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NTC Inrush Current Limiter - Thermistors

NTC Inrush Current Limiter

The main purpose of limiting inrush current is to prevent components in series with the input to the DC/DC convertor from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts.



  • Low cost, solid state device for inrush current suppression
  • Excellent mechanical strength
  • Wide operating temperature range: -58°F to 347°F (-50°C to 175°C)
  • Suitable for PCB mounting
  • Available as a standard with kinked or straight leads and on tape and reel to EIS RS-468A for automatic insertion



Control of the inrush current in switching power supplies, flourescent lamp, inverters, motors, etc.

  • Low steady resistance and accompanying power loss
  • Small size
  • Low cost solid state sensor


Type CL Specifications

NTC discs for inrush current limiting

Description Disc thermistor with uninsulated lead-wires.
Options For kinked leads, add suffix "A"
For tape and reel, add suffix "B"
Other tolerances in the range 0.7 Ω to 120 Ω
Other tolerances, tolerances at other temperatures
Alternative lead lengths, lead materials, insulations
Data *maximum rating at 77°F (25ºC) or Iderated = √(1.1425–0.0057 x TA) x Imax @ 77°F (25°C) for ambient temperatures other than 77°F (25ºC).
**maximum ratings
***R0=X1Y where X and Y are found in the table below

 

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Inrush CurrentLimiters In Switching Power Supplies The problem of current surges in switch-mode power supplies is caused by the large filter capacitors used to smooth the ripple in the rectified 60 Hz current prior to being chopped at a high frequency. The diagram above illustrates a circuit commonly used in switching power supplies.
In the circuit above the maximum current at turn-on is the peak line voltage divided by the value of R; for 120 V, it is approximately 120 x √2/RI. Ideally, during turn-on RI should be very large, and after the supply is operating, should be reduced to zero. The NTC thermistor is ideally suited for this application. It limits surge current by functioning as a power resistor which drops from a high cold resistance to a low hot resistance when heated by the current flowing through it. Some of the factors to consider when designing NTC thermistor as an inrush current limiter are:

Maximum permissible surge current at turn-on
Matching the thermistor to the size of the filter capacitors
Maximum value of steady state current
Maximum ambient temperature
Expected life of the power supply
Maximum Surge Current The main purpose of limiting inrush current is to prevent components in series with the input to the DC/DC convertor from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts. Since most thermistor materials are very nearly ohmic at any given temperature, the minimum no-load resistance of the thermistor is calculated by dividing the peak input voltage by the maximum permissible surge current in the power supply (Vpeak/Imax surge).
Energy Surge at Turn-On At the moment the circuit is energized, the filter caps in a switcher appear like a short circuit which, in a relatively short period of time, will store an amount of energy equal to 1/2CV2. All of the charge that the filter capacitors store must flow through the thermistor. The net effect of this large current surge is to increase the temperature of the thermistor very rapidly during the period the capacitors are charging. The amount of energy generated in the thermistor during this capacitor-charging period is dependent on the voltage waveform of the source charging the capacitors. However, a good approximation for the energy generated by the thermistor during this period is 1/2CV2 (energy stored in the filter capacitor). The ability of the NTC thermistor to handle this energy surge is largely a function of the mass of the device. This logic can be seen in the energy balance equation for a thermistor being self-heated:
Input Energy = Energy Stored + Energy Dissipated or in differential form: Pdt = HdT + δ(T – TA)dt
ntc-inrush-2
where:
P = Power generated in the NTC
t = Time H = Heat capacity of the thermistor
T = Temperature of the thermistor body
δ = Dissipation constant
TA = Ambient temperature
During the short time that the capacitors are charging (usually less than 0.1 second), very little energy is dissipated. Most of the input energy is stored as heat in the thermistor body. In the table of standard inrush limiters there is listed a recommended value of maximum capacitance at 120 V and 240 V. This rating is not intended to define the absolute capabilities of the thermistors; instead, it is an experimentally determined value beyond which there may be some reduction in the life of the inrush current limiter.
Maximum Steady-State Current The maximum steady-state current rating of a thermistor is mainly determined by the acceptable life of the final products for which the thermistor becomes a component. In the steady-state condition, the energy balance in the differential equation already given reduces to the following heat balance formula:
Power = I2R = δ(T – TA)
As more current flows through the device, its steady-state operating temperature will increase and its resistance will decrease. The maximum current rating correlates to a maximum allowable temperature.
In the table of standard inrush current limiters is a list of values for resistance under load for each unit, as well as a recommended maximum steady-state current. These ratings are based upon standard PC board heat sinking, with no air flow, at an ambient temperature of 77° (25°C). However, most power supplies have some air flow, which further enhances the safety margin that is already built into the maximum current rating. To derate the maximum steady state current for operation at elevated ambient temperatures, use the following equation:
Iderated = Iderated = √(1.1425–0.0057 x TA) x Imax @ 77°F (25°C)