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- Doctoral Thesis
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Medical X-ray systems have been under steady development since the discovery of X-rays by R¨ongten in 1895. However, the principle of X-ray generation has remained the same. Electrons are accelerated within an evacuated tube by a high-voltage difference from the cathode towards the anode. X-rays are emitted when electrons hits the anode. However, most of the energy of the electrons is dissipated by the anode in form of heat. Therefore, an X-ray power supply needs to provide not only voltages up to 150 kV but also a peak power of up to 100 kW. Since the cooling capability of the vacuum tube is typically only in the range of 1 kW, the power supply has to deliver high peak but only low average power. In the development of X-ray systems there are three major trends affecting the high-voltage generator. First, nowadays X-ray systems use passive three-phase rectifiers which limit the application with weak power grids, emit low-frequency current harmonics and usually require adaption transformers for use with different nominal mains voltages. It is desired to use an active rectifier instead in order to provide a wide input voltage range that allows world-wide use of the device without modifications to the hardware. Second, currently available high-voltage generators typically provide rise- and fall-times of the tube voltage in the range of 500 μs. In pulsed fluoroscopy applications short rise- and fall-times of the tube voltage would eliminate the need for the cathode control grid which is necessary to keep the patient dose at a minimum. In dual energy computer tomography shorter fall- and rise-times could replace nowadays systems using two X-ray tubes at two different voltage levels by a single tube which is alternating between two voltage levels. Finally, the additional features should be achieved in a cost effective manner in order to make the technology affordable for all patients. In order to achieve cost reductions the design of the inductive components is of particular importance. The high peak power but low average power of the application allows to reduce the volume and material cost of inductors and transformers substantially without exceeding the thermal limit. One basic element for the design of inductive components is the winding loss model which needs to take into account skin effect and proximity effect losses within the winding. An easy to use winding loss model is presented based on asymptotic approximations of the exact analytic equations for skin effect and proximity effect. Using appropriate asymptotic solutions for the low-frequency and for the high-frequency range, is usually sufficient. The error introduced by the approximations is only significant at the border between low-frequency and high-frequency range which is located approximately at the frequency where the conductor dimension equals twice the skin depth. It can be shown that windings with conductor dimensions close to this border should be avoided since they exhibit a higher AC resistance than windings with thicker or thinner conductors than twice the skin depth. Furthermore, the low-frequency asymptotic approximation allows to show that due to the proximity effect there is an optimum operating frequency of transformers which is inversely proportional to the conductor dimension and the winding width. Based on the winding loss model an active three-phase rectifier is optimized for minimum total cost of ownership, which is the sum of material cost and loss energy cost. Due to the high peak-to-average load profile a high power density (9.56 kW/dm3) and moderate efficiency (97.3%) design results. The prototype is designed for 65 kW nominal power and provides stabilized 800V DC for the required input voltage range of 290V-530V line-to-line RMS. Due to their competitive price, silicon IGBTs provide a lower total cost of ownership for the considered application than silicon carbide MOSFETs while also providing high reliability and availability. The Vienna rectifier as a unidirectional system exhibits a discontinuous conduction mode which is usually entered only at very low power levels. Due to the high power density of the boost inductors a relatively high current ripple occurs and discontinuous conduction occurs already at 20% load. Since the duty cycle to voltage relationship is no longer linear in discontinuous conduction mode, high input current distortion is encountered with traditional closed-loop current controllers. Therefore, a special control scheme is presented that maintains sinusoidal input currents also with low load. The duty cycle values to obtain sinusoidal input currents can be directly expressed by analytic equations depending on the mains voltages, the DC-link voltage and the current set value. The control scheme is demonstrated on a 65 kW rectifier prototype. In order to reach a high tube voltage control bandwidth a nonresonant high-voltage generator topology is proposed. The topology extends a single active bridge converter using two coupled transformers which are feeding two series connected rectifier stages at the output. Compared to commonly used resonant converter solutions the proposed topology mainly provides the advantage of zero voltage switching with constant switching frequency and high output voltage control bandwidth. Compared to a conventional single active bridge, which also shares these advantages, the proposed topology additionally provides lower inverter RMS currents and a higher maximum output current. The concept is demonstrated on a 60 kW DC-DC converter prototype. With two interleaved inverters using silicon IGBTs operating at 50 kHz output voltage rise-times and fall-times of ≈ 100 μs, i.e. five switching cycles, are achieved. The conclusion of the thesis summarizes the main findings and contributions and provides an outlook on future research topics emanating mainly from the construction of the high-voltage transformer Show more
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ContributorsExaminer: Kolar, Johann W.
Examiner: Nee, Hans-Peter
SubjectThree-Phase PFC Rectifier; High-Voltage Generator; Eddy Currents in Windings
Organisational unit02140 - Dep. Inf.technologie und Elektrotechnik / Dep. of Inform.Technol. Electrical Eng.
03573 - Kolar, Johann W. / Kolar, Johann W.
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