Converters require a soft start on the voltage error amplifier to prevent output overshoot on start-up. Figure 9-2 shows a common implementation of a secondary-side soft start that works with the typical TLV431 error amplifier. The soft-start components consist of D_SS, R_SS, and C_SS. They serve to control the output rate-of-rise by pulling V_COMP down as C_SS charges through R_OB, the optocoupler, and D_SS. This has the added advantage that the TLV431 output and C_IZ 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 TPS23734 provides an adjustable primary-side soft start, which persists long enough 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 COMP-derived current demand. The actual output voltage rise time is usually much shorter than the internal soft-start period. Initially the primary 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 primary soft-start period is over. Since the VCC startup source stays on longer after converter's output voltage is ramped up (VCC startup turns off only when 2.1V is reached on the SS pin), a large bias winding hold up capacitor is not necessary like in some traditional PWM controllers. Instead, this allows for a small 1 µF ceramic capacitor to be used on VCC.

Figure 9-2 Error Amplifier Soft Start