Behavioral Modelling of the Transient and CdV/dt Induced Turn-on of a Hybrid High Power Switch Unit
The use of hybrid power switches is a practical and effective method of improving the performance of high power switching devices. An emerging strategy is to combine high power Si IGBTs with SiC MOSFETs to take advantage of the SiC MOSFETs superior switching speed and lower switching losses. While this approach has a number of advantages, when it is used in a half bridge arrangement the possibility of false turn becomes a concern. The SiC MOSFETs improved transient characteristics leads to higher dv/dt during switching, which can induce a spike in the gate voltage of the low side Si IGBT through the parasitic capacitances. This behavior poses a serious risk of lowering efficiencies in operation as well as safety concerns. In this study, a set of behavior models were developed for both the SiC MOSFET and Si IGBT, with the specific target of modelling the transient behavior. These models were then integrated together into a half bridge capable of performing hybrid switching. Lastly, a model was developed to assess the risk of false turn on in the low side IGBT, based on the developed hybrid switching models. The models were limited to only parameters which are readily available on device datasheets, allowing the system to be useful and accessible to designers without the need to perform parameter extraction. The individual switch models accuracy were assessed against a set Spice models, covering a range of power conditions. The induced gate voltage in a synchronous buck converter with two hybrid SiC MOSFET Si IGBT switches was measured and used to verify the validity of the induced voltage model. The design approach for the development of a behavioral transient model and the limitations of the model were explored and discussed.