freewheeling diode. The reverse recovery related turn-on losses however may be eliminated if the converter is operated in boundary conduction mode (BCM), i.e. at the boundary of discontinuous conduction mode (DCM) and CCM. This paper shows that the Vienna Rectifier (VR) can be operated in BCM with zero current turn-on and that the reverse recovery current of the freewheeling diodes can be used to achieve partial zero voltage switching (ZVS). The principle of three-phase, three-level BCM control is described using space vectors, the effect of the variable switching frequency on the design of the input filter is investigated in detail and dimensioning criteria for the filter elements as well as the semiconductors are given. A current slope detector circuit using auxiliary windings on the input boost inductors is proposed for turning on the switches at minimum voltage. Further, a method to compensate the delay caused by the freewheeling diode reverse recovery time and an efficient implementation of the modulation with FPGAs using a quadratic counter is proposed. The proposed control scheme is finally experimentally verified on a hardware prototype.