Inrush Current Limit NTC Thermistor

Power NTC thermistor can be a cost effective device to limit the amount of inrush current in a switching power supply or other devices when the power is first turned on. Power NTC thermistor 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.
Inrush-current limiters power NTC thermistor protect circuits from undesirably high currents, suppressing high inrush current surges, while its resistance remains negligible low during continuous operation. Thanks to their low resistance in the operating state, Power NTC thermistor have a considerably lower power dissipation than the fixed resistors frequently used for this application.

In the cold state, i.e. at room temperature, the high initial resistance of the inrush current limiter effectively absorbs the power of peak inrush currents. As a result of the current load and subsequent heating, the resistance of the inrush current limiter then drops by a factor >30 – 50 to a few percent of its value at room temperature. The power consumption of the inrush current limiters is thus negligible in continuous operation – an outstanding advantage of NTC thermistor over fixed resistors.

Resuming operation after cooling down

After a load has been switched off, the NTC thermistor must be allowed to cool down to room temperature if its capacity for inrush current limiting is to be fully used. This can take from 30 seconds to two minutes depending on the disk size. In the case of switched mode power supplies, these cooling times are often a minor consideration because electrolytic capacitors in the circuit usually take longer to discharge fully. The NTC thermistor will therefore be cool enough to resume operation in the event of another short-term turn-on.

limiting surge current, suitable for the protection of switch mode power supply, UPS power, transformers, motors, various electric heating utensil, energy saving lights, ballast, various power circuit, amplifiers, colored displayer, monitors, color TV, filament protection, etc.
Power NTC thermistor components can also be used for the soft starting of motors, for example in vacuum cleaners with continuous currents of up to 20 A.

Comparison curve With Without Inrush Current Limiting Power NTC thermistor

·Low cost solid state device for inrush current suppression.
·Minimize line current distortion and radio noise.
·Protect switches, rectifier diodes and smoothing capacitors against premature failures.
·Prevent fuse from blowing in error.

Power NTC Thermistor Load Temperature Characteristics

·Resin coated disk NTC thermistor with uninsulated lead-wires.
·Suitable for both AC and DC circuits up to a voltage of 265 V(rms).
·Wide range of resistance, current and dimension.
·Excellent mechanical strength.
·Suitable for PCB mounting.

Typical Power Supply Circuit

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 NTC 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 converter from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts. Since most NTC thermistor materials are very nearly ohmic at any given temperature, the minimum no-load resistance of the NTC thermistoris 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 NTC 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

where:

• P = Power generated in the NTC thermistor
• t = Time
• H = Heat capacity of the NTC 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 thermistor; 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)

A few items of data are needed to scale an inrush current limiter NTC thermistor:

• Load capacitance of device to be protected (determination of minimum size of the component)
• Steady-state current and maximum ambient temperature
• Required reduction of inrush current

Load capacitance of device to be protected

The high inrush current of devices results from the higher energy required to turn on. In power supplies the energy requirement is primarily caused by load capacitors, in transformers by magnetizing energy. The associated turn-on operations load the inrush current limiter as a current pulse. So this energy must be known to select the right component. It can be converted into capacitance for a given voltage.

Steady-state current and maximum ambient temperature

Select the component so that the steady-state current does not exceed the maximum admissible current (Imax) of the inrush current limiter. The maximum admissible current is produced from the figure for Imax and the derating in 2.4 with the maximum ambient temperature.

When scaling a design, remember the possibility of line voltage fluctuations and different operating states (steady-state currents) of the device itself, and incorporate appropriate precautionary measures.

Required reduction of inrush current

Within this component model the maximum steady-state current then determines the highest possible cold resistance (R25) that can be used for an application.
The higher the cold resistance (R25) of the inrush current limiter, the more the inrush current is dampened. If the current limiting effect of a component is inadequate, choose a larger model.

• Maximum operating current > Actual operating current in the power loop
• Rated zero power resistance at 25C
  
  of which, E: loop voltage, Im: Surge current
  For conversion power, reversion power, switch power, UPS power, Im=100 times operating current
  For filament, heater, Im =30 times operating current
• The larger Beta value, the smaller residual resistance, the smaller operating temperature rising.
• Generally, the larger product of time constant and dissipation coefficient, the larger NTC thermal capacity, the more powerful NTC Thermistor surge current restraining capacity.

• For inrush current limiting, the NTC thermistor must be connected in series with the load circuit. Several inrush current limiters can also be connected in series for higher damping.
  Inrush current limiters must NOT be connected in parallel.
• In general inrush current limiters require time to get back to cold state, in which they can provide adequate inrush current limiting due to their high resistance. The cooling down time depends on ambient conditions.
• It should be considered that the surrounding area of NTC thermistor may become quite hot. Ensure the adjacent components are placed at sufficient distance from a thermistor to allow for proper cooling time of the thermistor.
• Make sure that adjacent materials are designed for operation at temperatures comparable to the surface temperature of the thermistor. Make sure that surrounding parts and materials can withstand this temperature.
• Make sure that thermistor are adequately ventilated to avoid overheating.
• Avoid contamination of the thermistor surface.
• Avoid contact of NTC thermistor  with any liquids and solvents. Ensure that no water enters an NTC thermistor

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