Summary
Inrush current is the large, brief current spike a circuit draws the instant it is powered on, far above its steady-state current. The most common cause is charging bulk input capacitance: an uncharged capacitor looks like a near short circuit, so current is limited only by the (very low) source and wiring resistance until the cap charges up. Other sources are transformer magnetizing inrush and motor startup.
It matters because the spike can blow fuses, weld relay/switch contacts, sag the supply rail, and stress capacitors and rectifiers.
How to mitigate it
NTC thermistor in series (most common, low-cost) — use an NTC thermistor.
- Cold at power-on → high resistance → limits the inrush.
- Current heats it up → resistance drops → low loss during normal operation.
- Downsides: wastes some power as heat in steady state, and it stays hot, so a quick power cycle does not fully reset its protection.
Note: a PTC is the opposite (resistance rises with temperature). PTCs are used for overcurrent / short-circuit protection (resettable fuse behaviour), not as the primary inrush limiter.
Active inrush limiter — a series MOSFET with an RC soft-start on the gate, so it turns on gradually and ramps the capacitor charging current. More efficient than an NTC, no steady-state heat, resets instantly.
Precharge resistor + bypass relay — charge through a resistor first, then close a relay/contactor to short it out. Common in high-power and EV/battery systems.
Dedicated soft-start / hot-swap controller ICs — integrate the MOSFET control and current limiting.
Role of the Schmitt trigger
A schmitt trigger does not limit inrush current itself — it lives in the control / enable path, not the power path. It is a comparator with hysteresis (upper trip point V₊, lower V₋), so it produces clean, single edges from slow or noisy signals.
Why that matters at power-up:
- Undervoltage lockout (UVLO): only enable the converter / close the bypass relay / release the load after the bus has charged to a healthy level. Hysteresis turns on high and off low, so it commits cleanly.
- No chatter during the inrush dip: the rail sags while caps charge. A plain (no-hysteresis) comparator at a single threshold would toggle on/off right there, re-triggering inrush or oscillating. The Schmitt trigger’s gap prevents this.
- Clean enable from a slow RC ramp and basic debouncing, e.g. to sequence the MOSFET soft-start.