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Fundamental Characteristics of Thyristors Application Note

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Teccor® brand Thyristors AN1001 Fundamental Characteristics of Thyristors The connections between the two transistors trigger Introduction the occurrence of regenerative action when a proper The Thyristor family of semiconductors consists of several gate signal is applied to the base of the NPN transistor. very useful devices. The most widely used of this family Normal leakage current is so low that the combined hFE are silicon controlled rectifiers (SCRs), Triacs, SIDACs, and of the specially coupled two-transistor feedback amplifier DIACs. In many applications these devices perform key is less than unity, thus keeping the circuit in an off-state functions and are real assets in meeting environmental, condition. A momentary positive pulse applied to the gate speed, and reliability specifications which their electro- biases the NPN transistor into conduction which, in turn, mechanical counterparts cannot fulfill. biases the PNP transistor into conduction. The effective hFE momentarily becomes greater than unity so that the This application note presents the basic fundamentals specially coupled transistors saturate. Once saturated, of SCR, Triac, SIDAC, and DIAC Thyristors so the user current through the transistors is enough to keep the understands how they differ in characteristics and combined hFE greater than unity. The circuit remains “on” parameters from their electro-mechanical counterparts. until it is “turned off” by reducing the anode-to-cathode Also, Thyristor terminology is defined. current (IT) so that the combined hFE is less than unity and regeneration ceases. This threshold anode current is the holding current of the SCR. SCR Geometric Construction Basic Operation Figure AN1001.3 shows cross-sectional views of an SCR Figure AN1001.1 shows the simple block construction of an chip and illustrations of current flow and junction biasing in SCR. both the blocking and triggering modes. Anode Anode Gate Cathode Cathode P (+) IGT (-) (-) J1 Forward Blocking N N J2 P Junction Gate P J3 Gate N N P (+) Cathode Cathode (+) IT Anode Anode Block Construction Schematic Symbol Forward Bias and Current Flow Equivalent Diode Relationship Figure AN1001.1 SCR Block Construction Gate Cathode Cathode (+) The operation of a PNPN device can best be visualized as Reverse Biased (+) Gate Junction a specially coupled pair of transistors as shown in Figure P N AN1001.2. N P Reverse Biased Anode Load Anode (-) Junction (-) Anode P Anode Reverse Bias Equivalent Diode N P J1 Relationship N N J2 J2 N Gate P P P J3 Figure AN1001.3 Cross-sectional View of SCR Chip N Gate N Cathode Cathode Two-transistor Two-transistor Block Schematic Construction Equivalent Figure AN1001.2 Coupled Pair of Transistors as a SCR ©2013 Littelfuse, Inc Fundamental Characteristics of Thyristors 389 Specifications are subject to change without notice. Revised: 09/23/13 Teccor® brand Thyristors AN1001 Triac Basic Operation Geometric Construction Figure AN1001.4 shows the simple block construction of a Figure AN1001.6 show simplified cross-sectional views of a Triac. Its primary function is to control power bilaterally in Triac chip in various gating quadrants and blocking modes. an AC circuit. Main MT1(-) Terminal 1 GATE(+) Main N I (MT1) GT Terminal 2 P N P Gate N N P (MT2) N N MT1(-) N MT2 Block Construction P N IT MT2(+) Blocking QUADRANT I Junction GATE(-) MT1(-) Gate IGT N N P MT2(+) N MT1 Equivalent Diode P N Schematic Symbol Relationship MT2(+) Figure AN1001.4 Triac Block Construction QUADRANT II Operation of a Triac can be related to two SCRs connected in parallel in opposite directions as shown in Figure AN1001.5. GATE(-) MT1(+) IGT N N Although the gates are shown separately for each SCR, P a Triac has a single gate and can be triggered by either N polarity. P N MT1(+) MT2(-) IT MT1 QUADRANT III Blocking GATE(+) MT1(+) Junction IGT N N P N P N MT2(-) MT2(-) IT Equivalent Diode Relationship QUADRANT IV Figure AN1001.6 Simplified Cross-sectional of Triac Chip MT2 Figure AN1001.5 SCRs Connected as a Triac Since a Triac operates in both directions, it behaves essentially the same in either direction as an SCR would behave in the forward direction (blocking or operating). ©2013 Littelfuse, Inc Fundamental Characteristics of Thyristors 390 Specifications are subject to change without notice. Revised: 09/23/13 Teccor® brand Thyristors AN1001 SIDAC DIAC Basic Operation Basic Operation The SIDAC is a multi-layer silicon semiconductor switch. The construction of a DIAC is similar to an open base Figure AN1001.7 illustrates its equivalent block construction NPN transistor. Figure AN1001.9 shows a simple block using two Shockley diodes connected inverse parallel. construction of a DIAC and its schematic symbol. Figure AN1001.7 also shows the schematic symbol for the SIDAC. MT1 MT1 N P N MT1 MT2 MT1 MT2 Block Construction Schematic Symbol P1 N2 N2 Figure AN1001.9 DIAC Block Construction P3 P3 N4 The bidirectional transistor-like structure exhibits a high- N4 impedance blocking state up to a voltage breakover point P5 (VBO) above which the device enters a negative-resistance region. These basic DIAC characteristics produce a bidirectional pulsing oscillator in a resistor-capacitor AC MT2 MT2 Equivalent Diode Relationship Schematic Symbol circuit. Since the DIAC is a bidirectional device, it makes a good economical trigger for firing Triacs in phase control Figure AN1001.7 SIDAC Block Construction circuits such as light dimmers and motor speed controls. Figure AN1001.10 shows a simplified AC circuit using a The SIDAC operates as a bidirectional switch activated DIAC and a Triac in a phase control application. by voltage. In the off state, the SIDAC exhibits leakage currents (IDRM) less than 5 µA. As applied voltage exceeds the SIDAC VBO, the device begins to enter a negative Load resistance switching mode with characteristics similar to an avalanche diode. When supplied with enough current (IS), the SIDAC switches to an on state, allowing high current to flow. When it switches to on state, the voltage across the device drops to less than 5 V, depending on magnitude of the current flow. When the SIDAC switches on and drops into regeneration, it remains on as long as holding current is less than maximum value (150 mA, typical value of 30 mA to 65 mA). The switching current (IS) is very near the holding current (IH) value. When the SIDAC Figure AN1001.10 AC Phase Control Circuit switches, currents of 10 A to 100 A are easily developed by discharging small capacitor into primary or small, very high- Geometric Construction voltage transformers for 10 µs to 20 µs. The main application for SIDACs is ignition circuits or MT1 MT1 inexpensive high voltage power supplies. N Geometric Construction P N MT1 MT2 MT2 P1 Cross-section of Chip Equivalent Diode N2 Relationship P3 Figure AN1001.11 Cross-sectional View of DIAC Chip N4 P5 MT2 Figure AN1001.8 Cross-sectional View of a Bidirectional SIDAC Chip with Multi-layer Construction ©2013 Littelfuse, Inc Fundamental Characteristics of Thyristors 391 Specifications are subject to change without notice. Revised: 09/23/13 Teccor® brand Thyristors AN1001 Electrical Characteristic Curves of Thyristors +I +I Voltage Drop (VT) at I Specified Current (iT) T Latching Current (IL) I H RS Off - State Leakage Reverse Leakage Current - (IDRM) at IS Current - (IRRM) at Specified VDRM Specified V Minimum Holding RRM IBO Current (IH) IDRM -V +V -V +V VBO Specified Minimum VT V Specified Minimum S Off - State (VBO - VS) Reverse Blocking = Blocking RS VDRM Voltage (VRRM) Voltage (VDRM) (IS - IBO) Reverse Forward Breakdown Breakover Voltage Voltage -I -I Figure AN1001.12 V-I Characteristics of SCR Device Figure AN1001.15 V-I Characteristics of a SIDAC Chip +I Methods of Switching on Thyristors Voltage Drop (vT) at Three general methods are available for switching Specified Current (iT) Latching Current (IL) Thyristors to on-state condition: • Application of gate signal Off-state Leakage Current – (IDRM) at • Static dv/dt turn-on Specified VDRM Minimum Holding • Voltage breakover turn-on Current (IH) -V +V Application Of Gate Signal Specified Minimum Gate signal must exceed I and V requirements of the Off-state GT GT Blocking Thyristor used. For an SCR (unilateral device), this signal Voltage (VDRM) must be positive with respect to the cathode polarity. A Triac (bilateral device) can be turned on with gate signal of Breakover either polarity; however, different polarities have different Voltage -I requirements of IGT and VGT which must be satisfied. Since DIACs and SIDACs do not have a gate, this method of turn- Figure AN1001.13 V-I Characteristics of Triac Device on is not applicable. In fact, the single major application of DIACs is to switch on Triacs. Static dv/dt Turn-on +I Static dv/dt turn-on comes from a fast-rising voltage 10 mA ∆V applied across the anode and cathode terminals of an SCR or the main terminals of a Triac. Due to the nature of Thyristor construction, a small junction capacitor is formed Breakover across each PN junction. Figure AN1001.16 shows how Current IBO typical internal capacitors are linked in gated Thyristors. -V +V Breakover Voltage VBO -I Figure AN1001.14 V-I Characteristics of Bilateral Trigger DIAC Figure AN1001.16 Internal Capacitors Linked in Gated Thyristors ©2013 Littelfuse, Inc Fundamental Characteristics of Thyristors 392 Specifications are subject to change without notice. Revised: 09/23/13 Teccor® brand Thyristors AN1001 When voltage is impressed suddenly across a PN junction, The most common quadrants for Triac gating-on are a charging current flows, equal to: Quadrants I and III, where the gate supply is synchronized with the main terminal supply (gate positive -- MT2 positive, i = C d v__ ( dt) gate negative -- MT2 negative). Gate sensitivity of Triacs is most optimum in Quadrants I and III due to the inherent When C d v__ becomes greater or equal to Thyristor I , Thyristor chip construction.
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