Capacitor Inrush Current

Reducing Inrush Current to Capacitors : Calculating the amount of current flowing to a capacitor, then protecting your load from this initial flow of current is important for any electronic device. The ability to reduce this inrush, caused at powerup, can typically be accomplished by the use of an NTC (negative temperature coefficient) thermistor inrush current limiter.

Cause of the Inrush Current : Filter capacitors are devices designed to reduce the effect of ripples when AC waveforms are converted to DC waveforms. In a typical power supply, the AC current flows through the diode bridge rectifier, converting the voltage to DC, then flows into the filter capacitor. At power on, an inrush of current occurs and while in its charging phase the filter capacitor acts like a dead short. This state continues until the filter capacitor is completely charged, leaving the potential of the inrush current to fully hit the load.

Inrush Current Protection : Safeguarding against the filter capacitor’s charging period’s initial current inrush flow is crucial for the performance of your device. Temporarily introducing high resistance between the input power and rectifier can increase the resistance of the powerup, leading to reduction of the inrush current. Using an inrush current limiter for this purpose helps because it can provide the initial resistance needed.

Placement of Inrush Current Limiter : The diagram below illustrates a typical set up of a circuit, with power entering at “AC IN” and the filter capacitors located at the “FILTER CAP.” To limit the onset inrush current, an NTC thermistor inrush current limiter is placed IN SERIES with input power at “A,” or “B,” or optionally in-series after diode bridge at “C,” or “D.” This allows the filter capacitor time to charge without the inrush current fully hitting the load.

Inrush Current Limiter Proving Resistance : The placement of an inrush current limiter between the input power and load, demonstrated in the diagram, gives the inrush current limiter the ability to provide resistance. When energized, the inrush current limiter self-heats and causes its body temperature to rise. This then leads the inrush current limiter to lower its resistance. As the resistance drops to a low value, the current can pass through at a standard level, without adversely affecting the normal operation or power efficiency. By the time resistance reaches the circuit’s steady-state condition, the filter capacitor will be fully charged and ready to deliver DC power to the load. At this time, the inrush current limiter will remain at this steady-state condition, allowing the current to flow through unaffected.