Sidactor Protection Thyristor Products Page 12 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Overvoltage Protection Comparison The four most commonly used technologies for overvoltage protection are as follows: SIDACtor devices Gas Discharge Tubes (GDTs) Metal Oxide Varistors (MOVs) TVS diodes All four technologies are connected in parallel with the circuit being protected, and all exhibit a high off-state impedance when biased with a voltage less than their respective blocking voltages. SIDACtor devices A SIDACtor device is a PNPN device that can be thought of as a thyristor device without a gate. Upon exceeding its peak off-state voltage (V DRM ), a SIDACtor device will clamp a transient voltage to within the device's switching voltage (V S ) rating. Then, once the current flowing through the SIDACtor device exceeds its switching current, the device will crowbar and simulate a short-circuit condition. When the current flowing through the SIDACtor device is less than the device's holding current (I H ), the SIDACtor device will reset and return to its high off-state impedance. Advantages Advantages of the SIDACtor device include its fast response time (Figure 1.1), stable electrical characteristics, long term reliability, and Low capacitance. Also, because the SIDACtor device is a crowbar device, it cannot be damaged by voltage. Restrictions Because the SIDACtor device is a crowbar device, it cannot be used directly across the AC line; it must be placed behind a load. Failing to do so will result in exceeding the SIDACtor device's maximum on-state current rating, which may cause the device to enter a permanent short-circuit condition. Applications Although found in other applications, SIDACtor devices are primarily used as the principle overvoltage protector in telecommunications and data communications circuits. For applications outside this realm, follow the design criteria in " SIDACtor Device Selection Criteria" . Gas Discharge Tubes Gas discharge tubes (GDTs) are either glass or ceramic packages filled with an inert gas and capped on each end with an electrode. When a transient voltage exceeds the DC breakdown rating of the device, the voltage differential causes the electrodes of the gas tube to fire, resulting in an arc, which in turn ionizes the gas within the tube and provides a low impedance path for the transient to follow. Once the transient drops below the DC holdover voltage and current, the gas tube returns to its off state. Advantages Gas discharge tubes have high surge current and Low capacitance ratings. Current ratings can be as high as 20 kA, and capacitance ratings can be as low as 1 pF with a zero-volt bias. Applications Gas discharge tubes are typically used for primary protection due to their high surge rating. However, their low interference for high frequency components make them a candidate for high speed data links. Metal Oxide Varistors Metal Oxide Varistors (MOVs) are two-leaded, through- hole components typically shaped in the form of discs. Manufactured from sintered oxides and schematically equivalent to two back-to-back PN junctions, MOVs shunt transients by decreasing their resistance as voltage is applied. Advantages Since MOVs surge capabilities are determined by their physical dimensions, high surge current ratings are available. Also, because MOVs are clamping devices, they can be used as transient protectors in secondary AC power line applications. Applications Although MOVs are restricted from use in many telecom applications (other than disposable equipment), they are useful in AC applications where a clamping device is required and tight voltage tolerances are not.

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