SLUA963B June 2020 – October 2022 UCC21710-Q1 , UCC21732-Q1 , UCC5870-Q1
As mentioned in the previous sections, DESAT and OCP/SCP send back a fault indication and triggers the driver to turn off the IGBT or SiC MOSFET. The driver initiates either 2-level turn-off or soft turn-off to safely shut down the IGBT or MOSFET, preventing large voltage overshoot across the device as a result of the high current transient.
The two-level turn-off (2LTOFF) function limits the transistor current during shutoff during certain fault conditions. When 2LTOFF is triggered, the gate of the power transistor is controlled to operate the transistor in the linear region where the channel current is controlled by the voltage level on the gate terminal. The power transistor current is reduced by controlling the gate voltage to a intermediate voltage, or plateau voltage, (V2LOFF) for t2LOFF, and then ramping the gate down to turn the power transistor off. While 2LTOFF is active, OUTL sinks current to discharge the gate capacitor of the power switch to the plateau voltage. The plateau voltage level and duration are configurable. After holding the plateau voltage for the programmed time, the gate is discharged fully using the soft turn-off current or pulled low as normal with the OUTL driver. The soft turn off current can be enabled and level chosen in the Configuration Registers.
The soft turn-off (STO) function is another method of protecting the power transistors from OV damage because of parasitic loop inductance induced voltage spikes on VCE. The STO slows down the turn-off process to limit the di/dt rate, and limiting the loop inductance-induced voltage spikes. During STO, the OUTL drive strength is reduced to the threshold programmed using SPI. The STO function is enabled for SC/OC and/or DESAT faults.
The inverter benefits not only to prevent the damage or destruction of the power switches, but also prevents high-voltages from being applied to the motor windings, which can also reduce the lifetime of the motor itself.