Study of Inrush Current

The selection of over-current protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows…

Inrush current, input surge current or switch-on surge is the maximum, instantaneous input current drawn by an electrical device when first turned on. Alternating current electric motors and transformers may draw several times their normal full-load current when first energized for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady state currents due to the charging current of the input capacitance. The selection of over-current protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.

Figure 1: An example of inrush current transients during capacitor bank energization


  When a transformer is first energize, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles. Toroidal transformers using less copper for the same power handling can have up to 60 times inrush to running current. Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage, (which for a pure inductance would be the current maximum in the AC cycle) and if the polarity of the voltage half cycle has the same polarity as the remanence in the iron core has. (The magnetic remanence was left high from a preceding half cycle). Unless the windings and core are sized to normally never exceed 50% of saturation, (and in an efficient transformer they never are, such a construction would be overly heavy and inefficient) then during such a start up the core will be saturated. This can also be expressed as the remnant magnetism in normal operation is nearly as high as the saturation magnetism at the “knee” of the hysteresis loop. Once the core saturates however, the winding inductance appears greatly reduced, and only the resistance of the primary side windings and the impedance of the power line are limiting the current. As saturation occurs for part half cycles only, harmonic rich waveforms can be generated, and can cause problems to other equipment.

  For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away (decay time proportional to ~XL/R)and the regular AC equilibrium is established. To avoid magnetic inrush, only for transformers with an air gap in the core, the inductive load needs to be synchronously connected near a supply voltage peak, in contrast with the zero voltage switching which is desirable to minimize sharp edged current transients with resistive loads such as high power heaters. But for toroidal transformers, only a pre-magnetizing procedure before switching on allows to start those transformers without any inrush current peak.

Inrush current can be divided in three categories:

  Energization inrush current: – Energization inrush current result of re-energization of transformer. The residual flux in this case can be zero or depending on energization timing. 
Recovery inrush current: – Recovery inrush current flow when transformer voltage is restored after having been reduced by system disturbance.

  Sympathetic inrush current: – Sympathetic inrush current flow when multiple transformers connected in same line and one of them energized.

Figure 2: An example of an inrush current transient during a 100VA toroid transformer energization. Inrush peak circa 50 times of nominal current


  When an electric motor, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary, until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed. For high power motors, the winding configuration may be changed (star at start and then delta) during start-up to reduce the current drawn.


  A resistor in series with the line can be used to limit the current charging input capacitors. However, this approach is not very efficient, especially, in high power devices, since the resistor will have a voltage drop and dissipate some power. Inrush current can also be reduced by inrush current limiters. Negative temperature coefficient (NTC) thermistors are commonly used in switching power supplies, motor drives and audio equipment to prevent damage caused by inrush current. A thermistor is a thermally-sensitive resistor with a resistance that changes significantly and predictably as a result of temperature changes. The resistance of an NTC thermistor decreases as its temperature increases. As the inrush current limiter self-heats, the current begins to flow through it and warm it. Its resistance begins to drop and a relatively small current flow charges the input capacitors. After the capacitors in the power supply become charged, the self heated inrush current limiter offers little resistance in the circuit, with a low voltage drop with respect to the total voltage drop of the circuit. A disadvantage is that immediately after the device is switched off, the NTC resistor is still hot and has a low resistance. It cannot limit the inrush current unless it cools for more than 1 minute to get a higher resistance. Another disadvantage is that the NTC thermistor is not short circuit proof. Another way to avoid the transformer inrush current is a transformer switching relay. This does not need time for cool down. It can deal also with power line half-wave voltage-dips and is short circuit proof. This technique is important for IEC 61000-4-11 tests. Another option, particularly, for high voltage circuits, is to use a pre-charge circuit. The circuit would support a current limited pre-charge mode during the charging of capacitors, and then switch to an unlimited mode for normal operation when the voltage on the load is 90% of full charge.

Inrush Current Limiter

  An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid blowing fuses or tripping circuit breakers. Negative temperature coefficient (NTC) thermistors and fixed resistors are often used to limit inrush current. NTC thermistors can be used as inrush-current limiting devices in power supply circuits when added in series with the circuit being protected. They present a higher resistance initially, which prevents large currents from flowing at turn-on. As the current continues to flow, NTC thermistors heat up allowing higher current flow during normal operation. NTC thermistors are usually much larger than measurement type thermistors, and are purposely designed for power applications.

a) Thermistor

  An NTC thermistor’s resistance is high at low temperatures. When the circuit is closed, the thermistor’s resistance limits the initial current. After some time current flow heats the thermistor, and its resistance changes to a lower value, allowing current to flow uninterrupted. It is inherently impossible for 100% of supply voltage to appear on the protected circuit as the thermistor must continue to dissipate power (producing heat) in order to maintain a low resistance. The resulting voltage drop from the operating resistance and the power consumption of the thermistor must be taken into account.

b) Fixed resistor

  Fixed resistors are also widely used to limit inrush current. These are inherently less efficient, since the resistance never falls from the value required to limit the inrush current. Consequently, they are generally chosen for lower power circuitry, where the additional ongoing power waste is minor. Inrush limiting resistors are much cheaper than thermistors. They are found in most compact fluorescent lamps (light bulbs).They can be switched out of the circuit using a relay or MOSFET though, after inrush current is complete.

Applications of Inrush Current Limiter

  A typical application of inrush current limiters is in the input stage of non-power factor corrected switching supplies, to reduce the initial surge of current from the line input to the reservoir capacitor. The most popular application is the inrush protection of the AC current in switching power supplies (SPS). The primary reason for having surge current suppression in a SPS is to protect the diode bridge rectifier as the input or charging capacitor is initially charged. This capacitor draws significant current during the first half AC cycle and can subject the components in line with the capacitor to excessive current. The initial equivalent series resistance (ESR) of the capacitor provides very little protection for the diode bridge rectifier.

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