GE
Sensing
Inrush CurrentLimiters 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 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/R
I
. Ideally, during turn-on R
I
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 thermistor to the size of the filter capacitors
Maximum value of steady state current
Maximum ambient temperature
Expected life of the power supply
~
R
I
-t
o
DC/DC
converter
Typical power
supply circuit
or in differential form:
Pdt = HdT +
δ(T
– T
A
)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
T
A
= 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
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
– T
A
)
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 = I
derated
=
√(1.1425–0.0057
x T
A
) x I
max
@ 77°F
(25°C)
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 (V
peak/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 1/2CV
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:
Input Energy = Energy Stored + Energy Dissipated
GE
Sensing
Power Thermistor
Specification
For the Reduction of Inrush Current
A power thermistor is a type of NTC thermistor used for
the reduction of large inrush currents. These large inrush
currents are typically caused by charging of filter
capacitors in switching power supplies.
Lead Wire Style
Inside Kink: E
5W
P: Th
C
P: Th
+
-
EH
+
-
P: Th
C
C
L
~
+
-
L
~
Power thermistor application circuits
Parts
7
9
11
13
15
18
D
7.0±1.5
9.0±1.5
11.0±1.5
13.5±1.5
15.0±1.5
18.0±1.5
T(max.)
5.2
6.0
6.5
8.0
9.0
9.0
Normal
no load
resistance
Ω
(Ω)
7
8
10
16
22
5
8
10
16
4.7
5
8
10
4.7
5
8
10
3
5
8
10
4
5
8
10
L
18.5
18.5
18.5
18.5
18.5
18.5
d
0.6
0.6
0.8
0.8
0.8
1.0
F
5.0
5.0
7.5
7.5
7.5
10.0
H1(±2.5)
15.5
17.0
19.5
21.5
23.5
27.0
W1(min.)
1.5
1.5
2.0
2.0
2.0
3.0
Time
Constant
(sec)
70
70
80
100
120
110
120
130
160
90
130
160
170
110
125
160
180
165
170
180
200
170
180
220
260
Parts Type
Diam
(mm)
TP7D7
TP8D7
TP10D7
TP16D7
TP22D7
TP5D9
TP8D9
TP10D9
TP16D9
TP4R7D11
TP5D11
TP8D11
TP10D11
TP4R7D13
TP5D13
TP8D13
TP10D13
TP3D15
TP5D15
TP8D15
TP10D15
TP4D18
TP5D18
TP8D18
TP10D18
7Φ
Outside Kink: F
9Φ
Straight: D
11Φ
Y-Form
No: M
13Φ
Yes: J
15Φ
Units:
in (mm)
W1
D
d
18Φ
F
Normal
Dissipation Max.
β
constant factor
Permissible
(K)
(mW/°C)
Current at
77°F (25°C)
3000
9.8
2.4
3000
10.0
2.3
3000
10.3
2.0
3000
10.5
1.6
3100
9.5
1.4
3000
11.0
3.0
3000
14.2
2.7
3000
12.9
2.3
3100
10.2
1.7
3000
15.0
3.7
3000
15.0
3.3
3000
17.6
2.6
3100
17.4
2.4
3000
15.0
4.3
3000
15.0
3.4
3100
17.0
2.7
3100
13.8
2.5
3000
16.5
4.0
3100
17.7
3.7
3100
21.7
3.1
3100
19.9
2.9
3000
22.2
4.1
3000
24.0
3.8
3100
26.8
3.1
3100
27.8
2.8
H1
L
* The resistance tolerance is ± 10% for standard devices.
* The b constant is determined by the equation :
β
= 1779.7 ln (R25/R85) ...R25 and R85 represent
the thermistor resistance at 77°F and 185°F (25ºC and 85ºC) respectively.
* For non-standard devices consult Thermometrics global business.
Code Designation
T
Resin coated
Pb/Sn-plated
copper wire
TP
1
1
2
3
4/5
6
7
8
9
10
11
©2006 GE. All rights reserved.
920-325A
8
D 13 L
K
B
E
S
M
N
R
Power thermistor standard dimensions
g
2
3
4
5
6
7
8
9
10
11
Shape: Power thermistor
Resistance at 77°F (25°C): 8 = 8
S,
4R7 = 4.7
S
Diameter size: D7, D9, ...D18
Resistance and B constant tolerance: K: ±10%, L: ±15%
Lead wire center-to-center: (F): A: 0.19 in (5 mm), B: 0.29 in (7.5 mm),
C: 0.39 in (10 mm)
Lead wire style: D: Straight, E: Inside kink, F: Outside kink
Lead wire length: G: 0.19 in (5 mm), H: 0.27 in (7 mm), I: 0.35 in (9 mm),
...S: Other dimensions
Y-Form: J: Yes, M: No
MK part number marking: N: No, O: Yes
Packing form: Taping P: 15 pitch, Q: 30 pitch, Others: R: Bulk, S: Paper pad,
T: Element
All specifications are subject to change for product improvement without notice.
GE
®
is a registered trademark of General Electric Co. Other company or product
names mentioned in this document may be trademarks or registered trademarks
of their respective companies, which are not affiliated with GE.
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