Power NTC Thermistors for Inrush Current Limiting Surge Suppression

What is Inrush Current Limiting (ICL) Surge Suppression Power NTC Thermistor?

Inrush Current Limit NTC Thermistor

Inrush Current Limit NTC Thermistor

Inrush Current Limiting (ICL) Surge Suppression Power NTC Thermistor is polycrystalline mixed oxide ceramics made semiconductor type NTC Thermistor disc soldered with radial metal wire, coated with epoxy resin, function as a power resistor to limit the inrush current surge current in the turn-on stage.

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.

How Does  NTC Thermistor Inrush Current Protection Work?

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.


Inrush Current Limit Power NTC Thermistor Application

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.

Surge Suppression Power NTC Thermistor Advantages:

Comparison curve With Without Inrush Current Limiting Power NTC thermistor

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 Features:

Power NTC Thermistor Load Temperature Characteristics

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 Application of  Power NTC Thermistors for Circuit Protection Diagram

NTC thermistors in a protective circuit mounting positions

NTC thermistors in a protective circuit mounting positions

NTC Thermistor for diode protection

NTC Thermistor for diode protection

Power thermistor application circuits

Power thermistor application circuits

NTC Inrush Current Limiters In Switching Power Supplies

Typical Power Supply Circuit

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)

Notes on scaling an inrush current limit  NTC thermistor

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.

AMWEI Inrush Current Limiting Power NTC Thermistors Dimensions (mm)

Inrush Current Limit NTC Thermistor Dimension

AMWEI Inrush Current Limit Power NTC Thermistors Data Sheet

AMWEI Part No. Resistance
R25
(ohm)
Max
Stable
Current
(A)
Approx.
Resistance
value
@max
current
(Ω)
Dissipation
Factor
(mW/oC)
Thermal
Time
Constant
(sec)
Dimensions
(mm)
Dmax Tmax F±1
ICL09-3R4A 3 ohm 4A 0.120 11 34 11 5.5 7.5 /5
ICL09-5R3A 5 ohm 3A 0.210 11 34 11 5.5 7.5 /5
ICL09-8R2A 8 ohm 2A 0.400 11 32 11 5.5 7.5/5
ICL09-10R2A 10 ohm 2A 0.458 11 32 11 5.5 7.5/5
ICL09-16R1A 16 ohm 1A 0.802 11 31 11 5.5 7.5/5
ICL09-22R1A 22 ohm 1A 0.950 11 30 11 5.5 7.5/5
ICL09-33R1A 33 ohm 1A 1.124 11 30 11 5.5 7.5/5
ICL09-50R1A 50 ohm 1A 1.252 11 30 11 5.5 7.5/5
ICL09-80R0.8A 80 ohm 0.8A 2.010 11 30 11 5.5 7.5/5
ICL11-3R5A 3 ohm 5A 0.100 13 43 13 5.5 7.5/5
ICL11-5R4A 5 ohm 4A 0.156 13 45 13 5.5 7.5/5
ICL11-8R3A 8 ohm 3A 0.255 14 47 13 5.5 7.5/5
ICL11-10R3A 10 ohm 3A 0.275 14 47 13 5.5 7.5/5
ICL11-12R2A 12 ohm 2A 0.462 14 48 13 5.5 7.5/5
ICL11-16R2A 16 ohm 2A 0.470 14 50 13 5.5 7.5/5
ICL11-20R2A 20 ohm 2A 0.512 15 52 13 5.5 7.5/5
ICL11-22R2A 22 ohm 2A 0.563 15 52 13 5.5 7.5/5
ICL11-33R1.5A 33 ohm 1.5A 0.734 15 52 13 5.5 7.5/5
ICL11-50R1.5A 50 ohm 1.5A 1.021 15 52 13 5.5 7.5/5
ICL11-60R1.5A 60 ohm 1.5A 1.215 15 52 13 5.5 7.5/5
ICL13-1.3R7A 1.3 ohm 7A 0.062 13 60 15.5 6 7.5
ICL13-3R6A 3 ohm 6A 0.092 14 60 15.5 6 7.5
ICL13-5R5A 5 ohm 5A 0.125 15 68 15.5 6 7.5
ICL13-10R4A 10 ohm 4A 0.206 15 65 15.5 6 7.5
ICL13-15R3A 15 ohm 3A 0.335 16 60 15.5 6 7.5
ICL13-30R2.5A 30 ohm 2.5A 0.517 16 65 15.5 6 7.5
ICL13-47R2A 47 ohm 2A 0.810 17 65 15.5 6 7.5
ICL15-1.3R8A 1.3 ohm 8A 0.048 18 68 17.5 6 10/7.5
ICL15-1.5R8A 1.5 ohm 8A 0.052 18 69 17.5 6 10/7.5
ICL15-3R7A 3 ohm 7A 0.075 18 76 17.5 6 10/7.5
ICL15-5R6A 5 ohm 6A 0.112 20 76 17.5 6 10/7.5
ICL15-8R5A 8 ohm 5A 0.178 20 80 17.5 6 10/7.5
ICL15-10R5A 10 ohm 5A 0.180 20 80 17.5 6 10/7.5
ICL15-15R4A 15 ohm 4A 0.268 20 85 17.5 6 10/7.5
ICL15-20R4A 20 ohm 4A 0.288 20 85 17.5 6 10/7.5
ICL15-30R3.5A 30 ohm 3.5A 0.438 21 85 17.5 6 10/7.5
ICL15-47R3A 47 ohm 3A 0.680 21 86 17.5 6 10/7.5
ICL20-0.7R11A 0.7 ohm 11A 0.018 24 89 22.5 7 10/7.5
ICL20-1.3R9A 1.3 ohm 9A 0.037 24 88 22.5 7 10/7.5
ICL20-3R8A 3 ohm 8A 0.055 24 88 22.5 7 10/7.5
ICL20-5R7A 5 ohm 7A 0.087 24 87 22.5 7 10/7.5
ICL20-8R6A 8 ohm 6A 0.142 25 105 22.5 7 10/7.5
ICL20-10R6A 10 ohm 6A 0.162 25 102 22.5 7 10/7.5

NTC thermistor part with increased max. operating current

AMWEI Part Resistance
R25
(ohm)
Max Stable Current(A) Approx. Resistance Value at Maximum Current (Ω) Max Rated Power Pmax.(W) Dissipation Factor (mW/oC) Thermal Time Constant (s) Dimensions (mm)
Dmax Tmax
ICL73T15-2.5R9.5A 2.5 ohm 9.5A 0.044 5W 22 min 75 max 17.5 6
ICL73T15-5R8A 5 ohm 8A 0.058 5W 22 min 75 max 17.5 6
ICL73T15-10R7A 10 ohm 7A 0.098 5W 22 min 75 max 17.5 6
ICL73T20-1R16A 1 ohm 16A 0.027 7W 28 min 110 max 22.5 7
ICL73T20-5R12A 5 ohm 12A 0.047 7W 28 min 110 max 22.5 7
ICL73T20-10R8A 10 ohm 8A 0.085 7W 28 min 110 max 22.5 7
ICL73T25-1R20A 1 ohm 20A 0.021 9W 30 min 130 max 29 8
ICL73T25-5R14A 5 ohm 14A 0.047 9W 30 min 130 max 29 8
ICL73T25-10R10A 10 ohm 10A 0.084 9W 30 min 130 max 29 8
ICL73T30-1R30A 1 ohm 30A 0.014 13W 40 min 190 max 35 10
ICL73T30-10R13A 10 ohm 13A 0.056 13W 40 min 190 max 35 10

Inrush Current Limiter NTC Thermistor Surge Suppression Reliability Data

Test Item Standard Test Conditions Specifications
Lead wire tensile IEC 60068-2-21 Test Ua: Force 20N,
On 10 Seconds;
ΔR25/R25 <15%,
No visible damage
Solderability IEC 60068-2-20 245 ±3℃, 3 ± 0.3 sec At least 95% of terminal electrode is covered by new solder
Resistance to Soldering Heat IEC 60068-2-20 260 ± 3℃, 10 ± 1 sec ΔR25/R25 <15%,
No visible damage
Storage in dry heat IEC 60068-2-2 Storage at upper category  temperature
Temperature 200C
Time 1000h
ΔR25/R25 <20%,
No visible damage
Storage in damp heat, steady state IEC 60068-2-78 Temperature of air: 40C
Relative humidity of air: 93%
Duration: 21 days
ΔR25/R25 <20%,
No visible damage
Rapid change of temperature IEC 60068-2-14 Lower test temperature: -55C
t: 30 min
Upper test temperature: 200C
t: 30 min
Time to change from lower to
upper temperature: < 30 s
Number of cycles: 10
ΔR25/R25 <20%,
No visible damage
Endurance with
max. current
IEC 60539-1 Ambient temperature: 25 ±5C
I=Imax
t: 1000h
ΔR25/R25 <20%,
No visible damage
Cyclic endurance IEC 60539-1 I=Imax
On-time=1 min
Cooling time=6 min
Number of cycles: 1000
ΔR25/R25 <20%,
No visible damage

Consideration in Selecting Power NTC Thermistors for Inrush Current Limiting Surge Suppression 

  • Maximum operating current > Actual operating current in the power loop
  • Rated zero power resistance at 25C
  • 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.

ICLs NTC Cautions Warnings on Storage Handling Soldering Mounting Operation

Storage

  • Store thermistors only in original packaging. Do not open the package before storage.
  • Storage conditions in original packaging: storage temperature -25 °C to +45 °C, relative humidity ≤75% annual mean, maximum 95%, dew precipitation is inadmissible.
  • Avoid contamination of thermistors surface during storage, handling and processing.
  • Avoid storage of thermistor in harmful environments like corrosive gases (SOx, Cl etc).

Handling

  • NTC inrush current limiters must not be dropped. Chip-offs must not be caused during
    handling of NTC inrush current limiters.
  • Components must not be touched with bare hands. Gloves are recommended.
  • Avoid contamination of thermistor surface during handling.
  • In case of exposure of the NTC inrush current limiters to water, electrolytes or other aggressive
    media, these media can penetrate the coating and reach the surface of the ceramic. Low-ohmic or high-ohmic behavior may occur due to the formation of an electrolyte with metals (silver/lead/tin from metallization or solder). Low-ohmic behavior is caused by electrochemical migration, high-ohmic behavior by dissolving of the electrode. In either case, the functionality of the NTC inrush current limiters can not be assured.
  • Washing processes may damage the product due to the possible static or cyclic mechanical loads (e.g. ultrasonic cleaning). They may cause cracks to develop on the product and its parts, which might lead to reduced reliability or lifetime.

Bending / twisting leads

  • A lead wire may be bent at a minimum distance of twice the wire’s diameter plus 4 mm from the component head or housing. When bending ensure the wire is mechanically relieved at the component head or housing. The bending radius should be at least 0.75 mm.
  • Twisting (torsion) by 180°of a lead bent by 90° is permissible at 6 mm from the bottom of the thermistor body.

Mounting

  • When thermistors are sealed, potted or over-molded, there must be no mechanical stress caused by thermal expansion during the production process (curing/ over-molding process) and during later operation.  The upper category temperature of the thermistor must not be exceeded. Ensure that the materials used (sealing / potting compound and plastic material) are chemically neutral.
  • Electrode must not be scratched before/during/after the mounting process.
  • Contacts and housings used for assembly with thermistor have to be clean before mounting.
  • During operation, the inrush current limiters surface temperature can be very high. Ensure that adjacent components are placed at a sufficient distance from the thermistor to allow for proper cooling of the NTC inrush current limiters.
  • Ensure that adjacent materials are designed for operation at temperatures comparable to the surface temperature of the thermistor. Be sure that surrounding parts and materials can withstand this temperature.
  • Make sure that inrush current limiters are adequately ventilated to avoid overheating.
  • Avoid contamination of thermistor surface during processing.

Operation

  • Use NTC inrush current limiters only within the specified operating temperature range.
  • Use NTC inrush current limiters only within the specified voltage and current ranges.
  • Environmental conditions must not harm the NTC inrush current limiters. Use NTC inrush current limiters only in normal atmospheric conditions.
  • Contact of NTC inrush current limiters with any liquids and solvents should be prevented. It must be ensured that no water enters the NTC inrush current limiters (e.g. through plug terminals). For measurement purposes (checking the specified resistance vs. temperature), the component must not be immersed in water but in suitable liquids (e.g. Galden).
  • In case of exposure of the NTC inrush current limiters to water, electrolytes or other aggressive media, these media can penetrate the coating and reach the surface of the ceramic. Low-ohmic or high-ohmic behavior may occur due to the formation of an electrolyte with metals (silver/lead/tin from metallization or solder). Low-ohmic behavior is caused by electrochemical
    migration, high-ohmic behavior by dissolving of the electrode. In either case, the functionality of the NTC inrush current limiters can not be assured.
  • Be sure to provide an appropriate fail-safe function to prevent secondary product damage caused by malfunction (e.g. use a metal oxide varistor for limitation of overvoltage condition).

Other Related AMWEI PTC Thermistors Products for Current Surge Protection

PTC Thermistor Resettable Fuse, Transformer Meter Overload Short Circuit Protection

PTC Thermistor Resettable Fuse, Transformer Meter Overload Short Circuit Protection

PTC Thermistor for Telecom Equipment High Voltage Current Surge Power Line Contact Protection

PTC Thermistor for Telecom Equipment High Voltage Current Surge Power Line Contact Protection