An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid tripping the supply's fuse or circuit breaker. Negative temperature coefficient (NTC) thermistors and fixed resistors are often used for this. Less often, other components are also used. For example, NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application. The inrush current limiter is placed in series with the circuit whose inrush current needs to be limited.
An NTC thermistor's resistance is initially high. When the equipment is turned on, the thermistor's resistance limits the initial current. Current flow heats the thermistor, and when hot its resistance is a lower value, allowing most of the input voltage to be appear on the following circuit. It is inherently impossible for 100% of supply voltage to appear on the following circuit, as the thermistor must continue to dissipate power to stay hot and therefore low resistance.
- Inrush current limiting thermistors are usually disk-shaped, with a radial lead on each side.
- Their power handling is proportional to their size.
- They are rated according to their resistance at room temperature.
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 CFL lamps (light bulbs).
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 inherent equivalent series resistance (ESR) of the capacitor provides very little protection for the diode bridge rectifier.
•IMAX - The first critical consideration in the selection of a thermistor is the maximum steady state current (AC or DC) of the power supply. thermistor are rated for maximum continuous current. The input power (Pin) is calculated as Pin = Pout/efficiency. In the case of a 75 Watt SPS with 0.70 efficiency, 100% load is calculated to be 107.14 Watts. The maximum input current is at the minimum input voltage. The effective input current (Ie) is equal to the maximum load divided by the minimum input voltage. In this case, a 75 Watt SPS, Ie = Pin/Vin(low) = 107.14 Watts/90 Volts = 1.2 Amps. Therefore, the thermistor must have an IMAX rating of at least 1.2 Amps. •R@25°C. - The second step is to determine the minimum R value of the thermistor to be selected that will limit the one cycle maximum current rating of the diode bridge rectifier to 50% of its rating to ensure adequate surge protection. Several additional calculations must be made to determine the estimated resistance value required at the point in time of the maximum current surge. RTI provides for a maximum AC voltage rating of 265V RMS on most thermistor. (Reference the Specifications) If the desired maximum inrush current is less than 100 Amps (50% of the diode bridge with a peak current rating of 200 Amps), then solving for R would produce a value of 2.65 ohms. If the MAX Operating Temperature is other than 25°C then the zero power resistance value must be calculated using the NTC.
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