CL-11 0.7 12 0.77 0.22 0.328 18 2700 600 0.50 -1.18 4.0112 14 .06 .04 .02 25
CL-21 1.3 8 0.55 0.21 0.328 18 800 200 0.60 -1.25 3.018.0 .25 .09 .06 .04 15
CL-30 2.5 8 0.77 0.22 0.328 18 6000 1500 0.81 -1.25 2.518.0 .34 .14 .09 .06 25
CL-40 5 6 0.77 0.22 0.328 18 5200 1300 1.09 -1.27 1.516.0 .65 .27 .16 .11 25
CL-50 7 5 0.77 0.26 0.328 18 5000 1250 1.28 -1.27 1.515.0 .96 .40 .24 .16 25
CL-60 10 5 0.77 0.22 0.328 18 5000 1250 1.45 -1.30 1.215.0 1.09 .44 .26 .18 25
CL-70 16 4 0.77 0.22 0.328 18 5000 1250 1.55 -1.26 1.014.0 1.55 .65 .39 .27 25
CL-80 47 3 0.77 0.22 0.328 18 5000 1250 2.03 -1.29 0.513.0 2.94 1.20 .71 .49 25
CL-90 120 2 0.93 0.22 0.328 18 5000 1250 3.04 -1.36 0.512.0 7.80 3.04 1.75 1.18 30
CL-101 0.5 16 0.93 0.22 0.328 18 4000 1000 0.44 -1.12 4.0116 .09 .04 .03 .02 30
CL-110 10 3.2 0.40 0.17 0.250 24 600 150 0.83 -1.29 0.713.2 1.10 .45 .27 .18 8
CL-120 10 1.7 0.40 0.17 0.250 24 600 150 0.61 -1.09 0.411.7 1.55 .73 .46 .34 4
CL-130 50 1.6 0.45 0.17 0.250 24 600 150 1.45 -1.38 0.411.6 5.13 1.97 1.13 .75 8
CL-140 50 1.1 0.45 0.17 0.250 24 600 150 1.01 -1.28 0.211.1 5.27 2.17 1.28 .89 4
CL-150 5 4.7 0.55 0.18 0.328 22 1600 400 0.81 -1.26 1.014.7 .66 .27 .16 .11 15
CL-160 5 2.8 0.55 0.18 0.328 22 1600 400 0.60 -1.05 0.812.8 .87 .42 .27 .20 9
CL-170 16 2.7 0.55 0.18 0.328 22 1600 400 1.18 -1.28 0.512.7 1.95 .80 .48 .33 15
CL-180 16 1.7 0.55 0.18 0.328 22 1600 400 0.92 -1.18 0.411.7 2.52 1.11 .69 .49 9
CL-190 25 2.4 0.55 0.18 0.328 22 800 200 1.33 -1.34 0.512.4 2.63 1.04 .60 .41 15
CL-200 25 1.7 0.55 0.18 0.328 22 800 200 0.95 -1.24 0.411.7 2.74 1.18 .70 .49 9
CL-210 30 1.5 0.40 0.20 0.250 24 600 150 1.02 -1.35 0.311.5 3.83 1.50 .87 .60 8
Max*
Steady
Res @ State
25°C Current
TYPE ±25% AMPS
Fig. 1 (ohms) (RMS)
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
OPTIONS:
For kinked leads, add suffix “A
For tape and reel, add suffix “B”
For tape and reel, add suffix “AB”
Other tolerances in the range 0.7to 120
Other tolerances, tolerances at other temperatures
Alternative lead lengths, lead materials, insulations
DATA:
*maximum rating at 25ºC or
Iderated = (1.14250.0057 x T
A) x Imax @ 25°C
for ambient temperatures other than 25ºC.
**maximum ratings
***R0=X1Y where X and Y are found in the table above
BOWTHORPE THERMOMETRICS THERMOMETRICS, INC. KEYSTONE THERMOMETRICS CORPORATION
Crown Industrial Estate, Priorswood Road 808 US Highway 1 967 Windfall Road
Taunton, Somerset TA2 8QY UK Edison, New Jersey 08817-4695 USA St. Marys, Pennsylvania 15857-3397 USA
Tel +44 (0) 1823 335200 Tel +1 (732) 287 2870 Tel +1 (814) 834 9140
Fax +44 (0) 1823 332637 Fax +1 (732) 287 8847 Fax +1 (814) 781 7969
Time
Const.
(sec.)
100
60
100
100
120
100
100
100
120
120
30
90
30
90
110
130
110
130
110
130
30
Diss.
Const.
(mW/°C)
Equation constants for
resistance under load *** Approx. Res. Under Load at %
Max. Rated Current
Cx(max)**
µFarads
Lead
Spacing
(Ref.)
(in.)
Disc
Thick.
(Max)
(in.)
Disc
Dia.
(Max)
(in.)
Lead
Dia.
AWG @120
VAC @240
VAC XY
Current
Range
Min.I/ Max.I25% 50% 75% 100%
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 rip-
ple in the rectified 60Hz current prior to being chopped at a
high frequency. The diagram above illustrates a circuit com-
monly 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 120v, it is
approximately 120 x
2/RI. Ideally, during turn-on RIshould 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 ther-
mistor 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).
BOWTHORPE THERMOMETRICS THERMOMETRICS, INC. KEYSTONE THERMOMETRICS CORPORATION
Crown Industrial Estate, Priorswood Road 808 US Highway 1 967 Windfall Road
Taunton, Somerset TA2 8QY UK Edison, New Jersey 08817-4695 USA St. Marys, Pennsylvania 15857-3397 USA
Tel +44 (0) 1823 335200 Tel +1 (732) 287 2870 Tel +1 (814) 834 9140
Fax +44 (0) 1823 332637 Fax +1 (732) 287 8847 Fax +1 (814) 781 7969
NTC THERMISTORS: TYPE CL
NTC DISCS FOR INRUSH CURRENT LIMITING
TYPICAL POWER SUPPLY CIRCUIT
-tº
RI
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 cur-
rent 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 wave-
form of the source charging the capacitors. However, a good
approximation for the energy generated by the thermistor dur-
ing 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(TTA)dt
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 (usu-
ally 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 rec-
ommended 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 experimental-
ly determined value beyond which there may be some reduc-
tion 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 val-
ues for resistance under load for each unit, as well as a rec-
ommended 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 cur-
rent rating. To derate the maximum steady state current for
operation at elevated ambient temperatures, use the following
equation:
Iderated = (1.14250.0057 x T
A) x Imax @ 25°C
Inrush Current Limiters
BOWTHORPE THERMOMETRICS THERMOMETRICS, INC. KEYSTONE THERMOMETRICS CORPORATION
Crown Industrial Estate, Priorswood Road 808 US Highway 1 967 Windfall Road
Taunton, Somerset TA2 8QY UK Edison, New Jersey 08817-4695 USA St. Marys, Pennsylvania 15857-3397 USA
Tel +44 (0) 1823 335200 Tel +1 (732) 287 2870 Tel +1 (814) 834 9140
Fax +44 (0) 1823 332637 Fax +1 (732) 287 8847 Fax +1 (814) 781 7969