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Power Adapter Transient Suppression Devices and Metal Oxide Varistors

Issue Time:2006-12-01
Transient suppression device
The ideal power adapter transient suppression device will open at normal voltage, can conduct electricity without delay under slight overvoltage, and does not allow voltage to increase during the clamping period, and can withstand unlimited current and power. After the stress has passed, it can return to the open state and the water is not damaged.

As of this point, no transient suppression device has been able to meet the above-mentioned ideal requirements under the various stresses specified in the IEEE 587 standard. The current effective transient protection requires the use of several devices, which require careful selection to provide effective protection under various voltage and current stresses.

In the low-intensity category A, silicon varistors are commonly used with transient suppression diodes, filter inductors and capacitors. In the higher power category B, these devices are equipped with higher rated current gas discharge tubes or sparks. The clearance is used in conjunction with a fast-responding fuse or circuit breaker when using a gas discharge device.

In order to effectively match the various suppression devices of the power adapter, their general performance parameters should be fully understood.

Metal oxide varistor
As the name suggests, metal oxide varistors (MOv exhibit voltage-dependent resistance characteristics. The device has high resistance and low load under the transition voltage. When the voltage across the transition exceeds the transition voltage, its resistance is drastically reduced. And the current increases sharply.

The main advantages of varistor are low cost and its relatively high transient energy absorption capability. The main disadvantage is that it gradually ages under repeated overpressure and has considerable dynamic resistance.

Limitations in varistor transient suppression applications are quite noticeable in medium to high-risk locations. At high exposure levels, the device ages rapidly, reducing its effective clamping. The varistor aging is not obvious and is not easy to measure, to some extent an internal process. Moreover, the relatively high dynamic resistance of the varistor means that its clamping effect on large current transients is very small, even for low-voltage varistors, the voltage at both ends is only a few tens of amps in transient current. Will exceed 1000V. As a result, if a metal oxide varistor is used alone, a destructive high voltage will be added to the device being protected. However, varistor is very useful if used in conjunction with other transient suppression devices.

Transient protection diode
There are a variety of transient protection diodes available as existing products, both unidirectional or bidirectional. The silicon transient protection diode consists of an avalanche voltage clamp device configured for high transient capability. In a bipolar protector, two junctions are connected in series back-to-back, and an avalanche diode is characterized by a common diode in the forward direction.

Transient protection diodes have two main advantages: one is that the clamping action is very fast, the avalanche condition can be established in a few nanoseconds, and the other is that the dynamic resistance in the conduction range is very low.

In the work area, the dynamic resistance is very small, and the voltage at both ends is only a few volts when the transient current is as high as several hundred amperes. From the surface, the transient protection diode provides a very reliable and efficient voltage clamp when any transient stress rises to the maximum current capacity of the diode. The characteristics of a typical 200V bipolar transient protection diode are shown in the figure. It should be noted that the terminal voltage is only 220V at 200A.

The main disadvantages of transient protection diodes are their relatively high price and limited current capacity. However, if the diode is over-voltage, it will be incapable of short-circuit conditions. In this case, to maintain the protection of the equipment, the external fuse is usually blown or the circuit breaker is disconnected.

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