CL-21

**NTC THERMISTORS:  TYPE CL** 

## **NTC DISCS FOR INRUSH CURRENT LIMITING** 

## **DESCRIPTION:** 

Disc thermistor with uninsulated lead-wires. 

## **FEATURES:** 

- [Low cost solid state device for inrush current ] suppression 

- [Excellent mechanical strength] 

- [Wide operating temperature range: -50°C to 175°C] 

- [Suitable for PCB mounting] 

- [Available as a standard with kinked leads and on ] tape and reel to EIA RS-468A for automatic insertion 

|TYPE<br>Fig. 1<br>~~a~~|Res @<br>25°C<br>±25%<br>(ohms)<br>~~a~~|Max*<br>Steady<br>State<br>Current<br>AMPS<br>(RMS)<br>~~a~~|Disc<br>Dia.<br>(Max)<br>(in.)<br>~~ee~~|Disc<br>Thick.<br>(Max)<br>(in.)<br>~~ee~~|Lead<br>Spacing<br>(Ref.)<br>(in.)<br>~~ee~~|Lead<br>Dia.<br>AWG<br>~~ee~~|Cx (max)**<br>µFarads<br>~~ee~~|Cx (max)**<br>µFarads<br>~~ee~~|Equation constants for<br>resistance under load ***<br>~~ee~~|Equation constants for<br>resistance under load ***<br>~~ee~~|Equation constants for<br>resistance under load ***<br>~~ee~~|Approx. Res. Under Load at %<br>Max. Rated Current<br>~~ee~~|Approx. Res. Under Load at %<br>Max. Rated Current<br>~~ee~~|Approx. Res. Under Load at %<br>Max. Rated Current<br>~~ee~~|Approx. Res. Under Load at %<br>Max. Rated Current<br>~~ee~~|Diss.<br>Const.<br>(mW/°C)<br>~~ee~~|Time<br>Const.<br>(sec.)<br>~~ee~~|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
||||||||@120<br>VAC<br>~~ee~~|@240<br>VAC<br>~~ee~~|X<br>~~ee~~|Y<br>~~ee~~|Current<br>Range<br>Min.I / Max.I<br>~~ee~~|25%<br>~~ee~~|50%<br>~~ee~~|75%<br>~~ee~~|100%<br>~~ee~~|||
|CL-11<br>~~a~~|0.7<br>~~a~~|12<br>~~a~~|0.77<br>~~ee~~|0.22<br>~~ee~~|0.328<br>~~ee~~|18<br>~~ee~~|2700<br>~~ee~~|600<br>~~ee~~|0.50<br>~~ee~~|-1.18<br>~~ee~~|4.0≤1≤12<br>~~ee~~|14<br>~~ee~~|.06<br>~~ee~~|.04<br>~~ee~~|.02<br>~~ee~~|25<br>~~ee~~|100<br>~~ee~~|
|CL-21<br>~~a~~|1.3<br>~~a~~|8<br>~~a~~|0.55<br>~~ee~~|0.21<br>~~ee~~|0.328<br>~~ee~~|18<br>~~ee~~|800<br>~~ee~~|200<br>~~ee~~|0.60<br>~~ee~~|-1.25<br>~~ee~~|3.0≤1≤8.0<br>~~ee~~|.25<br>~~ee~~|.09<br>~~ee~~|.06<br>~~ee~~|.04<br>~~ee~~|15<br>~~ee~~|60<br>~~ee~~|
|CL-30|2.5|8|0.77|0.22|0.328|18|6000|1500|0.81|-1.25|2.5≤1≤8.0|.34|.14|.09|.06|25|100|
|CL-40|5|6|0.77|0.22|0.328|18|5200|1300|1.09|-1.27|1.5≤1≤6.0|.65|.27|.16|.11|25|100|
|CL-50|7|5|0.77|0.26|0.328|18|5000|1250|1.28|-1.27|1.5≤1≤5.0|.96|.40|.24|.16|25|120|
|CL-60|10|5|0.77|0.22|0.328|18|5000|1250|1.45|-1.30|1.2≤1≤5.0|1.09|.44|.26|.18|25|100|
|CL-70|16|4|0.77|0.22|0.328|18|5000|1250|1.55|-1.26|1.0≤1≤4.0|1.55|.65|.39|.27|25|100|
|CL-80|47|3|0.77|0.22|0.328|18|5000|1250|2.03|-1.29|0.5≤1≤3.0|2.94|1.20|.71|.49|25|100|
|CL-90|120|2|0.93|0.22|0.328|18|5000|1250|3.04|-1.36|0.5≤1≤2.0|7.80|3.04|1.75|1.18|30|120|
|CL-101|0.5|16|0.93|0.22|0.328|18|4000|1000|0.44|-1.12|4.0≤1≤16|.09|.04|.03|.02|30|120|
|CL-110|10|3.2|0.40|0.17|0.250|24|600|150|0.83|-1.29|0.7≤1≤3.2|1.10|.45|.27|.18|8|30|
|CL-120|10|1.7|0.40|0.17|0.250|24|600|150|0.61|-1.09|0.4≤1≤1.7|1.55|.73|.46|.34|4|90|
|CL-130|50|1.6|0.45|0.17|0.250|24|600|150|1.45|-1.38|0.4≤1≤1.6|5.13|1.97|1.13|.75|8|30|
|CL-140|50|1.1|0.45|0.17|0.250|24|600|150|1.01|-1.28|0.2≤1≤1.1|5.27|2.17|1.28|.89|4|90|
|CL-150|5|4.7|0.55|0.18|0.328|22|1600|400|0.81|-1.26|1.0≤1≤4.7|.66|.27|.16|.11|15|110|
|CL-160|5|2.8|0.55|0.18|0.328|22|1600|400|0.60|-1.05|0.8≤1≤2.8|.87|.42|.27|.20|9|130|
|CL-170|16|2.7|0.55|0.18|0.328|22|1600|400|1.18|-1.28|0.5≤1≤2.7|1.95|.80|.48|.33|15|110|
|CL-180|16|1.7|0.55|0.18|0.328|22|1600|400|0.92|-1.18|0.4≤1≤1.7|2.52|1.11|.69|.49|9|130|
|CL-190|25|2.4|0.55|0.18|0.328|22|800|200|1.33|-1.34|0.5≤1≤2.4|2.63|1.04|.60|.41|15|110|
|CL-200|25|1.7|0.55|0.18|0.328|22|800|200|0.95|-1.24|0.4≤1≤1.7|2.74|1.18|.70|.49|9|130|
|CL-210|30|1.5|0.40|0.20|0.250|24|600|150|1.02|-1.35|0.3≤1≤1.5|3.83|1.50|.87|.60|8|30|



## **OPTIONS:** 

## **DATA:** 

- [For kinked leads, add suffix “A”] 

*maximum rating at 25ºC or 

- [For tape and reel, add suffix “B”] 

Iderated = √ (1.1425–0.0057 x TA)  x  Imax @ 25°C 

- [For tape and reel, add suffix “AB”] 

for ambient temperatures other than 25ºC. **maximum ratings 

- [Other tolerances in the range 0.7][Ω][to 120][Ω] 

- [Other tolerances, tolerances at other temperatures] 

***R0=X1[Y ] where X and Y are found in the table above 

- Alternative lead lengths, lead materials, insulations 

## **BOWTHORPE THERMOMETRICS** 

**THERMOMETRICS, INC.** 808 US Highway 1 

## **KEYSTONE THERMOMETRICS CORPORATION** 

967 Windfall Road St. Marys, Pennsylvania 15857-3397 USA **Tel +1 (814) 834 9140 Fax +1 (814) 781 7969** 

Crown Industrial Estate, Priorswood Road 

Edison, New Jersey 08817-4695 USA **Tel +1 (732) 287 2870 Fax +1 (732) 287 8847** 

Taunton, Somerset TA2 8QY UK **Tel +44 (0) 1823 335200 Fax +44 (0) 1823 332637** 

## **NTC THERMISTORS:  TYPE CL** 

## **NTC DISCS FOR INRUSH CURRENT LIMITING** 

**==> picture [151 x 113] intentionally omitted <==**

**----- Start of picture text -----**<br>
R I<br>i [A]<br>-tº<br>zZos<br>TYPICAL POWER SUPPLY CIRCUIT<br>**----- End of picture text -----**<br>


## INRUSH CURRENT LIMITERS 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 60Hz current prior to being chopped at a high frequency. The diagram above illustrates a circuit commonly used in switching power supplies. 

- 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 

In the circuit above the maximum current at turn-on is the peak line voltage divided by the value of R; for 120v, 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: 

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). 

## **BOWTHORPE THERMOMETRICS** 

Crown Industrial Estate, Priorswood Road Taunton, Somerset TA2 8QY UK **Tel +44 (0) 1823 335200 Fax +44 (0) 1823 332637** 

**THERMOMETRICS, INC.** 808 US Highway 1 Edison, New Jersey 08817-4695 USA **Tel +1 (732) 287 2870 Fax +1 (732) 287 8847** 

## **KEYSTONE THERMOMETRICS CORPORATION** 

967 Windfall Road 

St. Marys, Pennsylvania 15857-3397 USA **Tel +1 (814) 834 9140 Fax +1 (814) 781 7969** 

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 1⁄2CV[2] . 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 1⁄2CV[2] (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: 

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 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 = √ (1.1425–0.0057 x TA)  x  Imax @ 25°C 

Input Energy  =  Energy Stored 

+  Energy Dissipated 

or in differential form: 

**==> picture [124 x 11] intentionally omitted <==**

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 volts and 240 volts. 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  =  I[2] R  = δ (T – TA) 

Inrush Current Limiters 

**BOWTHORPE THERMOMETRICS** Crown Industrial Estate, Priorswood Road Taunton, Somerset TA2 8QY UK **Tel +44 (0) 1823 335200 Fax +44 (0) 1823 332637** 

**THERMOMETRICS, INC.** 808 US Highway 1 Edison, New Jersey 08817-4695 USA **Tel +1 (732) 287 2870 Fax +1 (732) 287 8847** 

## **KEYSTONE THERMOMETRICS CORPORATION** 

967 Windfall Road 

St. Marys, Pennsylvania 15857-3397 USA **Tel +1 (814) 834 9140 Fax +1 (814) 781 7969**