|
As an example, if the MAX Operating Temp. is 50°C, and
the SURGE-GARD™ selected
has an R-T Curve A, the RT/R25 factor
is 0.464. 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 RT/R25 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 a
SURGE-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 IOP/IMAX Ratio - Next,
estimate the actual operating current, IOP
, and calculate the IOP/IMAX ratio.
The nominal resistance of a SURGE-GARD™
when operated at its IMAX rating is specified
in the Specifications under the RIMAX heading. The
device's resistance when it is operated at a current less than
its IMAX rating can be estimated by multiplying its
RIMAX rating by the factor, M. As an
example, a SURGE-GARD™
with an IMAX of 3.0 Amps and an RIMAX
of 0.20 ohms that is operated at 1.2 Amps, the IOP/IMAX
current ratio is 1.2 Amps/3.0 Amps = 0.40. The corresponding
M 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 IMAX ratings but different R@25°C
values and they meet the circuit requirements, then select the
one with the lowest RIMAX nominal value.
- Lastly, if the MAX Operating Temp.
range is >65°C or <0°C, refer to the SURGE-GARD™
Recommended IMAX De-rating Curve, Figure C.
|
|
