NTC Inrush
Current Limiter
Applications
Control of the inrush current in switching power
supplies, fluorescent lamp, inverters, motors, etc.
Thermometrics
Thermistors
Features
• UL Approval (UL 1434 File# E82830)
• Small physical size offers design-in benets
over larger passive components
• Low cost, solid state device for inrush current
suppression
• Best-in-class capacitance ratings
• Low steady resistance and accompanying
power loss
• Excellent mechanical strength
• Wide operating temperature range: -58°F to
347°F (-50°C to 175°C)
• Suitable for PCB mounting
• Available with kinked or straight leads and tape
and reel to EIS RS-468A for automatic insertion
Amphenol
Advanced Sensors
Inrush Current Limiters In Switching Power
Supplies
The problem of current surges in switch-mode power supplies
is caused by the large lter capacitors used to smooth the
ripple in the rectied 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 owing 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 lter
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 lter 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 lter 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
lter 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:
~
DC/DC
Converter
Typical Power Supply Circuit
Input Energy = Energy Stored + Energy Dissipated
or in differential form:
Pdt = HdT + δ(T – TA)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
(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 dene 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 nal
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 ows 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 ow, at an ambient temperature of 77° (25°C).
However, most power supplies have some air ow, 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 @ 77°F (25°C)
CxMax **
(μ Farads)
Equation Constants for resistance
under load ***
Approximate Resistance Load at %
Maximum Rated
Type
Resistance
@ 25°C (Ω)
±25%
*Max.
Steady
State
Current
(Amps
RMS)
Max.
Disc
Dia.
in
(mm)
Max.
Disc
Thick.
in
(mm)
@120
(VAC
Rms)
@240
(VAC
Rms)
Max.
Energy
(Joules) X Y
Current Range
Min I Max I 25% 50% 75% 100%
Dissip.
Constant
(mW/°C)
Time
Constant
(sec.)
Max .
Current
Flow
@ 25°C
and 240
V Rms
(Amps)
CL-11 0.7 12 0.77
(19.56) 0.22
(5.59) 2700 675 19.44 0.5 -1.18 4<1<12 0.14 0.06 0.04 0.03 25 100 457
CL-21 1.3 8 0.55
(13.97 0.21
(5.33) 800 200 5.76 0.6 -1.25 3<1<8 0.25 0.11 0.06 0.04 15 60 246
CL-30 2.5 8 0.77
(19.56) 0.22
(5.59) 6000 1500 43.20 0.81 -1.25 2.5<1<8 0.34 0.14 0.09 0.06 25 100 128
CL-40 5 6 0.77
(19.56) 0.22
(5.59) 5200 1300 37.44 1.09 -1.27 1.5<1<6 0.65 0.27 0.16 0.11 25 100 64
CL-50 7 5 0.77
(19.56) 0.26
(6.60) 5000 1250 36.00 1.28 -1.27 1.5<1<5 0.96 0.40 0.24 0.17 25 120 46
CL-60 10 5 0.77
(19.56) 0.22
(5.59) 5000 1250 36.00 1.45 -1.3 1.2<1<5 1.08 0.44 0.26 0.18 25 100 32
CL-70 16 4 0.77
(19.56) 0.22
(5.59) 5000 1250 36.00 1.55 -1.26 1<1<4 1.55 0.65 0.39 0.27 25 100 20
CL-80 47 3 0.77
(19.56) 0.22
(5.59) 5000 1250 36.00 2.03 -1.29 0.5<1<3 2.94 1.20 0.71 0.49 25 100 7
CL-90 120 2 0.93
(23.62) 0.22
(5.59) 5000 1250 36.00 3.04 -1.36 0.5<1<2 7.80 3.04 1.75 1.18 30 120 3
CL-101 0.5 16 0.93
(23.62) 0.22
(5.59) 4000 1000 28.80 0.44 -1.12 4<1<16 0.09 0.04 0.03 0.02 30 120 640
CL-110 10 3.2 0.40
(10.16) 0.17
(4.32) 600 150 4.32 0.83 -1.29 0.7<1<3.2 1.11 0.45 0.27 0.19 8 30 32
CL-120 10 1.7 0.40
(10.16) 0.17
(4.32) 600 150 4.32 0.61 -1.09 0.4<1<1.7 1.55 0.73 0.47 0.34 4 90 32
CL-130 50 1.6 0.45
(11.43) 0.17
(4.32) 600 150 4.32 1.45 -1.38 0.4<1<1.6 5.13 1.97 1.13 0.76 8 30 6
CL-140 50 1.1 0.45
(11.43) 0.17
(4.32) 600 150 4.32 1.01 -1.28 0.2<1<1.1 5.27 2.17 1.29 0.89 4 90 6
CL-150 5 4.7 0.55
(13.97) 0.18
(4.57) 1600 400 11.52 0.81 -1.26 1<1<4.7 0.66 0.28 0.17 0.12 15 110 64
CL-160 5 2.8 0.55
(13.97) 0.18
(4.57) 1600 400 11.52 0.6 -1.05 0.8<1<2.8 0.87 0.42 0.28 0.20 9 130 64
CL-170 16 2.7 0.55
(13.97) 0.18
(4.57) 1600 400 11.52 1.18 -1.28 0.5<1<2.7 1.95 0.80 0.48 0.33 15 110 20
CL-180 16 1.7 0.55
(13.97) 0.18
(4.57) 1600 400 11.52 0.92 -1.18 0.4<1<1.7 2.53 1.11 0.69 0.49 9 130 20
CL-190 25 2.4 0.55
(13.97) 0.18
(4.57) 800 200 5.76 1.33 -1.34 0.5<1<2.4 2.64 1.04 0.61 0.41 15 110 13
CL-200 25 1.7 0.55
(13.97) 0.18
(4.57) 800 200 5.76 0.95 -1.24 0.4<1<1.7 2.74 1.16 0.70 0.49 9 130 13
CL-210 30 1.5 0.40
(10.16) 0.2
(5.08) 600 150 4.32 1.02 -1.35 0.3<1<1.5 3.83 1.50 0.87 0.59 8 30 11
Type CL Specications
NTC discs for inrush current limiting
Description
Disc thermistor with uninsulated lead-wires.
Options
• For kinked leads, add sufx “A”
• For tape and reel, add sufx “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
Selection Criteria for Thermometrics CL-Products
1. I max - Thermometrics CLs are rated for maximum steady state current. The maximum steady current is mainly determined
by the acceptable life of the nal products for which the thermistor becomes a component. The differential equation Pdt
= HdT + δ(T – TA)dt reduces to Power = I2R = δ(T – TA). An example in the case of a 100 watt power supply with an efciency
rating of 80%, 100% load is calculated to be 125 watts. The maximum input current is calculated from the minimum
supply voltage. For a standard 120V supply, this could be rated as low at 110V. Therefore, input current would be
calculated by 125 watts/110 V = 1.14 Amps. Selection of the CL should have an I max rating of at least 1.14 Amps.
2. The second step of selection of the CL is to understand the desired maximum inrush current allowable. This is generally
speciced by the components in line of the CL, such as the diode bridge. In the case of the diode bridge rated at 200 Amps,
one would should select a CL that would limit max surge current to 50% of the rating, therefore limit surge to a maximum
of 100 Amps. The listed maximum current ow is rated at 25°C, so derating is required if the ambient temperture is greater
than 25°C.
3. The next selection of criteria for the CL is to understand the bulk Capacitance of the device to be protected. On power, the
bulk capacitance of the device appears as a short to the system. The designer needs to understand the bulk capacitance
at the RMS voltage rating of the system. Assuming the input capacitance is approximately 500 μFds, the selection of the
CL needs to be able to absorb input energy.
Using the above criteria, the selection of the CL provides multiple solutions. One would opt for the smallest size CL to achieve
the required protection. The selection criteria is as follows:
1. I max >1.14 Amps
2. Max allowable Inrush current 100 Amps
3. Bulk Capacitance listed as 500 μfd
4. Choose smallest physical size that will allow protection for the device.
Criteria indicates that either the CL-150 or CL-160 would be suitable for the application. In the case of the CL-150 less heat is
dissipated allowing the operating resistance to drop but at a higher temperature. This increases efciency of the system but
may lead to shorter component life.
AAS-920-325D-03/2014
www.amphenol-sensors.com
© 2014 Amphenol Corporation. All Rights Reserved. Specications are subject to change without notice.
Other company names and product names used in this document are the registered trademarks or
trademarks of their respective owners.
Amphenol
Advanced Sensors
