Schlüsselwörter
(Englisch)
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SST, SiC, Efficiency, Reliability, Battery-Storage
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Kurzbeschreibung
(Deutsch)
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Es wird ein Funktionsmuster eines 50 kW AC/DC Umrichters komplett auf SiC-Basis mittels Simulationen ausgelegt und optimiert, und anschliessend aufgebaut und getestet. Das Entwicklungsziel ist eine hohe Effizienz bei gleichzeitig hoher Zuverlässigkeit. Die erreichte Effizienz soll mit einem herkömmlichen Silizium-basierten Umrichter an verschiedenen Arbeitspunkten verglichen werden.
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Kurzbeschreibung
(Englisch)
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A functional model of a 50 kW AC/DC converter, completely based on SiC devices is designed and optimized by means of simulations, and subsequently the converter is built and characterized. Development goals are a high efficiency and a good reliability. The reached efficiency will be compared to that of a conventional silicon-based converter at different working points.
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Schlussbericht
(Englisch)
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A Dual Active Bridge (DAB) DC/DC converter, based on SiC MOSFETs as active semiconductor switches has been developed, built and fully characterized. The DAB converter is compatible to an active rectifier, which was developed by the Ostschweizer Fachhochschule OST in a parallel project. The entire topology consists of an input EMC mains filter, a three-phase active rectifier, a DC link and the DAB converter which provides galvanic isolation. The setup features the possibility of bidirectional power flow. Hence, the flexibility of this topology enables not only a grid connection suitable for EV batteries, but for any DC bus to be connected to the mains with galvanic isolation and bidirectional power flow. The DAB converter specifications are: output power 22 kW; DC link (= primary side) voltage: 750 V; DC output (= secondary side) voltage: 320…450 V; MOSFET switching frequency: 70 kHz. The converter consists of two main printed circuit boards (PCB): The first board includes the AC input and the 3-phase two-level active rectifier (OST part), DC-link capacitors, the primary side H-bridge of the DAB converter and voltage/current measurements needed for control. The second board includes the secondary side of the DAB converter, output capacitors, the DC output to the load and further voltage/current measurements. Both PCBs are identical for a modular converter design and are connected via a series inductance and a transformer, which both had been purchased from an external supplier. The control of the MOSFETs has been realized by means of proven control topologies. The DAB converter has been comprehensively characterized with respect to its efficiency at different operating points. The highest measured total efficiency is 95.58 %. It has turned out that the biggest part of the converter losses arises from the magnetic components (i.e., the choke and the transformer). The semiconductor losses of the DAB converter make up only 1 % of the transferred power. Since the magnetic components have a significant impact on the total converter losses, it is recommended to further investigate the optimization of magnetic components to be operated above 50 kHz. The SiC DAB converter cannot directly be compared to a silicon-based system, since the switching frequency of 70 kHz is not reached by comparable Si IGBTs. When operated at 10 kHz - which is rather low for SiC MOSFETs but more suitable for Si IGBTs, a SiC DAB converter would reach a maximum efficiency above 99%; whereas a Si DAB converter would reach only 97 % (in both cases: only semiconductor losses considered). In particular in partial load regime, the efficiency benefit of SiC becomes even more pronounced. Furthermore, a reliability model for DAB converters has been established and applied to the developed converter. From an input mission profile of the converter, the model calculates the junction temperature profiles of all semiconductors, which are then used to estimate the semiconductor lifetimes. For the present DAB converter, the model showed that the MOSFETs on the secondary side are more stressed due to a higher current load, which predicts a reduced lifetime. In the specific application example of the converter as a bidirectional charger, it is shown that the MOSFETs on the secondary side are also sufficiently dimensioned to ensure reliable long-term operation. The reliability model can be used to optimize converter design with respect to reliability and designing before the converter is physically built, and it can be adapted to other converter types. With the knowledge gained, follow-up projects in the field of SiC converter development and SiC reliability could already be started in cooperation with industry.
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