Thyristor Semiconductor Products Page 22 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1001 Triac Gating Modes Of Operation When voltage is impressed suddenly across a PN junction, a charging current flows, equal to: When C ( dv __ dt ) becomes greater or equal to Thyristor I GT , the Thyristor switches on. Normally, this type of turn-on does not damage the device, providing the surge current is limited. Generally, Thyristor application circuits are designed with static dv/dt snubber networks if fast-rising voltages are anticipated. i = C ( dv __ dt ) Voltage Breakover Turn-on This method is used to switch on SIDACs and DIACs. However, exceeding voltage breakover of SCRs and Triacs is definitely not recommended as a turn-on method. In the case of SCRs and Triacs, leakage current increases until it exceeds the gate current required to turn on these gated Thyristors in a small localized point. When turn-on occurs by this method, localized heating in a small area may melt the silicon or damage the device if di/dt of the increasing current is not sufficiently limited. Triacs can be gated in four basic gating modes as shown in Figure AN1001.17. DIACs used in typical phase control circuits are basically protected against excessive current at breakover as long as the firing capacitor is not excessively large. When DIACs are used in a zener function, current limiting is necessary. SIDACs are typically pulse-firing, high-voltage transformers and are current limited by the transformer primary. The SIDAC should be operated so peak current amplitude, current duration, and di/dt limits are not exceeded. MT2 POSITIVE (Positive Half Cycle) MT2 NEGATIVE (Negative Half Cycle) MT1 MT2 + I GT REF QII MT1 I GT GATE MT2 REF MT1 MT2 REF MT1 MT2 REF QI QIV QIII ALL POLARITIES ARE REFERENCED TO MT1 (-) I GT GATE (+) I GT - I GT GATE (-) I GT GATE (+) + - NOTE: Alternistors will not operate in Q IV Figure AN1001.17 Gating Modes The most common quadrants for Triac gating-on are Quadrants I and III, where the gate supply is synchronized with the main terminal supply (gate positive -- MT2 positive, gate negative -- MT2 negative). Gate sensitivity of Triacs is most optimum in Quadrants I and III due to the inherent Thyristor chip construction. If Quadrants I and III cannot be used, the next best operating modes are Quadrants II and III where the gate has a negative polarity supply with an AC main terminal supply. Typically, Quadrant II is approximately current sensitivity in Quadrant II is lowest. Therefore, it is difficult for Triacs to latch on in Quadrant II when the main terminal current supply is very low in value. General Terminology The following definitions of the most widely-used Thyristor terms, symbols, and definitions conform to existing EIA- JEDEC standards: Special consideration should be given to gating circuit design when Quadrants I and IV are used in actual application, because Quadrant IV has the lowest gate sensitivity of all four operating quadrants. Breakover Point - Any point on the principal voltage-current characteristic for which the differential resistance is zero and where the principal voltage reaches a maximum value Principal Current - Generic term for the current through the collector junction (the current through main terminal 1 and main terminal 2 of a Triac or anode and cathode of an SCR) Principal Voltage - Voltage between the main terminals: (1) In the case of reverse blocking Thyristors, the principal voltage is called positive when the anode potential is higher than the cathode potential and negative when the anode potential is lower than the cathode potential. (2) For bidirectional Thyristors, the principal voltage is called positive when the potential of main terminal 2 is higher than the potential of main terminal 1. Off State - Condition of the Thyristor corresponding to the high-resistance, low-current portion of the principal voltage- current characteristic between the origin and the breakover point(s) in the switching quadrant(s) On State - Condition of the Thyristor corresponding to the low-resistance, low-voltage portion of the principal voltage- current characteristic in the switching quadrant(s). Fundamental Characteristics of Thyristors (continued)

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