8.2.2.14 Softstart

Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 35 shows a common implementation of a secondary-side softstart that works with the typical TL431 error amplifier. The softstart components consist of D_SS, R_SS, and C_SS. They serve to control the output rate-of-rise by pulling V_CTL down as C_SS charges through R_OB, the optocoupler, and D_SS. This has the added advantage that the TL431 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 does not become active until there is sufficient voltage on the secondary. The TPS23751 and TPS23752 provide a primary-side softstart which persists long enough (approximately 3ms) for secondary side voltage-loop softstart to take over. The primary-side current-loop softstart 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 softstart ramp or the CTL-derived current demand. The actual output voltage rise time is usually much shorter than the internal softstart period. Initially the internal softstart ramp limits the maximum current demand as a function of time. The current limit, secondary-side softstart, or output regulation assume control of the PWM before the internal softstart period is over.

Figure 35. Error Amplifier Soft Start