Abstract
Silicon carbide (SiC) devices can exhibit simultaneously high electro-thermal conductivity and extremely fast switching. To perform optimal designs showing the benefits of SiC in achieving efficiency, size, weight, and cost objectives for electric converters design, it is necessary to establish better models for calculating device losses and an efficient thermal model that can be calibrated to consider the nuances of measurement based thermal equivalent circuits. This work's outputs can be used in a power converter multi-objective optimization process or real-time temperature prediction of drives to improve reliability and maximize performance. The proposed modified loss calculation and thermal model demonstrate the SiC power module's advantages to reduce peak junction temperature and power cycling effects. A detailed power loss calculation and thermal model is developed and tested on a 480 V, 186 A, 150 HP variable frequency drive (VFD) with SiC modules. A comparison between the SiC module and an equivalent rated Si module demonstrates the reduction in power cycling effects, particularly at low-speed operation. Infrared (IR) imaging results and analytical explanations of the phenomenon is provided. Power cycle tests show that higher thermal conductivity is not the only reason contributing to the lower temperature ripple in low-speed operation.
Highlights
With Silicon carbide (SiC) based power modules that inherently exhibit low switching losses, high PWM switching frequencies can be obtained
These results show that the external diode in parallel with SiC MOSFET does not significantly contribute to the module losses as it mainly conducts only during the dead-time, and this external diode can be neglected in calculations of losses
TI,max is the maximum IGBT junction temperature and TI,av is the average IGBT junction temperature. These results show that the temperature swing is smaller in SiC MOSFET modules for lower operating frequencies compared to Si IGBT modules
Summary
With SiC based power modules that inherently exhibit low switching losses, high PWM switching frequencies can be obtained. Si IGBT converter is run at 8 kHz due to the IGBT junction temperature limit being exceeded at 186 A output current and 12 kHz PWM frequency These results show that the external diode in parallel with SiC MOSFET does not significantly contribute to the module losses as it mainly conducts only during the dead-time, and this external diode can be neglected in calculations of losses. A method is proposed to modify the thermal equivalent circuits of power modules to match the predictions with measurements in a converter test setup These modifications are based on an RC thermal network proposed in [14] that account for the thermal interaction between MOSFET and diode chips. For modeling the thermal behavior of the module, the inverter was run in different operating frequencies, and the average junction temperature was measured.
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