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 for an isolated converter with a controller on the primary side and a limited duty cycle or current.
Secondary-side Soft-start Circuit for an Isolated Converter When first applying the input, the converter output (VOUT) starts to rise. The capacitor (CSS) is charging up. The CSS charging current (ISS) flows through the resistor (RSS). When ISS is high, then VBE(on)/RSS.
The CSS charging current (ISS) flows through the resistor (RSS). When ISS is high, then VBE(on)/RSS. QSS 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.
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Although a smooth soft start is required for systems with power-on reset (POR), this is difficult for 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.
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 12V. A 50% load current is applied at 10V, the POR threshold of the system. As soon as a load is applied, the output drops and triggers system shutdown, causing the system power cycle several times.