Most dc-dc converters require a soft-start circuit to limit the in-rush current at startup. Although a smooth soft start is required for systems with power-on reset (POR), this is difficult to implement in an isolated converter with a controller on the primary side and a limited duty cycle or current.
Figure 1 shows the soft start of a forward converter with a duty-cycle soft start from the primary side. The steady-state output of the converter is 12 V. A 50% load current is applied at 10 V, which is the POR threshold of the system. As soon as a load is applied, the output drops and triggers system shutdown, causing the system power to cycle several times. At the end of soft start, the output overshoots 10%, which is not desirable.
A simple circuit can implement a smooth soft start for an isolated converter in a system using the LM5025 active-clamp, voltage-mode PWM controller. Figure 2 shows the concept of this secondary-side soft-start approach.
When first applying the input, the converter output (VOUT) starts to rise, capacitor CSS is charging up, and the CSS charging current (ISS) flows through resistor RSS. When ISS is higher than VBE(on)/RSS, QSS then turns on and starts to pull current from the secondary-side comp node (SEC COMP), thus reducing the duty cycle. During soft start, the error amplifier saturates and the soft-start circuit dominates the feedback loop. The converter, CSS, RSS, QSS, and optocoupler form a closed loop. When the output rises to regulation, the error amplifier starts to regulate and ISS goes down, thus turning QSS off.