Converters require a soft start on the voltage error amplifier to prevent output overshoot on start-up. Figure 35 shows a common implementation of a secondary-side soft start that works with the typical TL431 error amplifier. The soft-start components consist of DSS, RSS, and CSS. They serve to control the output rate-of-rise by pulling VCTL down as CSS charges through ROB, the optocoupler, and DSS. This has the added advantage that the TL431 output and CIZ are preset to the proper value as the output voltage reaches the regulated value, preventing voltage overshoot due to the error amplifier recovery. The secondary-side error amplifier will not become active until there is sufficient voltage on the secondary. The TPS23754 device provides a primary-side soft start, which persists long enough (about 4 ms) for secondary side voltage-loop soft start to take over. The primary-side current-loop soft start controls the switching MOSFET peak current by applying a slowly rising ramp voltage to a second PWM control input. The PWM is controlled by the lower of the soft-start ramp or the CTL-derived current demand. The actual output voltage rise time is usually much shorter than the internal soft-start period. Initially the internal soft-start ramp limits the maximum current demand as a function of time. Either the current limit, secondary-side soft start, or output regulation assume control of the PWM before the internal soft-start period is over. Figure 24 shows a smooth handoff between the primary and secondary-side soft start with minimal output voltage overshoot.