SURGE-GARD™ Features
Inrush Current Limiting
Devices | Applications | Selection
Consideration for SURGE-GARDs™
SURGE-GARD™ Specifications | NTC
Resistance Temperature Conversion Tables
SURGE-GARD™ Features
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.
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
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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)
SURGE-GARD™ Selection Procedure
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.
SURGE-GARD™ - Installation Options
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