Metal Oxide Varistor Degradation

发表自: www.kestar.com.cn 时间:2008-9-18

Metal Oxide Varistor Degradation
It is well-known that MOVs( metal oxide varistor) experience degradation due to single and multiple current impulses. The test results documented in Mardira, Saha and Sutton show that MOVs can be degraded from an 8/20us surge current at 1.5 times the rated MOV surge current. A 20 mm MOV with a 10 kA surge current rating will be degraded if a 15 kA single pulse surge current is applied.

When MOVs degrade they become more conductive after they have been stressed by either continuous current or surge current. MOVs generally experience degradation due to excessive surges exceeding the MOV’s rating while in operation. However, many MOVs show no signs of degradation when operated below a specified threshold voltage. The degradation of MOVs is primarily dependent on their composition and fabrication, as well as their application or duty.

Degraded MOVs were found to have smaller average grain size and change in the diffraction peak position compared to a new sample.5 The non-uniform temperature distribution in the material is due to the development of localized hot spotting during the current impulse and the dissolving in some other phases.

In high current conditions the zinc oxide junctions of the MOV begin to degrade resulting in a lower measured MCOV or turn-on voltage. As the degradation continues, and the MOV’s MCOV continues to drop until it conducts continuously, shorting or fragmenting within several seconds.

One of the key parameters related to measuring degradation of a varistor is leakage current. Leakage current in the pre-breakdown region of an MOV is important for two reasons:

1. Leakage determines the amount of watt loss an MOV is expected to generate upon application of a nominal steady-state operating voltage.

2. The leakage current determines the magnitude of the steady-state operating voltage that the MOV can accept without generating an excessive amount of heat.

The total leakage current is composed of a resistive current and a capacitive current. The resistive component of current is thermally stimulated and is significant, since it is responsible for the joule heating within the device. The capacitive current is a function of the MOV’s capacitance value and the applied ac voltage. If an MOV is subjected to an elevated voltage at a specific temperature, the internal current increases with time. Conversely, if the MOV is subjected to an elevated temperature at a specific applied voltage, the internal current increases with time. This phenomenon is accelerated by higher operating stress, and is further aggravated by elevated temperatures. The life of an MOV is primarily determined by the magnitude of the internal current and its increase in temperature, voltage, and time. As the current increases, the amount of heat (if not allowed to dissipate) can rapidly raise the temperature of the device. This condition may result in thermal runaway that can cause destruction of the MOV.

Tests were performed to induce thermal runaway. As for a 40 mm MOV is with an MCOV rating of 130 volts ac. during the test 240 V ac were applied at 15 amps and the MOV ignited.

MOVs exhibit greater power dissipation at higher temperatures given a fixed voltage. This characteristic can lead to thermal runaway. If the increase in power dissipation of the MOV occurs more rapidly than the MOV can transfer heat to the environment, the temperature of the MOV will increase until it is destroyed.

MOVs degrade gradually when subjected to surge currents above their rated capacity. The end-of-life is commonly specified when the measured varistor voltage (Vn) has changed by + 10 percent.4 MOVs usually are functional after the end-of-life, as defined. However, if an MOV experiences sequential surge events, each causing an additional 10 percent reduction of Vn, the MOV may soon reach a Vn level below the peak recurring value for the applied Vrms. When this state is reached the MOV draws in excess of 1 mA of current during each half-cycle of the sine wave voltage, a condition tantamount to thermal runaway. In nearly all cases, the value of Vn decreases with exposure to surge currents. The degradation manifests itself as an increase in idle current at the maximum normal operating voltage in the system. Excessive idle current during normal, steady-state operation will cause heating in the varistor. Because the varistor has a negative temperature coefficient, the current will increase as the varistor becomes hotter. Thermal runaway may occur, with consequent failure of the varistor.

Littelfuse publishes varistor pulse rating curves that are: The pulse rating curves plot the maximum surge current versus the impulse duration in seconds. It is noted that stresses above the conditions may cause permanent damage to the device.

Power Dissipation Ratings
If transients occur in rapid succession, the average power dissipation is the energy (watt-seconds) per pulse times the number of pulses per second. The power generated must be within the specifications shown in the chart above. Operating values must be derated at high temperatures. Note the rapid drop in rated value at temperature greater that 85C.

Varistors can dissipate a relative small amount of average power compared to surge power and are not suitable for repetitive applications that involve substantial amounts of power dissipation.

In the ANSI/IEEE C62.33 (1982) Standard for Surge Protective Devices the following is stated: "Single and lifetime pulse current ratings are appropriate tests of varistor surge withstand capability. In the absence of special requirements, energy ratings are recommended for use only as supplements to the predominant current ratings, and for application problems, which are more conveniently treated in terms of energy."