Inrush current limiter
Encyclopedia
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) thermistor
s and fixed resistor
s 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.
Thermistor
An NTC thermistor
's resistance is initially high. When the equipment is turned on, the thermistor's resistance limits the initial current. Current flow heat
s 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.
s 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 more or less all CFL lamps
(light bulbs).
, 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.
•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
Inrush current
Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...
to avoid gradual damage to components and avoid tripping the supply's fuse or circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
. Negative temperature coefficient (NTC) thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
s and fixed resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s 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.
ThermistorThermistorA thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
An NTC thermistorThermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
's resistance is initially high. When the equipment is turned on, the thermistor's resistance limits the initial current. Current flow heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
s 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 powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
handling is proportional to their size. - They are rated according to their resistance at room temperatureRoom temperature-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
.
Fixed resistor
Fixed resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s 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 more or less all CFL lamps
Compact fluorescent lamp
A compact fluorescent lamp , also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps...
(light bulbs).
Applications
A typical application of inrush current limiters is in the input stage of non-Power Factor Corrected switching suppliesSwitched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
, to reduce the initial surge of current from the line input to the reservoir capacitor
Filter capacitor
Filter capacitors are any capacitors used for filtering. Filter capacitors are common in electrical and electronic work, and cover a number of applications, such as:* Glitch removal on Direct current power rails...
. 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.
Selection Consideration
•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