In-rush Current Limiters (Surge-Gard)
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In-rush Current Limiters (Surge-Gard)
SURGE-GARD™
Circuit Protection Devices
NTC Thermistors
CSA Approved
NTC Thermistors
SURGE-GARD™
Features
• Lowers rectifier
cost by
reducing
required peak
forward surge
current rating
• Reduces Noise
• Reduces Fuse
Failures
• High Current
Capability
Negative Temperature 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.
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 Terminology 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
SURGE-GARD™
Selection
Procedure
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
• Calculate I
MAX
• Calculate R@25°C
capacitor draws significant current during the first half AC cycle and can subject
• Select
SURGE-
the components in line with the capacitor to excessive current. The inherent
GARD™
specified
equivalent series resistance (ESR) of the capacitor provides very little protection
to handle the input
for the diode bridge rectifier. Use of the proper
SURGE-GARD™
will provide
energy &
maximum current protection when the power supply is turned on and allow the
maximum current
design engineer to select lower peak current rated diode bridge rectifiers for use in
with a R@25°C
their SPS.
value capable of
limiting the inrush
If the resistance of one
SURGE-GARD™
does not provide sufficient inrush current
current
limiting for an existing application, two or more may be used in series or in
• Evaluate Joules
separate legs of the power supply circuit.
SURGE-GARDs™
should not be used in
Rating
parallel since one unit will tend to conduct nearly all the current available.
SURGE-
http://www.rtie.com/category-s/48.htm
5/14/2014
In-rush Current Limiters (Surge-Gard)
Page 2 of 5
• Calculate
GARDs™
may be used in the AC input side or in the circuit on the DC line between
theSURGE-
the charging capacitors and the diode bridge rectifier circuit. (Reference Figure A)
GARD™resistance
at I
OP
using the 'M'
Selection Considerations for SURGE-GARDs™
curve in Figure B
• Check Figure C if
•
I
MAX
- The first critical consideration in the selection of a
SURGE-GARD™
is
de-rating is
the maximum steady state current (AC or DC) of the power
required for high
ambient operating
supply.
SURGE-GARDs™
are rated for maximum continuous current. The
temperature
input power (Pin) is calculated as Pin = Pout/efficiency. In the case of a
SURGE-GARD™
Installation
Options
• Thru-hole Leads
• Insulated/
Uninsulated
• Standoffs
• Preformed Leads
• See Figure D
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
the
SURGE-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
.
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. To simplify our selection of the
minimum R 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 (+/-Tol)
10-6
*(265*1.414)
2
=
15.44 J
(nominal). The Joules 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
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