Selecting a Littelfuse Varistor A pplication Note J u ly 1 9 9 9 AN9771.1 Introduction The varistor must operate under both a continuous operating (standby) mode as well as the predicted transient (normal) mode. The selection process, therefore, requires a knowledge of the electrical environment. When the environment is not fully defined, some approximations can be made. For most applications, the selection is a five-step process: 1. Determine the necessary steady-state voltage rating (working voltage) 2. Establish the transient energy absorbed by the varistor 3. Calculate the peak transient current through the varistor 4. Determine power dissipation requirements 5. Select a model to provide the required voltage-clamping characteristic A final consideration is to choose the appropriate package style to suit the application. Steady-State Voltage Rating Consider the maximum continuous voltage that will be applied to the varistor including any high line conditions (i.e., 110% or more of nominal voltage). Ratings are given for continuous sinusoidal AC and DC voltages. If a nonsinusoidal waveform is applied, the recurrent peak voltage should be limited to √2x VM(AC). Specifications for the UltraMOV™ Series varistor, for example, are shown in Table 1 for 140V AC rated devices to illustrate the use of the ratings and specifications table. VM(AC) - These models can be operated continuously with up to 140VRMS at 50Hz - 60Hz applied. They would be suitable for 120VAC nominal line operation and would allow for about a 120% high line condition. VM(DC) - Operation up to 180VDC applied continuously is allowed. Energy Transient energy ratings are given in the WTM column of the specifications in joules (watt-second). The rating is the maximum allowable energy for a single impulse of 10/1000µs current waveform with continuous voltage applied. Energy ratings are based on a shift of VN of less than ±10% of initial value. When the transient is generated from the discharge of an inductance (i.e., motor, transformer) or a capacitor, the source energy can be calculated readily but, in most cases the transient is from a source external to the equipment and is of unknown magnitude. For this situation an approximation technique can be used to estimate the energy of the transient absorbed by the varistor. The method requires finding the transient current and voltage applied to the varistor. To determine the energy absorbed the following equation applies: E = τ ∫0 VC ( t )I ( t )∆t = KV C Iτ where I is the peak current applied, VC is the clamp voltage which results, τ is the impulse duration and K is a constant. K values are given in Figure 1 for a variety of waveshapes frequently encountered. The K value and pulse width correspond to the current waveform only, assuming the varistor voltage waveform is almost constant during the current impulse. For complex waveforms, this approach also can be used by dividing the shape into segments that can be treated separately. TABLE 1. ULTRAMOV RATINGS AND SPECIFICATIONS EXAMPLE MAXIMUM RATING (85oC) CONTINUOUS DEVICE MODEL NUMBER MODEL BRANDNUMBER ING CHARACTERISTICS (25oC) TRANSIENT VARISTOR VOLTAGE AT 1mA DC TEST CURRENT RMS VOLTS DC VOLTS ENERGY 2ms PEAK CURRENT 8 x 20µs VM(AC) VM(DC) WTM ITM ITM 2 x PULSE 1 x PULSE (V) (V) (J) (A) VNOM MIN (A) VNOM MAX (V) MAXIMUM CLAMPING VOLTAGE 8 x 20µs TYPICAL CAPACITANCE VC IPK f = 1MHz (V) (A) (pF) V07E140 7V140 140 180 13.5 1200 1750 200 240 360 10 160 V10E140 10V140 140 180 27.5 2500 3500 200 240 360 25 400 V14E140 14V140 140 180 55 4500 6000 200 240 360 50 900 V20E140 20V140 140 180 110 6500 10000 200 240 360 100 1750 10-121 UltraMOV™ is a trademark of Littelfuse, Inc. 1-800-999-9445 or 1-847-824-1188 | Copyright © Littelfuse, Inc. 1999 Application Note 9771 WAVESHAPE K† EQUATION 0.637 IPK I Π PK sin ----t τ Section (1) E = kVC Iτ = (0.5) (500) (100) (5) (10-6) t τ Peak Current t I PK -- τ 0.86 IPK 0.5 IPK I PK sin ( πt )e t – t /τ 1.4 IPK I PK e -t/1.44τ 0.5 IPK t τ IPK The peak current rating can be checked against the transient current measured in the circuit. If the transient is generated by an inductor, the peak current will not be more than the inductor current at the time of switching. Another method for finding the transient current is to use a graphical analysis. When the transient voltage and source impedance is known, a Thevenin equivalent circuit can be modeled. Then, a load line can be drawn on the log - log, V-I characteristic as shown in Figure 3. The two curves intersect at the peak current value. The rated single pulse current, ITM, is the maximum allowable for a single pulse of 8/20µs exponential waveform (illustrated in Application Note AN9767, Figure 21). For longer duration pulses, ITM should be derated to the curves in the varistor specifications. Figure 4 shows the derating curves for 7mm size, LA series devices. This curve also provides a guide for derating current as required with repetitive pulsing. The designer must consider the total number of transient pulses expected during the life of the equipment and select the appropriate curve. 1.0 Where the current waveshape is different from the exponential waveform of Figure 11 of AN9767, the curves of Figure 4 can be used by converting the pulse duration on the basis of equivalent energy. This is easily done using the constants given in Figure 1. For example, suppose the actual current measured has a triangular waveform with a peak current of 10A, a peak voltage of 340V and an impulse duration of 500µs. IPK t τ 3.28J Total 0.5 t τ † = 0.13J Section (2) E = kVC Iτ = (1.4) (500) (100) (50-5) 10-6) = 3.15J IPK τ The waveform is divided into two parts that are treated separately using the factors of Figure 1: current waveform Section (1) 0 to 5µs and (2) 5µs to 50µs. The maximum voltage across the V130LA1 at 100A is found to be 500V from the V-I characteristics of the specification sheet. Based upon alpha of 25 to 40 FIGURE 1. ENERGY FORM FACTOR CONSTANTS ZS Consider the condition where the exponential waveform shown below is applied to a V130LA1 type Littelfuse Varistor. IV VOC VR 100A FIGURE 3A. EQUIVALENT CIRCUIT 50A t 0 5µs 50µs FIGURE 2. 10-122 The pulse rise portion of the waveform can be ignored when the impulse duration is five times or more longer. The maximum number of pulses for the above example would exceed 104 from the pulse derating curves shown in Figure 4. VR = VOC -IZS VOC Varistor Voltage VC CLAMP VOLTAGE VARISTOR V-I CHARACTERISTIC IV -VOC/ZS LOG VARISTOR CURRENT (A) FIGURE 3B. GRAPHICAL ANALYSIS TO DETERMINE PEAK I FIGURE 3. DETERMINING VARISTOR PEAK CURRENT FROM A VOLTAGE SOURCE TRANSIENT 2,000 RATED PEAK PULSE CURRENT (A) 1,000 500 1 2 NUMBER OF PULSES 10 200 MODEL SIZE 7mm V130LA1 - V300LA4 103 104 102 100 105 50 20 106 10 5 2 1 INDEFINITE NUMBER OF PULSES The varistor nominal voltage (VNOM or VN) represents the applied voltage where the varistor transitions from its “standby” mode to its low impedance “clamping” mode. It is measured at the 1mA conduction point. The minimum and maximum limit values are specified in the ratings table. Power Dissipation Requirements Transients generate heat in a suppressor too quickly to be transferred during the pulse interval. Power dissipation capability is of concern for a suppressor if transients will be occurring in rapid succession. Under this condition, the power dissipation required is simply the energy (watt-seconds) per pulse times the number of pulses per second. The power so developed must be within the specifications shown on the ratings tables for the specific device type. It is to be noted that varistors can only dissipate a relatively small amount of average power and are, therefore, not suitable for repetitive applications that involve substantial amounts of average power dissipation (likewise, varistors are not suitable as voltage regulation devices). Furthermore, the operating values need to be derated at temperatures above the absolute maximum limits as shown in Figure 5. CH, CP CS, RA SERIES 20 100 1,000 IMPULSE DURATION (µs) 10,000 FIGURE 4. PEAK CURRENT DERATING BASED ON PULSE WIDTH AND NUMBER OF APPLIED PULSES Then: E = (.5)(10)(340)(500)(10-6) = 850mJ The equivalent exponential waveform of equal energy is then found from: ETRIANGULAR = EEXP 850mJ = 1.4 VCIτEXP The exponential waveform is taken to have equal VC and I values. Then, 850mJ 1.4 (340) (10) = 179µs 100 PERCENT OF RATED VALUE LOG VARISTOR (V) Application Note 9771 90 80 70 60 50 40 30 BA/BB, CA, DA/DB, LA, “C” III, HA, NA, MA, UltraMOV, PA, ZA SERIES 20 10 0 -55 50 60 70 80 90 100 110 120 130 140 150 AMBIENT TEMPERATURE (oC) FIGURE 5. CURRENT, ENERGY, POWER DERATING vs TEMPERATURE τEXP = Voltage Clamping Selection Or: τEXP = K*τ∗ 1.4 Where: K* and τ* are the values for the triangular waveform and τEXP is the impulse duration for the equivalent exponential waveform. 10-123 Transient V-I characteristics are provided in the specifications for all models of varistors. Shown below in Figure 6 are curves for 130VAC rated models of the LA series. These curves indicate the peak terminal voltage measured with an applied 8/20µs impulse current. For example, if the peak impulse current applied to a V130LA2 is 10A, that model will limit the transient voltage to no higher than 340V. Application Note 9771 The ability of the varistor to limit the transient voltage is sometimes expressed in terms of a clamp ratio. For example, consider a varistor applied to protect the power terminals of electrical equipment. If high line conditions will allow a rise to 130VAC , then 184V peak would be applied. The device selected would require a voltage rating of 130VACRMS or higher. Assume selection of a V130LA2 model varistor. The V130LA2 will limit transient voltages to 340V at currents of 10A. The clamp ratio is calculated to be, 1000 8000 6000 5000 4000 MAXIMUM CLAMPING VOLTAGE COMPARED BY MODEL SIZE VM(AC) = 130V RATING TA = -55 TO 85oC MAXIMUM PEAK (V) 3000 2000 UL1449 CORD CONNECTED AND DIRECT PLUG-IN CATEGORY 1500 V130LA2 Clamp Ratio = 1000 800 V130LA10A 400 IMPULSE GENERATOR LOAD LINES (IMPLIED) UL1449 PERMANENTLY CONNECTED CATEGORY, AND ANSI/IEEE C61.41 (IEEE587) CATEGORY B 100 101 102 103 = 1.85 The clamp ratio can be found for other currents, of course, by reference to the V-I characteristic. In general, clamping ability will be better as the varistor physical size and energy level increases. This is illustrated in Figure 7 which compares the clamping performance of the different Littelfuse Varistor families. It can be seen that the lowest clamping voltages are obtained from the 20mm (LA series) and 60mm (BA series) products. In addition, many varistor models are available with two clamping selections, designated by an A, B, or C at the end of the model number. The A selection is the standard model, with B and C selections providing progressively tighter clamping voltage. For example, the V130LA20A voltage clamping limit is 340V at 100A, while the V130LA20B clamps at not more than 325V. V130LA20A 300 100 340V 184V = 600 500 200 VC at 10A Peak Voltage Applied V130LA5 104 PEAK AMPERES 8/20µs WAVESHAPE FIGURE 6. TRANSIENT V-I CHARACTERISTICS OF TYPICAL LA SERIES MODELS If the transient current is unknown, the graphical method of Figure 3 can be utilized. From a knowledge of the transient voltage and source impedance a load line is plotted on the V-I characteristic. The intersection of the load line with the varistor model curve gives the varistor transient current and the value of clamped peak transient voltage. MAXIMUM CLAMP RATIO AND MAXIMUM INSTANTANEOUS VOLTAGE 1000 4 800 MA4 LA4 600 3 2 500 400 300 1.5 LA10 PA, LA20 BA 200 1 RATIO NOTE: CLAMP RATIO EQUALS VARISTOR VOLTAGE DIVIDED BY VNOM OR 184V FOR 130VACRMS 100 0.01 0.05 0.1 0.5 1.0 5 10 50 100 500 1K 5K 10K INSTANTANEOUS CURRENT (A) FIGURE 7. VARISTOR V-I CHARACTERISTICS FOR FOUR PRODUCT FAMILIES RATED AT 130VAC 10-124 Application Note 9771 MAXIMUM STEADY-STATE APPLIED VOLTAGE PEAK CURRENT ENERGY (A) (J) 80 500 0.5 - 5.0 30 1000 0.1 - 25 40 - 100 0.07 1.7 50 - 6500 0.1 - 52 100 - 6500 0.4 - 160 1,200 10,000 11 - 400 6500 70 - 250 25,000 40,000 270 1,050 50,000 70,000 450 10,000 30,000 40,000 270 1050 20,000 70,000 200 10,000 65,000 100,000 2,200 12,000 VOLTS AC RMS 4 10 25 150 130 264 250 275 460 660 750 1,000 2,800 6,000 VOLTS DC 3.5 14 35 200 175 365 330 369 615 850 970 1,200 3,500 7,000 22, 20, 16 GAUGE CP, SERIES AUML †, ML †, MLE †, MLN CH SERIES 0603 0805 1206 1210 1812 2220 5 x 8mm †, MA SERIES 3mm 5, 7, 10, 14, 20 (mm) ZA SERIES 5 x 8, 10 x 16, 14 x 22 (mm) RA SERIES 7, 10, 14, 20 (mm) C-III, LA, UltraMOV SERIES 20mm PA SERIES 32, 34 40 (mm) HA, HB, DA/ DB SERIES BA/ BB SERIES 60mm NA SERIES 34mm SQ. CA SERIES 32, 40, 60 (mm) AS †† SERIES † Littelfuse multilayer suppression technology. FIGURE 8. VARISTOR PACKAGE STYLES AND RATINGS RANGE 10-125 DISC SIZES/ PACKAGES 32, 42, 60 (mm) Application Note 9771 Varistor Ordering Information The varistor part number includes ratings information. Some types include the working voltage, others indicate the nominal voltage. See the varistor ordering nomenclature guides below. ULTRAMOV TYPES V XX E XXX LX X X DEVICE FAMILY: NONSTANDARD LEAD SPACING OPTIONS (DO NOT ADD IF STANDARD) (NOTE 2): Varistor 5 7 1 DISC DIAMETER: = 5mm Lead Spacing = 7.5mm Lead Spacing = 10mm Lead Spacing 07, 10, 14, or 20 (mm) PACKAGING: ENCAPSULATION: LEAD FORMATION: E = Epoxy L1 L2 L3 L4 VM(AC)RMS: 130 to 625 (V) = = = = Straight Crimped In-Line Trim/Crimp (Bulk pack only) B = Bulk Pack T = Tape and Reel A = Ammo Pack OTHER VARISTOR TYPES BA, BB, CA, CP, CS, DA, DB, HA, HB, LA, NA, PA, VARISTOR SERIES V 130 LA 20 A CH, MA, ZA, VARISTOR SERIES V 220 MA 4 A Selection - Clamping Voltage (A or B) Selection - Clamping Voltage (A or B) Relative Energy Indicator or Disc Size Product Series Max RMS Applied Voltage V = Metal-Oxide Varistor (MOV) The five major considerations for varistor selection have been described. The final choice of a model is a balance of these factors with device packaging and cost trade-offs. In some applications a priority requirement such as clamp voltage or energy capability may be so important as to force the selection to a particular model. Figure 8 illustrates the Littelfuse varistor package styles in a matrix that compares energy and current ratings to the working voltage range. 10-126 Relative Energy Indicator Product Series VN(DC) Nominal Varistor Voltage MOV Varistor

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