Circuit Ensures Smooth “Soft Start” for Isolated Converter, Limits In-Rush Current (.PDF Download)

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 (V_OUT) starts to rise, capacitor C_SS is charging up, and the C_SS charging current (I_SS) flows through resistor R_SS. When I_SS is higher than V_BE(on)/R_SS, Q_SS  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, C_SS, R_SS, Q_SS, and optocoupler form a closed loop. When the output rises to regulation, the error amplifier starts to regulate and I_SS goes down, thus turning Q_SS off.