inrushlimiting

Inrush limiting refers to techniques used to reduce the peak current drawn when electrical equipment is energized. The inrush surge occurs because circuits contain uncharged capacitors, magnetizing components in transformers, or motors that draw a large initial current before settling to steady state. High inrush can stress power components, trip breakers, or cause voltage dips, so limiting it improves reliability and reduces EMI.

Passive methods are the simplest forms of inrush limiting. A common device is the NTC thermistor, placed in series with the load. It has a relatively high resistance when cold, which limits the initial current, and then its resistance drops as it heats, allowing normal operation with lower power loss during steady state. Other passive options include series resistors or pre-charge circuits that momentarily connect through a resistor to charge large capacitive banks before full operation. Polymeric resettable fuses (PPTCs) are primarily protective devices for overcurrent events and are not typically used as primary inrush limiters, though they can influence current under fault conditions.

Active methods use electronics to control the current ramp. Soft-start circuits gradually increase the supply voltage or current using switching devices such as SCRs or MOSFETs, often integrated into power supplies. This approach reduces peak current and can also improve EMI performance. In systems with very large capacitive loads, dedicated pre-charge circuitry or controlled pre-charge through a resistor is used to limit the initial surge before connecting the main power path.

Applications include switch-mode power supplies, motor drives, battery chargers, and lighting drivers. Selection factors include the expected peak surge, continuous current, supply voltage, temperature range, and the acceptable trade-offs between dissipation, size, and complexity.