RTI Surge-Gard Inrush Current Limiters

RTI Surge-Gard Inrush Current Limiters

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.

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SURGE-GARD CIRCUIT PROTECTION DEVICES

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Inrush Current Limiting Devices

RTI manufacturesSURGE-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’sSURGE-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 properSURGE-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 oneSURGE-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 Features

  • Lowers rectifier cost by reducing required peak forward surge current rating
  • Reduces Noise
  • Reduces Fuse Failures
  • High Current Capacity

Installation Options

  • Thru-Hole Leads
  • Insulated/Un-insulated
  • Standoffs
  • Preformed Leads. Refer to Figure D

Selection Procedure

  • Calculate IMAX
  • Calculate R@25°C
  • Select SURGE-GARD specified to handle the input energy & maximum current with a R@25°C value capable of limiting the inrush current
  • Evaluate Joules Rating
  • Calculate the SURGE-GARD™ resistance at IOP using the ‘M’ curve in Figure B
  • Check Figure C if de-rating is required for high ambient operating temperature
  • Evaluate Joules Rating

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Selection Considerations for SURGE-GARDs™

  • IMAX– The first critical consideration in the selection of aSURGE-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 (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, theSURGE-GARD™must have an IMAXrating 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 mostSURGE-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 theMAX Operating Temperatureis other than 25°C then the zero power resistance value must be calculated using theNTC Resistance/Temperature Conversion Tables.

 

  • Select aSURGE-GARD™– The third requirement is to select aSURGE-GARD™from theSpecifications. 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 selectedSURGE-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. TheSURGE-GARD™devices are rated inJoules. 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 theSURGE-GARD™selection process. Consult RTI’s application engineering personnel for additional information.)

 

  • Calculate IOP/IMAXRatio– Next, estimate the actual operating current,IOP, and calculate theIOP/IMAXratio. The nominal resistance of aSURGE-GARD™when operated at itsIMAXrating is specified in the Specifications under the RIMAXheading. The device’s resistance when it is operated at a current less than its IMAXrating can be estimated by multiplying itsRIMAXrating by the factor,M. As an example, aSURGE-GARD™with anIMAXof 3.0 Amps and anRIMAXof 0.20 ohms that is operated at 1.2 Amps, theIOP/IMAXcurrent ratio is 1.2 Amps/3.0 Amps = 0.40. The correspondingMfactor 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 beR= 3.2 * 0.20 ohms = 0.64 ohms. If two differentSURGE-GARDs™have similarIMAXratings but different R@25°C values and they meet the circuit requirements, then select the one with the lowestRIMAXnominal value.
  • Lastly, if theMAX Operating Temp. range is >65°C or <0°C, refer to theSURGE-GARD™Recommended IMAXDe-rating Curve, Figure C

 

Surge-Gard Specifications-Figures A, B, C, and D
   surge-gard-installations     surge-gard-resistance-curve    derating-curve   lead-style-config 
Part Number R@25°C
(Ohms)
Tol
%
Imax
(AMPS)
RImax
(Ohms)
Max. D
(Inches)
Max. T
(Inches)
Lead Dia.
(Inches)
NTC
Curve
Mouser Digikey
SG260 0.5 20 30 0.01 1.25 0.2 0.04 A 995-SG260 570-1142-ND
SG326 0.5 20 30 0.01 1.25 0.2 0.04 A 995-SG326 SG326-ND
SG100 1 15 20 0.015 0.9 0.3 0.04 A 995-SG100
SG301 1 15 20 0.015 0.9 0.3 0.04 A 995-SG301 SG301-ND
SG405 1 25 30 0.015 1.25 0.25 0.04 A 995-SG405 570-1144-ND
SG328 1 25 30 0.015 1.25 0.25 0.04 A 995-SG328 570-1145-ND
SG416 1.3 25 8 0.05 0.55 0.2 0.04 A 995-SG416
SG110 2 15 18 0.03 0.9 0.35 0.04 A 995-SG110 570-1146-ND
SG302 2 15 18 0.03 0.9 0.35 0.04 A 995-SG302 SG302-ND
SG420 2 25 23 0.025 1.25 0.3 0.04 A 995-SG420
SG355 2 25 23 0.025 1.25 0.3 0.04 A 995-SG335
SG120 2.5 15 3 0.15 0.6 0.25 0.032 A 995-SG120 570-1148-ND
SG303 2.5 15 3 0.15 0.6 0.25 0.032 A 995-SG303 SG303-ND
SG130 2.5 15 7 0.05 0.6 0.25 0.032 A 995-SG130 570-1149-ND
SG304 2.5 15 7 0.05 0.6 0.25 0.032 A 995-SG304 SG304-ND
SG140 2.5 15 9 0.04 0.6 0.25 0.032 A 995-SG140 570-1150-ND
SG305 2.5 15 9 0.04 0.6 0.25 0.032 A 995-SG305 SG305-ND
SG150 2.5 15 10 0.04 0.9 0.3 0.04 A 995-SG150 570-1151-ND
SG306 2.5 15 10 0.04 0.9 0.3 0.04 A 995-SG306 SG306-ND
SG160 2.5 15 15 0.03 0.9 0.3 0.04 A 995-SG160 570-1152-ND
SG307 2.5 15 15 0.03 0.9 0.3 0.04 A 995-SG307 SG307-ND
SG170 4 15 8 0.07 0.6 0.25 0.04 A 995-SG170 570-1153-ND
SG308 4 15 8 0.07 0.6 0.25 0.04 A 995-SG308 570-1154-ND
SG32 4 20 14 0.05 0.9 0.35 0.04 A 995-SG32 570-1155-ND
SG330 4 20 14 0.05 0.9 0.35 0.04 A 995-SG330 570-1156-ND
SG190 5 15 4 0.15 0.6 0.25 0.032 A 995-SG190 570-1157-ND
SG310 5 15 4 0.15 0.6 0.25 0.032 A 995-SG310 SG310-ND
SG200 5 15 7 0.07 0.6 0.25 0.032 A 995-SG200
SG311 5 15 7 0.07 0.6 0.25 0.032 A 995-SG311 SG311-ND
SG44 5 20 8 0.05 0.6 0.25 0.04 A 995-SG44
SG332 5 20 8 0.05 0.6 0.25 0.04 A 995-SG332
SG26 5 15 12 0.06 0.9 0.275 0.04 A 995-SG26
SG333 5 15 12 0.06 0.9 0.275 0.04 A 995-SG333 570-1162-ND
SG210 7 15 4 0.2 0.6 0.3 0.04 A 995-SG210 570-1165-ND
SG312 7 15 4 0.2 0.6 0.3 0.04 A 995-SG312 SG312-ND
SG64 7 15 10 0.08 0.95 0.275 0.04 J 995-SG64
SG336 7 15 10 0.08 0.95 0.275 0.04 J 995-SG336 570-1164-ND
SG337 10 15 2 0.3 0.5 0.25 0.032 A 995-SG337
SG220 10 15 3 0.2 0.45 0.3 0.032 A 995-SG220 570-1166-ND
SG313 10 15 3 0.2 0.45 0.3 0.032 A 995-SG313 570-1167-ND
SG42 10 15 5 0.2 0.6 0.35 0.04 A 995-SG45
SG338 10 15 5 0.2 0.6 0.35 0.04 A 995-SG338 SG338-ND
SG27 10 15 6 0.15 0.5 0.35 0.04 A 995-SG27 570-1169-ND
SG314 10 15 6 0.15 0.5 0.35 0.04 A 995-SG314 570-1170-ND
SG40 10 20 8 0.1 0.9 0.35 0.04 J 995-SG40
SG320 16 25 4 0.25 0.75 0.25 0.04 J 995-SG320
SG230 20 15 1.75 0.6 0.5 0.3 0.032 A 995-SG230 570-1173-ND
SG315 20 15 1.75 0.6 0.5 0.3 0.032 A 995-SG315 SG315-ND
SG240 40 15 2 0.6 0.625 0.25 0.032 B 995-SG240
SG316 40 15 2 0.6 0.625 0.25 0.032 B 995-SG316 SG316-ND
SG250 120 15 3 0.9 0.925 0.25 0.04 C 995-SG250 570-1176-ND
SG317 120 15 3 0.9 0.925 0.25 0.04 C 995-SG317 SG317-ND

For applications requiring ratings not shown, contact RTI Electronics, Inc. applications engineering.

  • Maximum operating voltage is 265V RMS.
  • *Maximum operating voltage is 120V RMS

NTC Resistance Temperature Conversion Tables

Temp
°C
R-T Curve A R-T Curve B R-T Curve C R-T Curve J
RT/R25 DEV RT/R25 DEV RT/R25 DEV RT/R25 DEV
-60 43.0 75.0 6.6 140.5 6.6 52.5
-55 31.9 54.1 6.1 96.4 6.1 39.0
-50 24.3 39.7 5.6 67.0 5.6 29.2 18.5
-45 18.6 29.2 5.2 47.2 5.2 22.1 17.0
-40 14.4 7.6 21.7 4.7 33.7 4.7 16.9 15.4
-35 11.3 6.9 16.4 4.3 24.3 4.3 13.0 14.0
-30 8.93 6.2 12.5 3.8 17.7 3.8 10.1 12.5
-25 7.10 5.6 9.58 3.4 13.0 3.4 7.90 11.2
-20 5.69 5.0 7.42 3.0 9.71 3.0 6.24 9.9
-15 4.56 4.4 5.75 2.6 7.30 2.6 4.96 8.7
-10 3.68 3.7 4.50 2.2 5.53 2.2 3.97 7.4
-5 2.99 3.1 3.55 1.9 4.23 1.9 3.20 6.2
0 2.45 2.5 2.82 1.5 3.27 1.5 2.60 5.0
5 2.02 2.0 2.26 1.2 2.54 1.2 2.12 3.9
10 1.68 1.6 1.83 0.8 1.99 0.8 1.74 2.7
15 1.42 1.1 1.48 0.5 1.57 0.5 1.44 1.6
20 1.18 0.6 1.22 0.2 1.25 0.2 1.20 0.5
25 1.00 0.0 1.00 0.0 1.00 0.0 1.00 0.0
30 0.854 0.6 0.828 0.4 0.806 0.4 0.841 1.4
35 0.732 1.1 0.689 0.7 0.653 0.7 0.710 2.3
40 0.628 1.6 0.576 1.0 0.533 1.0 0.602 3.2
45 0.537 2.0 0.482 1.3 0.437 1.3 0.513 4.3
50 0.464 2.5 0.406 1.5 0.360 1.5 0.439 5.0
55 0.403 3.0 0.343 1.8 0.299 1.8 0.377 5.9
60 0.350 3.4 0.292 2.0 0.249 2.0 0.326 6.7
65 0.305 3.8 0.247 2.3 0.208 2.3 0.282 7.5
70 0.267 4.2 0.212 2.5 0.175 2.5 0.245 8.2
75 0.236 4.6 0.182 2.8 0.148 2.8 0.214 9.0
80 0.208 4.9 0.157 3.0 0.126 3.0 0.188 9.8
85 0.183 5.3 0.137 3.2 0.107 3.2 0.165 10.5
90 0.163 5.6 0.120 3.4 0.0916 3.4 0.146 11.2
95 0.145 6.0 0.105 3.6 0.0787 3.6 0.129 11.9
100 0.130 6.3 0.0920 3.8 0.0679 3.8 0.114 12.6
105 0.117 6.7 0.0812 4.0 0.0588 4.0 0.102 13.3
110 0.105 7.0 0.0723 4.2 0.0511 4.2 0.0908 13.9
115 0.0943 7.3 0.0641 4.4 0.0445 4.4 0.0813 14.4
120 0.0852 7.6 0.0569 4.6 0.0389 4.6 0.0730 14.9
125 0.0771 7.9 0.0508 4.8 0.0342 4.8 0.0657 15.6
130 0.0700 8.2 0.0455 4.9 0.0301 4.9 0.0593 16.3
135 0.0636 8.4 0.0408 5.1 0.0265 5.1 0.0536 17.0
140 0.0579 8.6 0.0368 5.3 0.0235 5.3 0.0486 17.6
145 0.0529 9.0 0.0332 5.4 0.0208 5.4 0.0442 18.0
150 0.0483 9.3 0.0300 5.5 0.0185 5.5 0.0402 18.4

NTC Resistance Temperature Curve Characteristics

R-T Curve A B C J
Temp. Coeff@ 25°C -3.3%/°C -3.9%/°C -4.4%/°C -3.5%/°C
Beta, ß 3000 °K 3530 °K 3965 °K 3200 °K