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Cataloges
RTIE Surge-Gard™ Inrush Current Limiters
SURGE-GARD™
Circuit Protection Devices
NTC Thermistors
Stocking Distributors:
SURGE-GARD™
Features
NTC Thermistors
Negative T
emperature Coefficient (NTC) thermistors are thermally sensitive
semiconductor resistors which exhibit a decrease in resistance as absolute
temperature increases. Change in the resistance of the NTC thermistor can be
brought about either by a change in the ambient temperature or internally by self-
heating resulting from current flowing through the device. Most of the practical
applications of NTC thermistors are based on these material characteristics.
Lowers rectifier cost
by reducing required
peak forward surge
current rating
Reduces Noise
Reduces Fuse Failures
High Current Capability
Inrush Current Limiting Devices
RTI manufactures
SURGE-GARD™
Inrush current limiting devices using specially
formulated metal oxide ceramic materials. These devices are capable of suppressing
high inrush current surges. They are especially useful in power supplies where the
low impedance of the charging capacitor exposes the diode bridge rectifier to an
excessively high current surge at turn-on.
Thermistor T
erminology for Inrush Current Limiting Devices
I
MAX
- The maximum steady state RMS AC or DC current.
I
OP
- The actual operating current.
RI
MAX
- The approximate resistance under maximum steady state current
conditions.
MAX Operating Temperature
- RTI's recommended maximum ambient
temperature is 65°C without de-rating. (Ref. Fig. C for de-rating information)
Recovery Time
-
SURGE-GARD™
devices require time to return to their
ambient resistance state in order to provide adequate inrush current limiting at
each power turn-on. This time varies with each device, the mounting
configuration and the ambient operating temperature. RTI recommends a
minimum of 60 seconds. The selection of a capacitor bleeder resistor can
reduce the required cool down time requirement.
Applications
More Distributors
SURGE-GARD™
Selection
Procedure
Calculate I
MAX
Calculate R@25°C
Select
SURGE-GARD
™
specified to handle
the input energy &
maximum current with a
R@25°C value capable
of limiting the inrush
current
Evaluate Joules Rating
Calculate theSURGE-
GARD™resistance
at
I
OP
using the 'M' curve in
Figure B
Check Figure C if de-
rating is required for
high ambient operating
temperature
RTI's
SURGE-GARDs™
are used in many applications today that require limiting
inrush current when power is applied to a system. The most popular application is
the inrush protection of the AC current in switching power supplies (SPS). The
primary reason for having surge current suppression in a SPS is to protect the diode
bridge rectifier as the input or charging capacitor is initially charged. This capacitor
draws significant current during the first half AC cycle and can subject the
components in line with the capacitor to excessive current. The inherent equivalent
series resistance (ESR) of the capacitor provides very little protection for the diode
bridge rectifier. Use of the proper
SURGE-GARD™
will provide maximum current
protection when the power supply is turned on and allow the design engineer to
select lower peak current rated diode bridge rectifiers for use in their SPS.
If the resistance of one
SURGE-GARD™
does not provide sufficient inrush current
limiting for an existing application, two or more may be used in series or in separate
legs of the power supply circuit.
SURGE-GARDs™
should not be used in parallel
since one unit will tend to conduct nearly all the current available.
SURGE-GARDs
™may
be used in the AC input side or in the circuit on the DC line between the
charging capacitors and the diode bridge rectifier circuit. (Reference Figure A)
Selection Considerations for SURGE-GARDs™
I
MAX
- The first critical consideration in the selection of a
SURGE-GARD™
is the
maximum steady state current (AC or DC) of the power supply.
SURGE-GARDs
™
are rated for maximum continuous current. The input power (Pin) is calculated
as Pin = Pout/efficiency. In the case of a 75 Watt SPS with 0.70 efficiency,
100% load is calculated to be 107.14 Watts. The maximum input current is at
the minimum input voltage. The effective input current (I
e
) is equal to the
maximum load divided by the minimum input voltage. In this case, a 75 Watt
SPS, I
e
= P
in
/V
in(low)
= 107.14 Watts/90 Volts = 1.2 Amps. Therefore,
the
SURGE-GARD™
must have an I
MAX
rating of at least 1.2 Amps.
R@25°C.
- The second step is to determine the minimum R value of theSURGE-
GARD™
to be selected that will limit the one cycle maximum current rating of
the diode bridge rectifier to 50% of its rating to ensure adequate surge
protection. Several additional calculations must be made to determine the
estimated resistance value required at the point in time of the maximum current
surge. RTI provides for a maximum AC voltage rating of 265V RMS on
most
SURGE-GARDs™.
(Reference the Specifications) If the desired maximum
inrush current is less than 100 Amps (50% of the diode bridge with a peak
current rating of 200 Amps), then solving for R would produce a value of 2.65
ohms. If the
MAX Operating Temperature
is other than 25°C then the zero
power resistance value must be calculated using the
NTC
Resistance/Temperature Conversion Tables.
SURGE-GARD™
Installation
Options
Thru-hole Leads
Insulated/ Uninsulated
Standoffs
Preformed Leads
See Figure D
As an example, if the
MAX Operating Temp.
is 50°C, and
theSURGE-GARD™ selected has an
R-T Curve A,
the R
T
/R
25
factor
is0.464. This indicates in order for the
SURGE-GARD™
to have the
same effective current limiting characteristic at the elevated
temperature, it must have a higher resistance than the
R@25°C
value
previously determined. T simplify our selection of the minimum R
o
value divide the initial
R@25°C
value by the R
T
/R
25
factor. In this
case, the
Minimum R@25°C value
= 2.65 ohm/0.464 =
5.71
ohms.
Select a
SURGE-GARD™
- The third requirement is to select
a
SURGE-GARD™
from the
Specifications.
First find the
column labeled R@25°C. The resistance values are listed in
ascending order. If the exact R value calculated is not listed
round up to the next highest R value. In this example that would
be a 6 ohm, 5 Amp part, number SG418. Notice that the current
rating is higher than required. This current rating is mass
dependent therefore the part would be larger in size than the
circuit requires. Continue down the column until the closest
current rating is located. In this case it would be a 10 ohm, 3
Amp rated part, number SG220. This would be the
selected
SURGE-GARD™
of choice.
Evaluate Joules Rating
- The fourth step is to review the
amount of energy that can be absorbed or dissipated by
aSURGE-GARD™ before a failure may occur. The
SURGE-GARD
™
devices are rated in
Joules.
In order to calculate the Joules
rating the input capacitor value must be specified. Assume that
the input capacitor is 220µfd. The instantaneous energy is equal
to one half times the capacitance of the capacitor plus its
tolerance times the peak voltage squared. In this example, Ei =
0.5 (220 (+/-T
10-6
*(265*1.414)
2
=
15.44 J
(nominal). The Joules
ol)
rating for the SG220 selected is 17J.
(Please note that other criteria such as hold up time, ripple
current, capacitor discharge time, and the efficiency of the power
supply design may affect the
SURGE-GARD™selection
process.
Consult RTI's application engineering personnel for additional
information.)
Calculate I
OP
/I
MAX
Ratio
- Next, estimate the actual operating
current,
I
OP
, and calculate the
I
OP
/I
MAX
ratio. The nominal
resistance of a
SURGE-GARD™
when operated at itsI
MAX
rating
is specified in the Specifications under the RI
MAX
heading. The
device's resistance when it is operated at a current less than its
I
MAX
rating can be estimated by multiplying its
RI
MAX
rating by
the factor,
M.
As an example, a
SURGE-GARD™
with an
I
MAX
of
3.0 Amps and an
RI
MAX
of 0.20 ohms that is operated at 1.2
Amps, the
I
OP
/I
MAX
current ratio is 1.2 Amps/3.0 Amps = 0.40.
The correspondingM factor can be determined from the graph
shown in Figure C to be 3.2. Therefore the device's estimated
resistance at 1.2 Amps can be calculated to be
R
= 3.2 * 0.20
ohms = 0.64 ohms. If two different
SURGE-GARDs™
have
similar
I
MAX
ratings but different R@25°C values and they meet
the circuit requirements, then select the one with the
lowest
RI
MAX
nominal value.
Lastly, if the
MAX Operating Temp.
range is >65°C or <0°C,
refer to the
SURGE-GARD™
Recommended I
MAX
De-rating
Curve,
Figure C.
SURGE-GARD™
Specifications
Part Number
R@25°C
R
Imax RImax Max. D Max. T Lead NTC Surge
Style A Style B Style C (Ohms) Tolerance (AMPS) (Ohms) (Inches) (Inches) Dia. Curve Rating
(±%)
(Inches)
(Joules)
SG260 SG326
0.5
20
30
0.010 1.250 0.200 0.040
A
31*
SG415 SG327
0.7
25
12
0.030 0.770 0.200 0.040
A
45
SG100 SG301
1.0
15
20
0.015 0.900 0.300 0.040
A
48*
SG405 SG328
1.0
25
30
0.015 1.250 0.250 0.040
A
157
SG416 SG329
1.3
25
8
0.050 0.550 0.200 0.040
A
40
SG110 SG302
2.0
15
18
0.030 0.900 0.350 0.040
A
80
SG420 SG355
2.0
25
23
0.025 1.250 0.300 0.040
A
250
SG120
SG303
2.5
15
3
0.150 0.600 0.250 0.032
A
27
SG130
SG304
2.5
15
7
0.050 0.600 0.250 0.032
A
27
SG140
SG305
2.5
15
9
0.040 0.600 0.250 0.032
A
27
SG150 SG306
2.5
15
10
0.040 0.900 0.300 0.040
A
87
SG160 SG307
2.5
15
15
0.030 0.900 0.300 0.040
A
87
SG170 SG308
4.0
15
8
0.070 0.600 0.250 0.040
A
27
SG32 SG330
4.0
20
14
0.050 0.900 0.350 0.040
A
100
SG180
SG309
5.0
15
2
0.400 0.600 0.250 0.032
A
36
SG413
5.0
25
2.8
0.250 0.530 0.200 0.025
A
23
SG190
SG310
5.0
15
4
0.150 0.600 0.250 0.032
A
36
SG450
SG373
5.0
15
6
0.100 0.600 0.250 0.032
A
30
SG200
SG311
5.0
15
7
0.070 0.600 0.250 0.032
A
40
SG44 SG332
5.0
20
8
0.050 0.600 0.250 0.040
A
40
SG26 SG333
5.0
15
12
0.060 0.900 0.275 0.040
A
100
SG418
SG334
6.0
15
5
0.150 0.600 0.270 0.032
A
40
SG210 SG312
7.0
15
4
0.200 0.600 0.300 0.040
A
50
SG85 SG335
7.0
25
5
0.150 0.600 0.300 0.040
A
45
SG64 SG336
7.0
15
10
0.080 0.950 0.275 0.040
J
100
SG13
SG337
10
15
2
0.300 0.500 0.250 0.032
A
17
SG220
SG313
10
15
3
0.200 0.450 0.300 0.032
A
17
SG42 SG338
10
15
5
0.200 0.600 0.350 0.040
A
44
SG27 SG314
10
15
6
0.150 0.500 0.350 0.040
A
40
SG40 SG72
10
20
8
0.100 0.900 0.350 0.040
J
50
SG451 SG374
12
15
4
0.220 0.500 0.350 0.040
A
40
SG452
SG375
15
15
2.5
0.330 0.550 0.300 0.032
A
40
SG86
16
25
1.7
0.600 0.530 0.300 0.025
A
45
SG414
16
25
2.7
0.400 0.530 0.300 0.025
A
45
SG63 SG320
16
25
4.0
0.250 0.750 0.250 0.040
J
50
SG230
SG315
20
15
1.75 0.600 0.500 0.300 0.032
A
31
SG411
SG341
25
25
1.7
0.600 0.500 0.300 0.032
A
30
SG412
SG342
25
25
2.4
0.400 0.500 0.300 0.032
A
30
SG38 SG343
30
15
3.0
0.400 0.600 0.250 0.040
B
25
SG240
SG316
40
15
2.0
0.600 0.625 0.250 0.032
B
20
SG52 SG344
47
25
3.0
0.500 0.770 0.240 0.040
B
55
SG453
SG376
60
15
1.5
1.000 0.600 0.250 0.032
B
50
SG250 SG317
120
15
3.0
0.900 0.925 0.250 0.040
C
36
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