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 TPS23730 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 2.1 V 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