This invention relates to varistors, and more particularly to metal oxide varistors and methods used to reduce voltage increases (upturn) at high current levels, as well as to application employing these varistors.
In general, metal oxide varistor operation is governed by the following approximate equation: EQU I=(V/C).sup..alpha.
where I is the current through the varistor, V is the voltage across the varistor, .alpha. is the exponent or so-called coefficient of nonlinearity, and C is a parameter depending upon varistor geometry and material composition. For low values of current, the varistor behaves like a high resistance device. For higher values of current, the varistor behaves like a low resistance device. However, at very high current levels, there is a tendency for the varistor to become less conductive. This results in an increase in voltage across the varistor that is not predicted by the above equation but which is often interpreted as .alpha. being dependent on the current so that .alpha. decreases with increasing current at high current levels, resulting in an undesired voltage upturn. The value of .alpha. may be computed between any two points on the varistor current-voltage characteristic, such as, (I.sub.1, V.sub.1) and (I.sub.2, V.sub.2, by using the following formula, which may be readily found by elimination of the variable C from the above equation: EQU .alpha.=ln(I.sub.1 /I.sub.2)/ln (V.sub.1 /V.sub.2).
The varistor operating in accord with the above equations also exhibits the ability to clamp the voltage across it at a predeterminable voltage level known as the breakdown or clamping voltage. The clamping voltage is most apparently reflected in the value for C in the first equation above and is typically controlled by a judicious selection of the thickness of the varistor is which is typically manufactured in the form of a disk or other body of relatively uniform thickness.
At the low current end of the varistor characteristic curve, the device desirably exhibits a high resistance and a correspondingly low value of current for a given voltage. This behavior is often characterized by the so-called "leakage current" which is defined as the current at one-half of the clamping voltage. Unfortunately, improvement at the high end of the characteristic curve (to reduce voltage upturn) has a tendency to increase the leakage current.
Metal oxide varistors are typically manufactured from a powder mixture comprising one-half mole percent bismuth oxide, one-half mole percent cobalt oxide, one-half mole percent manganese oxide, one mole percent antimony oxide one-half mole percent tin oxide, 0.1 mole percent barium oxide and 0.1 mole percent boron oxide, the remaining powder being zinc xide. This powder is pressed into the shape of a disk or other shape of relatively uniform thickness. The pressed disk is then sintered to form a ceramic material. Typically, the pressed material is raised to a temperature between approximately 900.degree. C. and 1,500.degree. C. over a period of approximately ten hours. The material is then maintained at a relatively constant temperature for approximately three hours or sufficiently long to form a ceramic composition. The varistor ceramic is then cooled at approximately the same rate it was heated. That is to say, typical cooling from approximately 1,200.degree. C. to room temperature is carried out over a period of approximately ten hours at an approximate uniform rate of 120.degree. C. per hour. Additionally, dopants such as aluminum, indium, or gallium may be included in the varistor powder mixture prior to sintering.