Soft-start: why does a supply power up slowly?

The instant a power supply switches on, its output capacitors are empty and behave like a short. If the IC started at full power at once, the MOSFET would be stressed in the first second. A small capacitor prevents that.

What does it do? Soft-start begins the duty cycle at zero at switch-on and slowly raises it to its working value. The IC charges the capacitor on its SS pin with a small constant current; the SS voltage sets the widest pulse allowed. So there's no surge in the input current and the output rises without overshoot. Once SS reaches the top, the feedback loop takes over.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: SS empty: no pulses

Power has just arrived. Css is empty, so the SS pin sits at 0 V and the IC gives no pulses (or very narrow ones). The output is still 0 V.

1. SS pin (Css voltage)

Starts at 0 V at switch-on, rises linearly, then stays at the top (2.5–5 V depending on the IC). Shown in slow motion here; in reality the ramp lasts a few ms to a few tens of ms.

On the scope

Multimeter0 → 5 V ramp

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

🔍 How to spot it on a board

•A small capacitor (10 nF–1 µF) from the IC's SS (soft-start) pin to ground
•On a TL494, an RC network on the DTC pin; on an SG3525, pin 8
•On ICs without an SS pin like the UC3842, a small circuit of a capacitor, diode and transistor on the COMP pin

🔧 How to measure it

•Unplugged: Ω across Css; it must not be shorted.
•Remove it and check its value with a capacitance meter.
•Running, scope the SS pin: you should see a clean ramp rising from 0 V at switch-on.

⚠ Common faults

•Css shorted: SS is stuck at 0 V, the IC never pulses; the board looks dead.
•Css too small or dried out: start-up is harsh, current limit or over-voltage protection trips, the board hiccups.
•If the output capacitor was swapped for a bigger one, the ramp may not be enough and the board trips at switch-on.

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Go further

UC3842 pins SG3525: the SS pin Concept: start-up and auxiliary winding Concept: hiccup protection