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Cause: When an electrical device like a power supply is turned on, an inrush current will momentarily flow in that device. In the case of an AC/DC switching power supply, the inrush current is primarily due to the charging current of the large electrolytic storage capacitor(s) employed in the rectification stage. See the simplified schematic Figure 1 . These currents can easily exceed 50 amps. The reason for this is that there is very little impedance present at turn-on, typically just a few ohms. Consider the Ohm's law equation, V/R = I . Ex. If turn on is at the peak of the AC cycle, 120VAC and we assume an input impedance of 3 ohms, then (120 x √2)/3µ = 56.7A. If the nominal input voltage is 240Vac in rush currents in this example can be greater than 100A (Fig 1).

Figure 1
  1. High inrush currents can unduly stress the bridge rectifier, causing catastrophic failures
  2. Fuses become overheated and degraded over time
  3. Line circuit breakers can trip, especially in applications with multiple supplies sharing the AC mains
  4. Switch contacts are subjected to arcs resulting in premature failures
  5. High currents cause voltage dip disturbances which may affect other circuits

These effects cause system lifetimes to decrease, which in turn adds significant repair/replace costs. Unplanned downtime from tripped circuit breakers or power supply failures is expensive and more importantly can be dangerous as in medical or mission critical situations. Solutions are necessary to mitigate the inrush current and a few are shown here. Solutions: Solutions generally fall into two categories, Passive and Active. 1.) Passive... dissipate energy at turn on A.) Inrush limiting resistor: A fixed resistor in series with the line will help limit the inrush current. This approach should only be used in applications less than a few watts because there will be losses in efficiency due to dissipated power in the resistor. B.) NTC resistor: An NTC is a Negative Temperature Coefficient resistor. This is a simple method of inrush current limiting. The way it works is that at initial turn on the resistor is cold and has a high resistance to the inrush current. Shortly thereafter, the NTC resistor itself heats up due to the power it's dissipating. When that happens the resistance decreases significantly and dissipates much less power for the rest of the on time. When the power supply is turned off, the NTC cools off and goes back to it's initially high resistive state. It's now ready for the next turn on. The disadvantage is as the name implies, in the temperature coefficient of this component. Adequate time is needed between turn-ons for the NTC resistor to cool down. If not the NTC offers little resistance to the inrush current. NTC resistors are also affected by their ambient temperatures and will act accordingly. Never warming up in cold environments and never cooling down in hot climates. Despite the disadvantages, this simple inexpensive method is widely used. C.) Bypassed resistor: When the charging current of the electrolytic capacitor is reached the resistor will be bypassed by triac, relay or IGBTs. Greatly reduces efficiency losses. This method adds some expense (Fig 1a). D.) Bypassed NTC: Just as in the case of the fixed resistors , the NTC resistor can be bypassed after charging current is reached by triacs, IGBTs or relays. This makes the system more efficient and ensures that the NTC will have a high impedance at turn on. As expected, it also adds expense. These last two methods are sometimes known as Active Passive.

Figure 1a

Figure 1b

1.) Active ....controlled start up
A.) Zero crossing technique: The Vac input is monitored by a control circuit and the supply turned on when the line is approximately 30Vac or below. Synchronous solid state relays can be used or triacs. This eliminates random turn on at peak voltage and keeps inrush current low. No NTC is needed and there is no temperature dependency. Not recommended for designs over 500 watts as the inrush current can still be large at these power levels regardless of low line voltage (Fig 2a).
B.) Time delayed switching: This behavior is accomplished by a pre-charging module that controls the pulses used to slowly charge the storage capacitor. Since the current used to charge a capacitor depends upon the rate of change of the voltage I = C dv/dt, pre charging the capacitor with "soft" pulsing reduces the inrush current. Delaying the turn on this way also allows other consumer circuits to meet their inrush requirements uninhibited by the power supply turn-on. Not temperature dependent (Fig 2b).
C.) Mosfet limiting: Mosfet limiting is another way to effect the voltage rate of change to the bulk capacitor and mitigate inrush current. A Mosfet power switch will soft start the voltage to the capacitor. The change in the capacitor current is equal to the slew rate of the mosfet output voltage as configured below (Fig 2c).

Figure 2a

Figure 2b

Figure 2c

Inrush current can be lowered by additional impedance or by a controlled start up circuit, using discrete components or advanced semiconductor designs.
These are some of the accepted methods to reduce inrush current. They are others as well but the basic concepts are the same. Use the method that best suits your application and budget.
Note: These techniques are specific to AC/DC power supply inrush and may not be advisable for use with other electrical devices.

By John Benatti