Sidactor Protection Thyristor Products Page 9 SIDACtor Protection Thyristor Products

SIDACtor Protection Thyristors 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 02/23/17 Construction and Operation SIDACtor devices are thyristor devices used to protect sensitive circuits from electrical disturbances caused by lightning-induced surges, inductive-coupled spikes, and AC power fault conditions. The unique structure and characteristics of the thyristor are used to create an overvoltage protection device with precise and repeatable turn-on characteristics with low voltage overshoot and high surge current capabilities. Key Parameters Key parameters for SIDACtor devices are V DRM , I DRM , V S , I H , and V T (please refer to Figure 1.3 on page 11). V DRM is the repetitive peak off-state voltage rating of the device (also known as stand-off voltage) and is the continuous peak combination of AC and DC voltage that may be applied to the SIDACtor device in its off-state condition. I DRM is the maximum value of leakage current that results from the application of V DRM . Switching voltage (V S ) is the maximum voltage that subsequent components may be subjected to during a fast-rising (100 V/s) overvoltage condition. Holding current (I H ) is the minimum current required to maintain the device in the on state. On-state voltage (V T ) is the maximum voltage across the device during full conduction. Operation The device operates much like a switch. In the off state, the device exhibits leakage currents (I DRM ) less than 5 A, making it invisible to the circuit it is protecting. As a transient voltage exceeds the device's V DRM , the device begins to enter its protective mode with characteristics similar to an avalanche diode. When supplied with enough current (I S ), the device switches to an on state, shunting the surge from the circuit it is protecting. While in the on state, the device is able to sink large amounts of current because of the low voltage drop (V T ) across the device. Once the current flowing through the device is either interrupted or falls below a minimum holding current (I H ), the device resets, returning to its off state. If the I PP rating is exceeded, the device typically becomes a permanent short circuit. Physics The device is a semiconductor device characterized as having four layers of alternating conductivity: PNPN (Figure 1.2 below). The four layers include an emitter layer, an upper base layer, a mid-region layer, and a lower base layer. The emitter is sometimes referred to as a cathode region, with the lower base layer being referred to as an anode region. P N P Figure 1.2 Geometric Structure of Bidirectional SIDACtor devices As the voltage across the device increases and exceeds the device's V DRM , the electric field across the center junction reaches a value sufficient to cause avalanche multiplication. As avalanche multiplication occurs, the impedance of the device begins to decrease, and current flow begins to increase until the device's current gain exceeds unity. Once unity is exceeded, the device switches from a high impedance (measured at V S ) to a low impedance (measured at V T ) until the current flowing through the device is reduced below its holding current (I H ).

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