7.3.4 Fault Protection

The UCC28740 provides extensive fault protection. The protection functions include:

• Output overvoltage
• Input undervoltage
• Internal overtemperature
• Primary overcurrent fault
• CS-pin fault
• VS-pin fault

A UVLO reset and restart sequence applies to all fault-protection events.

The output-overvoltage function is determined by the voltage feedback on the VS pin. If the voltage sample of VS exceeds 4.6 V, the device stops switching and the internal current consumption becomes I_FAULT which discharges the VDD capacitor to the UVLO-turnoff threshold. After that, the device returns to the start state and a startup sequence ensues.

The UCC28740 always operates with cycle-by-cycle primary peak current control. The normal operating voltage range of the CS pin is 0.773 V to 0.194 V. An additional protection, not filtered by leading-edge blanking, occurs if the CS pin voltage reaches 1.5 V, which results in a UVLO reset and restart sequence.

Current into the VS pin during the MOSFET on-time determines the line-input run and stop thresholds. While the VS pin clamps close to GND during the MOSFET on-time, the current through R_S1 is monitored to determine a sample of V_BULK. A wide separation of the run and stop thresholds allows for clean startup and shutdown of the power supply with the line voltage. The run-current threshold is 225 µA and the stop-current threshold is 80 µA.

The internal overtemperature-protection threshold is 165°C. If the junction temperature reaches this threshold the device initiates a UVLO-reset cycle. If the temperature is still high at the end of the UVLO cycle, the protection cycle repeats.

Protection is included in the event of component failures on the VS pin. If complete loss of feedback information on the VS pin occurs, the controller stops switching and restarts.