SG307

In-rush Current Limiters (Surge-Gard) Page 1 of 5 You are here: Home > Products > 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 selfheating 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 • IMAX - The maximum steady state RMS AC or DC current. • IOP - The actual operating current. • RIMAX - 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 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 IMAX • Calculate R@25°C capacitor draws significant current during the first half AC cycle and can subject • Select SURGEthe 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- SURGE-GARD™ Selection Procedure 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 theSURGEthe charging capacitors and the diode bridge rectifier circuit. (Reference Figure A) GARD™resistance at IOPusing the 'M' Selection Considerations for SURGE-GARDs™ curve in Figure B • Check Figure C if • IMAX - 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 (Ie) is equal to the maximum load divided by the minimum input voltage. In this case, a 75 Watt SPS, Ie = Pin/Vin(low) = 107.14 Watts/90 Volts = 1.2 Amps. Therefore, the SURGE-GARD™ must have an IMAX 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. As an example, if the MAX Operating Temp. is 50°C, and theSURGEGARD™ selected has an R-T Curve A, the RT/R25 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 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 aSURGEGARD™ 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 itsIMAX rating is specified in the Specifications under the RIMAXheading. 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 RIMAXof 0.20 ohms that is operated at 1.2 Amps, the IOP/IMAXcurrent 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 IMAXratings but different http://www.rtie.com/category-s/48.htm 5/14/2014 In-rush Current Limiters (Surge-Gard) Page 3 of 5 R@25°C values and they meet the circuit requirements, then select the one with the lowest RIMAXnominal value. • Lastly, if the MAX Operating Temp. range is >65°C or