The UCC21738-Q1 implements a fast overcurrent and short circuit protection feature to protect the SiC MOSFET or IGBT from catastrophic breakdown during fault. The OC pin of the device has a typical 0.7-V threshold with respect to COM, source or emitter of the power semiconductor. When the input is in floating condition, or the output is held in low state, the OC pin is pulled down by an internal MOSFET and held in LOW state, which prevents the overcurrent and short circuit fault from false triggering. The OC pin is in high-impedance state when the output is in high state, which means the overcurrent and short circuit protection feature only works when the power semiconductor is in ON state. The internal pulldown MOSFET helps to discharge the voltage of OC pin when the power semiconductor is turned off.

The overcurrent and short circuit protection feature can be used to SiC MOSFET module or IGBT module with SenseFET, traditional desaturation circuit, and shunt resistor in series with the power loop for lower power applications. For the SiC MOSFET module or IGBT module with SenseFET, the SenseFET integrated in the module can scale down the drain current or collector current. With an external high precision sense resistor, the drain current or collector current can be accurately measured. If voltage of the sensed resistor higher than the overcurrent threshold V_OCTH is detected, a soft turn-off is initiated. A fault is reported to the input side FLT pin to the DSP/MCU. The output is held to LOW after the fault is detected, and can only be reset by the RST/EN pin. The state-of-art overcurrent and short circuit detection time helps to ensure a short shutdown time for the SiC MOSFET and IGBT.

The overcurrent and short circuit protection feature can also be paired with desaturation circuit and shunt resistors. The DESAT threshold can be programmable in this case, which increases the versatility of the device. Detailed application diagrams of the desaturation circuit and shunt resistor are given in Figure 8-5.

• High current and high dI/dt during the overcurrent and short circuit fault can cause a voltage bounce on the shunt resistor’s parasitic inductance and board layout parasitic, which results in a false trigger of the OC pin. A high precision, low ESL, and small value resistor must be used in this approach.
• A shunt resistor approach is not recommended for high power applications and short circuit protection of low power applications.

Detailed applications of the overcurrent and short circuit feature are discussed in Section 9.

Figure 8-5 Overcurrent and Short Circuit Protection