Press pack insulated gate bipolar transistor (IGBT) modules connected in series for high-voltage direct current (HVDC) converter applications are designed such that when a failure occurs, it occurs in a safe manner by the formation of a stable short circuit, while redundant modules take up the voltage blocking function of the failed module. One such design using individual pressure contacts is described and the events occurring from the initiation of the short circuit to its final failure due to open circuit are reported from an electronics packaging materials design point of view. Experiments to hasten the failure under accelerated test conditions on modules were performed and interrupted at various stages of operation under a short circuit condition. The formation and subsequent aging of the metallurgical alloy under short circuit conditions was investigated by analyzing cross sections of the alloy forming the short circuit. Liquid metal corrosion along with the formation of intermetallics with poor conductivities lead to the final failure by open circuit.Index Terms-high-voltage direct current (HVDC), insulated gate bipolar transistor (IGBT), intermetallics, liquid metal corrosion, power electronics packaging, press pack modules, short circuit failure mode.
Purpose: This paper summarizes the development of a high-power distributed x-ray source, or "multisource," designed for inverse-geometry computed tomography (CT) applications [see B. De Man et al., "Multisource inverse-geometry CT. Part I. System concept and development," Med. Phys. 43, 4607-4616 (2016)]. The paper presents the evolution of the source architecture, component design (anode, emitter, beam optics, control electronics, high voltage insulator), and experimental validation. Methods: Dispenser cathode emitters were chosen as electron sources. A modular design was adopted, with eight electron emitters (two rows of four emitters) per module, wherein tungsten targets were brazed onto copper anode blocks-one anode block per module. A specialized ceramic connector provided high voltage standoff capability and cooling oil flow to the anode. A matrix topology and low-noise electronic controls provided switching of the emitters. Results: Four modules (32 x-ray sources in two rows of 16) have been successfully integrated into a single vacuum vessel and operated on an inverse-geometry computed tomography system. Dispenser cathodes provided high beam current (>1000 mA) in pulse mode, and the electrostatic lenses focused the current beam to a small optical focal spot size (0.5 × 1.4 mm). Controlled emitter grid voltage allowed the beam current to be varied for each source, providing the ability to modulate beam current across the fan of the x-ray beam, denoted as a virtual bowtie filter. The custom designed controls achieved x-ray source switching in <1 µs. The cathode-grounded source was operated successfully up to 120 kV. Conclusions: A high-power, distributed x-ray source for inverse-geometry CT applications was successfully designed, fabricated, and operated. Future embodiments may increase the number of spots and utilize fast read out detectors to increase the x-ray flux magnitude further, while still staying within the stationary target inherent thermal limitations. C 2016 American Association of Physicists in Medicine. [http://dx
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