•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
SURGE-GARD™
Installation
Options
• Thru-hole Leads
• Insulated/
Uninsulated
• Standoffs
• Preformed Leads
• See Figure D
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
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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-Gard)
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