AN9768 LITTELFUSE | Alldatasheet
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the parallel-connected load. These types of crowbar devices can have two limitations. the price of the added resistance is less effective clamping. demonstrated by the following comparison.
- If the source impedance is Z
typical nonlinear V130LA20A varistor at 51.7A is 330V.
- If the source impedance is only 5
which justifies the “educated guess” of 500A in the circuit. left for the interested reader to work out. TABLE 1. 3000V “OPEN-CIRCUIT” TRANSIENT VOLTAGE
corresponding change is only 330V to 400V. In other words, a variation of only 21% in the protective level achieved with a nonlinear suppressor occurs for a 10 to 1 error in the assumption made on the transient parameters, in contrast to a 447% variation in the protective level with a linear suppressor for the same error in assumption. Nonlinear voltage-clamping devices give the lowest clamping voltage, resulting in the best protection against transients. Crowbar Devices This category of suppressors, primarily gas tubes or carbon- block protectors, is widely used in the communication field where power-follow current is less of a problem than in power circuits. Another form of these suppressors is the hybrid circuit which uses solid-state or MOV devices. In effect, a crowbar device short-circuits a high voltage to ground. This short will continue until the current is brought to a low level. Because the voltage (arc or forward-drop) during the discharge is held very low, substantial currents can be carried by the suppressor without dissipating a considerable amount of energy within it. This capability is a major advantage. Volt-Time Response - When the voltage rises across a spark gap, no significant conduction can take place until transition to the arc mode has occurred by avalanche breakdown of the gas between the electrodes. Power-Follow - The second characteristic is that a power current from the steady-state voltage source will follow the surge discharge (called “follow-current” or “power-follow”). Voltage-Clamping Devices To perform the voltage limiting function, voltage-clamping devices at the beginning of the section depend on their nonlinear impedance in conjunction with the transient source impedance. Three types of devices have been used: reverse selenium rectifiers, avalanche (Zener) diodes and varistors made of different materials, i.e., silicon carbide, zinc oxide, etc. [1]. Selenium Cells - Selenium transient suppressors apply the technology of selenium rectifiers in conjunction with a special process allowing reverse breakdown current at high- energy levels without damage to the polycrystalline structure. These cells are built by developing the rectifier elements on the surface of a metal plate substrate which gives them good thermal mass and energy dissipation performance. Some of these have self-healing characteristics which allows the device to survive energy discharges in excess of the rated values for a limited number of operations characteristics that are useful, if not “legal” in the unsure world of voltage transients. The selenium cells, however, do not have the clamping ability of the more modern metal-oxide varistors or avalanche diodes. Consequently, their field of application has been considerably diminished. Zener Diodes - Silicon rectifier technology, designed for transient suppression, has improved the performance of regulator-type Zener diodes. The major advantage of these diodes is their very effective clamping, which comes closest to an ideal constant voltage clamp. Since the diode maintains the avalanche voltage across a thin junction area during surge discharge, substantial heat is generated in a small volume. The major limitation of this type of device is its energy dissipation capability. Silicon Carbide Varistors - Until the introduction of metal- oxide varistors, the most common type of “varistor” was made from specially processed silicon carbide. This material was very successfully applied in high-power, high-voltage surge arresters. However, the relatively low a values of this material produce one of two results. Either the protective level is too high for a device capable of withstanding line voltage or, for a device producing an acceptable protective level, excessive standby current would be drawn at normal voltage if directly connected across the line. Therefore, a series gap is required to block the normal voltage. In lower voltage electronic circuits, silicon carbide varistors have not been widely used because of the need for using a series gap, which increases the total cost and reproduces some of the characteristics of gaps described earlier. However, this varistor has been used as a current-limiting resistor to assist some gaps in clearing power-follow current. Metal-Oxide Varistors - A varistor functions as a nonlinear variable impedance. The relationship between the current in the device, I and the voltage across the terminals, V is typically described by a power law: I kV α . While more accurate and more complete equations can be derived to reflect the physics of the device, [2, 3] this definition will suffice here. A more detailed discussion will be found in Properties, Terminology and Theory”. The term α (alpha) in the equation represents the degree of nonlinearity of the conduction. A linear resistance has an α = 1. The higher the value of a, the better the clamp, which explains why α is sometimes used as a figure of merit. Quite naturally, varistor manufacturers are constantly striving for higher alphas. This family of transient voltage suppressors are made of sintered metal oxides, primarily zinc oxide with suitable additives. These varistors have α values considerably greater than those of silicon carbide varistors, typically in the range of an effective value of 15 to 30 measured over several decades of surge current. The high exponent values ( α ) of the metal-oxide varistors have opened completely new fields of applications by providing a sufficiently low protective level and a low standby current. The opportunities for applications extend from low- power electronics to the largest utility-type surge arresters. Application Note 9768
TABLE 2. CHARACTERISTICS AND FEATURES OF TRANSIENT VOLTAGE SUPPRESSOR TECHNOLOGY
As a rule the source impedance of the signal and the frequency as well as the capacitance of the transient suppressor should be considered. The current through C P is a function of dv/dt and the distortion is a function of the signal's source impedance. Each case must be evaluated individually to determine the maximum allowable capacitance. The structural characteristics of metal-oxide varistors unavoidably result in an appreciable capacitance between the device terminals, depending on area, thickness and material processing. For the majority of power applications, this capacitance can be of benefit. In high-frequency applications, however, the effect must be taken into consideration in the overall system design. References For Littelfuse documents available on the web, see http://www.littelfuse.com/ Concept in Station Arrester Design,” IEEE Trans. PAS-96, No. 2, March-April 1977, pp. 647-656. [2] Philipp, H.R. and L.M. Levinson, “Low Temperature Electrical Studies in Metal Oxide Varistors - A Clue to Conduction Mechanisms,” Journal of Applied Physics, Vol. 48, April 1977, pp. 1621-1627. [3] Philipp, H.R. and L.M. Levinson, “Zinc Oxide for Transient Suppression,” IEEE Trans. PHP , December 1977. [4] “Surge Arresters for Alternating Current Power Circuits,” ANSI Standard C62.1, IEEE Standard 28. [5] “Lightning Arresters. Part I: Nonlinear Resistor Type Arresters for AC Systems,” IEC Recommendation 99-1,1970. [6] Matsuoka, M., T. Masuyama and Y . Iida, “Supplementary Journal of Japanese Society of Applied Physics,” Vol. 39, 1970, pp. 94-101. Golden, “Metal-Oxide Varistor: A New Way to Suppress Transients,” Electronics, October 2, 1972. [8] Martzloff, F .D., “The Development of a Guide on Surge Voltages in Low - Voltage AC Power Circuits,” Report 81CRD047, General Electric, Schenectady, New Y ork, 1981. [9] Martzloff, F .D., “Varistor versus Environment: Winning the Rematch,” Report 85CRD037, General Electric, Schenectady, New Y ork, May 1985. Application Note 9768