8.3.10 Soft Start

Soft-start is a process for boosting the output voltage of the PFC converter from the peak of the ac-line input voltage to the desired regulation voltage under controlled conditions. Instead of a dedicated soft-start pin, the UCC28064A uses the voltage error amplifier as a controlled current source to increase the PWM duty-cycle by way of increasing the COMP voltage. To avoid excessive start-up time-delay when the ac-line voltage is low, a higher current is applied until VSENSE exceeds 3 V at which point the current is reduced to minimize the tendency for excess COMP voltage at no-load start-up.

The PWM gradually ramps from zero on-time to normal on-time as the compensation capacitor from COMP to AGND charges from zero to near its final value. This process implements a soft-start, with timing set by the output current of the error amplifier and the value of the compensation capacitors. Soft-start ends when VSENSE pin voltage exceeds 95% of VSENSEreg. During soft-start the device will operate with both phases on and even if the COMP voltage is below the BRST pin voltage the device will not stop switching. In the event of a HVSEN failsafe OVP, brownout, external-disable, UVLO fault, or other protection faults, COMP is actively discharged and the UCC28064A will soft-start after the triggering event is cleared. Even if a fault event happens very briefly, the fault is latched into the soft-start state and soft-start is delayed until COMP is fully discharged to 20 mV and the fault is cleared. See Figure 26 for details on the COMP current. See Figure 27 which illustrates an example of typical system behavior during soft-start.

Expanded COMP output current curve including voltage error amplifier transconductance and modifications applicable to soft-start and overvoltage conditions.

Figure 26. Expanded COMP pin Output Current Curve

Figure 27. Soft-Start Timing with System Behavior