Intelligent Comprehensive Design and Operation Paradigm for WBG-Based Converters
Letís be honest: power semiconductors today are used by guess and tweak. Seeing shoot-through? Letís increase the dead-time. Too much overvoltage during switching? Maybe try slow everything down. Surprisingly, this trial and error method works quite well for silicon-based converters. However, the uncertainty behind these methods haunts us engineers, especially when dealing with the much faster WBG devices. The design and operation method developed here at UTK can boost the efficiency and fully utilize these superior semiconductors, but most importantly, it provides us unprecedented confidence -- what is better than being able to lean back and trust your design? Thanks to this pioneering work, two technical papers win Second Prize Paper Award of IEEE Transactions on Power Electronics in 2016 and First Prize William Portnoy Paper Award of IEEE Industry Applications Society in 2015, respectively.
SiC based three-phase voltage source inverter for motor drive application
The understanding and modeling of switching transient is crucial here. At the core of this problem is the nonlinear and high-order nature of power semiconductor switching behavior. Significant amount of work is spent into identification of components defining switching characteristics and verification of a simulation model. Furthermore, elaborate the switching behavior testing under different working condition and circuit parameters is done to further determine their contribution and prove the knowledge we gained from modeling. Based on these fundamentals, the design and operation of WBG based power converter can be scientifically optimized.
Overvoltage is the ringing spikes exerted on semiconductor during switching transients and the device voltage rating is usually limited by the maximum magnitude. While an empirical percentage value is usually used for the selection of silicon counterparts, the design guideline proposed here help using the most out of the device reliably.
Dead-time is usually inserted between the switching actions of the switching pair to avoid shorting the dc bus voltage to ground and often it is a fixed operation parameter. With the knowledge of current commutation inside the switching pair, we are able to optimize and adaptively tune this parameter by monitoring the switching time. Therefore, operation safety is guaranteed while reducing the diode conduction loss at the same time.
How WBG Can Help
As WBG pushes the switching speed beyond limit, the switching transient becomes super sensitive to load condition. While a fixed relatively large dead time is enough for safe operation, the dead time makes the diode conduction time much longer and therefore hurts the efficiency. The adaptive operation solves this problem all together by choosing the optimal dead time while on the fly. Another problem comes along with fast switching is the adverse ringing. As the slew rate increases, it is critical to keep the overall voltage ringing spike under the device maximum rating. During the design stage, the modeling knowledge can predict the maximum overvoltage and then the designer can confidently build their circuit afterwards.