Abstract
This article presents a medium voltage half-bridge circuit where 3.3 kV power switches are built with two 1.7 kV/325 A SiC MOSFETs series-connected inside power modules. Essence of this work is an active balancing method eliminating the voltage imbalances between the devices by means of gate signals modification. The advantage over commonly-used passive snubbers is low cost and limited amount of additional losses. Performance of the half-bridge is illustrated with a model-based Saber simulation at first, then, an experimental full-scale model is designed with on-the-shelf power modules capable operating at power level above 150 kVA. A series of laboratory tests at 1.5 kV DC confirmed reduction of voltage differences across devices in the stack to an acceptable level - the designed system is working correctly despite EMI caused by fast switching transients. Moreover, the observed current and voltage waveforms prove the positive impact of the method as not only voltages but also switching losses are better balanced among the devices. Experimental comparison to a 3.3 kV counterpart shows two times lower turn-off energy, therefore, the presented design with two 1.7 kV SiC MOSFET power modules can beneficently compete not only in terms of lower cost but also switching performance.
Highlights
Currently, the most common approaches for energy conversion in medium voltage (MV) range include the usage of classic Si devices in the form of high-voltage IGBTs or fully controlled thyristors [1]–[5]
In order to economically utilize the many advantages of SiC in the MV range more sophisticated approaches such as series connection and multilevel topologies may have to be employed
This article focuses on problems related to series connection of transistors, which has been extensively analyzed in the past, especially regarding Si devices [13]
Summary
The most common approaches for energy conversion in medium voltage (MV) range include the usage of classic Si devices in the form of high-voltage IGBTs or fully controlled thyristors [1]–[5]. The active voltage balancing methods are much more complex in terms of structure and usually require advanced measurement systems, but minimize the voltage imbalance between the series- connected devices at higher precision and with minimal additional power losses in comparison to passive approaches [22]. The approach described above makes the regulation system robust – it can diminish the voltage imbalance regardless of its source and which transistor turns-off faster This method is universal for all switched voltages and currents and have limited effect on the total switching power losses of the series-connected transistors. The imbalance, emulated in the simulations through the use of delay time in the span of nanoseconds between series connected transistors control signals, occurs in switches but due to active compensation the voltage mismatch between the transistors is reduced below 1% of the supply voltage after 7 switching periods.
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