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.

SURGE-GARD CIRCUIT PROTECTION DEVICES

          

 

 

 

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

• I_MAX– The maximum steady state RMS AC or DC current.
• I_OP– The actual operating current.
• RI_MAX– 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 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)

Selection Considerations for SURGE-GARDs™

• IMAX – 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 (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 Temperatures other than 25°C then the zero power resistance value must be calculated using the NTC Resistance/Temperature Conversion Tables.

 

• 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 220µf. 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_MAXRatio– Next, estimate the actual operating current,I_OP, and calculate theI_OP/I_MAXratio. The nominal resistance of a SURGE-GARD™ when operated at itsI_MAXrating is specified in the Specifications under the RI_MAXheading. The device’s resistance when it is operated at a current less than its I_MAXrating can be estimated by multiplying itsRI_MAXrating by the factor, M. As an example, a SURGE-GARD™with anI_MAXof 3.0 Amps and anRI_MAXof 0.20 ohms that is operated at 1.2 Amps, theI_OP/I_MAXcurrent 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 I_MAX ratings but different R@25°C values and they meet the circuit requirements, then select the one with the lowestRI_MAXnominal value.

• Lastly, if the MAX Operating Temp. range is >65°C or <0°C, refer to the SURGE-GARD™ recommended I_MAXDe-rating Curve, Figure C

 

Part Number	Resistance @ 25 °C	Tolerance (%)	lmax (Amps)	RImax (Ω)	Max. D (Inches)	Max. T (Inches)	Lead Diameter (Inches)	NTC Curve	Digi-Key	Mouser
SG260	0.5	20	30	0.01	1.25	0.2	0.04	A	570-1142-ND	995-SG260
SG326	0.5	20	30	0.01	1.25	0.2	0.04	A	SG326-ND	995-SG326
SG100	1	15	20	0.015	0.9	0.3	0.04	A	570-1143-ND	995-SG100
SG301	1	15	20	0.015	0.9	0.3	0.04	A	SG301-ND	995-SG301
SG405	1	25	30	0.015	1.25	0.25	0.04	A	570-1144-ND	995-SG405
SG328	1	25	30	0.015	1.25	0.25	0.04	A	570-1145-ND	995-SG328
SG416	1.3	25	8	0.05	0.55	0.2	0.04	A	570-1445-ND	995-SG405
SG110	2	15	18	0.03	0.9	0.35	0.04	A	570-1146-ND	995-SG110
SG302	2	15	18	0.03	0.9	0.35	0.04	A	SG302-ND	995-SG302
SG420	2	25	23	0.025	1.25	0.3	0.04	A	570-1147-ND	995-SG420
SG355	2	25	23	0.025	1.25	0.3	0.04	A	SG355-ND	995-SG355
SG120	2.5	15	3	0.15	0.6	0.25	0.032	A	570-1148-ND	995-SG120
SG303	2.5	15	3	0.15	0.6	0.25	0.032	A	SG303-ND	995-SG303
SG130	2.5	15	7	0.05	0.6	0.25	0.032	A	570-1149-ND	995-SG130
SG304	2.5	15	7	0.05	0.6	0.25	0.032	A	SG304-ND	995-SG304
SG140	2.5	15	9	0.04	0.6	0.25	0.032	A	570-1150-ND	995-SG140
SG305	2.5	15	9	0.04	0.6	0.25	0.032	A	SG305-ND	995-SG305
SG150	2.5	15	10	0.04	0.9	0.3	0.04	A	570-1151-ND	995-SG150
SG306	2.5	15	10	0.04	0.9	0.3	0.04	A	SG306-ND	995-SG306
SG160	2.5	15	15	0.03	0.9	0.3	0.04	A	570-1152-ND	995-SG160
SG307	2.5	15	15	0.03	0.9	0.3	0.04	A	SG307-ND	995-SG307
SG170	4	15	8	0.07	0.6	0.25	0.04	A	570-1153-ND	995-SG170
SG308	4	15	8	0.07	0.6	0.25	0.04	A	570-1154-ND	995-SG308
SG32	4	20	14	0.05	0.9	0.35	0.04	A	570-1155-ND	995-SG32
SG330	4	20	14	0.05	0.9	0.35	0.04	A	570-1156-ND	995-SG330
SG190	5	15	4	0.15	0.6	0.25	0.032	A	570-1157-ND	995-SG190
SG310	5	15	4	0.15	0.6	0.25	0.032	A	SG310-ND	995-SG310
SG200	5	15	7	0.07	0.6	0.25	0.032	A	570-1159-ND	995-SG200
SG311	5	15	7	0.07	0.6	0.25	0.032	A	SG311-ND	995-SG311
SG44	5	20	8	0.05	0.6	0.25	0.04	A	570-1160-ND	995-SG44
SG332	5	20	8	0.05	0.6	0.25	0.04	A	570-1446-ND	995-SG332
SG26	5	15	12	0.06	0.9	0.275	0.04	A	570-1161-ND	995-SG26
SG333	5	15	12	0.06	0.9	0.275	0.04	A	570-1162-ND	995-SG333
SG210	7	15	4	0.2	0.6	0.3	0.04	A	570-1165-ND	995-SG210
SG312	7	15	4	0.2	0.6	0.3	0.04	A	SG312-ND	995-SG312
SG64	7	15	10	0.08	0.95	0.275	0.04	J	570-1163-ND	995-SG64
SG336	7	15	10	0.08	0.95	0.275	0.04	J	570-1164-ND	995-SG336
SG337	10	15	2	0.3	0.5	0.25	0.032	A	570-1447-ND	995-SG337
SG220	10	15	3	0.2	0.45	0.3	0.032	A	570-1166-ND	995-SG220
SG313	10	15	3	0.2	0.45	0.3	0.032	A	570-1167-ND	995-SG313
SG42	10	15	5	0.2	0.6	0.35	0.04	A	570-1168-ND	995-SG42
SG338	10	15	5	0.2	0.6	0.35	0.04	A	SG338-ND	995-SG338
SG27	10	15	6	0.15	0.5	0.35	0.04	A	570-1169-ND	995-SG27
SG314	10	15	6	0.15	0.5	0.35	0.04	A	570-1170-ND	995-SG314
SG40	10	20	8	0.1	0.9	0.35	0.04	J	570-1171-ND	995-SG40
SG320	16	25	4	0.25	0.75	0.25	0.04	J	570-1448-ND	SG320
SG230	20	15	1.75	0.6	0.5	0.3	0.032	A	570-1173-ND	995-SG230
SG315	20	15	1.75	0.6	0.5	0.3	0.032	A	SG315-ND	995-SG315
SG240	40	15	2	0.6	0.625	0.25	0.032	B	570-1174-ND	995-SG240
SG316	40	15	2	0.6	0.625	0.25	0.032	B	SG316-ND	995-SG316
SG250	120	15	3	0.9	0.925	0.25	0.04	C	570-1176-ND	995-SG250
SG317	120	15	3	0.9	0.925	0.25	0.04	C	SG317-ND	995-SG317

For applications requiring ratings not shown, contact our technical support department.

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