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Thyristor Theory and Design Considerations Thyristor Theory and Design Considerations Handbook HBD855/D Rev. 1, Nov−2006 © SCILLC, 2005 Previous Edition © 2005 as Excerpted from DL137/D “All Rights Reserved’’ http://onsemi.com 1 ABOUT THYRISTORS Thyristors can take many forms, but they have certain capacitor discharge ignitors, engine ignition systems, and things in common. All of them are solid state switches many other kinds of equipment. which act as open circuits capable of withstanding the Although thyristors of all sorts are generally rugged, rated voltage until triggered. When they are triggered, there are several points to keep in mind when designing thyristors become low−impedance current paths and circuits using them. One of the most important is to remain in that condition until the current either stops or respect the devices’ rated limits on rate of change of drops below a minimum value called the holding level. voltage and current (dv/dt and di/dt). If these are Once a thyristor has been triggered, the trigger current can exceeded, the thyristor may be damaged or destroyed. On be removed without turning off the device. the other hand, it is important to provide a trigger pulse Silicon controlled rectifiers (SCRs) and triacs are both large enough and fast enough to turn the gate on quickly members of the thyristor family. SCRs are unidirectional and completely. Usually the gate trigger current should be devices where triacs are bidirectional. An SCR is at least 50 percent greater than the maximum rated gate designed to switch load current in one direction, while a trigger current. Thyristors may be driven in many triac is designed to conduct load current in either different ways, including directly from transistors or logic families, power control integrated circuits, by direction. optoisolated triac drivers, programmable unijunction Structurally, all thyristors consist of several alternating transistors (PUTs) and SIDACs. These and other design layers of opposite P and N silicon, with the exact structure considerations are covered in this manual. varying with the particular kind of device. The load is Of interest too, is a new line of Thyristor Surge applied across the multiple junctions and the trigger Suppressors in the surface mount SMB package covering current is injected at one of them. The trigger current surge currents of 50, 80 and 100 amps, with breakover allows the load current to flow through the device, setting voltages from 77 to 400 volts. NP Series Thyristor Surge up a regenerative action which keeps the current flowing Protector Devices (TSPD) protect telecommunication even after the trigger is removed. circuits such as central office, access, and customer These characteristics make thyristors extremely useful premises equipment from overvoltage conditions. These in control applications. Compared to a mechanical switch, are bidirectional devices so they are able to have a thyristor has a very long service life and very fast turn functionality of 2 devices in one package, saving valuable on and turn off times. Because of their fast reaction times, space on board layout. These devices will act as a crowbar regenerative action and low resistance once triggered, when overvoltage occurs and will divert the energy away thyristors are useful as power controllers and transient from circuit or device that is being protected. Use of the overvoltage protectors, as well as simply turning devices NP Series in equipment will help meet various regulatory on and off. Thyristors are used in motor controls, requirements including: GR−1089−CORE, IEC incandescent lights, home appliances, cameras, office 61000−4−5, ITU K.20/21/45, IEC 60950, TIA−968−A, equipment, programmable logic controls, ground fault FCC Part 68, EN 60950, UL 1950. See ON interrupters, dimmer switches, power tools, Semiconductor application note AND8022/D for telecommunication equipment, power supplies, timers, additional information. http://onsemi.com 2 CHAPTER 1 Theory and Applications Sections 1 thru 9 Page Page Section 1: Symbols and Terminology . 4 Interfacing Digital Circuits to Thyristor Section 2: Theory of Thyristor Operation . 10 Controlled AC Loads. 118 Basic Behavior. 10 DC Motor Control with Thyristors. 127 Switching Characteristics. 13 Programmable Unijunction Transistor (PUT) False Triggering. 15 Applications. 132 Theory of SCR Power Control. 16 Triac Zero−Point Switch Applications. 136 Triac Theory. 22 AN1045 — Series Triacs in AC High Voltage Switching Circuits. 141 Methods of Control. 24 AN1048 — RC Snubber Networks for Thyristor Zero Point Switching Techniques. 25 Power Control and Transient Suppression. 152 Section 3: Thyristor Drivers and Triggering . 29 AND8005 — Automatic AC Line Voltage Pulse Triggering of SCRs. 29 Selector. 174 Effect of Temperature, Voltage and Loads. 33 AND8006 — Electronic Starter for Flourescent Using Negative Bias and Shunting. 35 Lamps. 177 Snubbing Thyristors. 38 AND8007 — Momentary Solid State Switch Using Sensitive Gate SCRs. 40 for Split Phase Motors. 181 Drivers: Programmable Unijunction AND8008 — Solid State Control Solutions Transistors. 44 for Three Phase 1 HP Motor. 186 Section 4: The SIDAC, A New High Voltage AND8015 — Long Life Incandescent Lamps Bilateral Trigger . 49 using SIDACs. 194 Section 5: SCR Characteristics . 60 AND8017 — Solid State Control for SCR Turn−Off Characteristics. 60 Bi−Directional Motors. 198 SCR Turn−Off Mechanism. 60 Section 7: Mounting Techniques for Thyristors . 201 Mounting Surface Considerations. 202 SCR Turn−Off Time tq . 60 Thermal Interface. 203 Parameters Affecting tq . 65 Characterizing SCRs for Crowbar Applications. 71 Insulation Considerations. 204 Fastening Techniques. 209 Switches as Line−Type Modulators. 79 Insulated Packages. 210 Parallel Connected SCRs. 85 Surface Mount Devices. 212 RFI Suppression in Thyristor Circuits. 89 Thermal System Evaluation. 214 Section 6: Applications . 93 Section 8: Reliability and Quality . 218 Phase Control with Thyristors. 93 Using Transient Thermal Resistance Data in Motor Control. 94 High Power Pulsed Thyristor Applications. 218 Phase Control with Trigger Devices. 102 Thyristor Construction. 230 Cycle Control with Optically Isolated In−Process Controls and Inspections. 230 Triac Drivers. 105 Reliability Tests. 231 AC Power Control with Solid−State Relays. 110 Stress Testing. 233 Triacs and Inductive Loads. 114 Environmental Testing. 233 Inverse Parallel SCRs for Power Control. 117 Section 9: Appendices . 234 http://onsemi.com 3 SECTION 1 SYMBOLS AND TERMINOLOGY SYMBOLS The following are the most commonly used schematic symbols for Thyristors: Name of Device Symbol Silicon Controlled G Rectifier (SCR) A K Triac MT2 MT1 G Thyristor Surge Protective MT1 MT2 Devices & Sidac Programmable Unijunction G Transistor (PUT) A K http://onsemi.com 4 THYRISTOR TERMINOLOGY (The following terms are used in SCR and TRIAC specifications.) Symbol Terminology Definition di/dt CRITICAL RATE OF RISE OF ON−STATE The maximum rate of change of current the device will CURRENT withstand after switching from an off−state to an on−state when using recommended gate drive. In other words, the maximum value of the rate of rise of on−state current which a Triac or SCR can withstand without damage. (di/dt)c RATE OF CHANGE OF COMMUTATING Is the ability of a Triac to turn off itself when it is driving an CURRENT (Triacs) inductive load and a resultant commutating dv/dt condi- tion associated with the nature of the load. dv/dt CRITICAL RATE OF RISE OF OFF−STATE Also, commonly called static dv/dt. It is the minimum VOLTAGE value of the rate of rise of forward voltage which will cause switching from the off−state to the on−state with gate open. IDRM PEAK REPETITIVE BLOCKING CURRENT The maximum value of current which will flow at VDRM and specified temperature when the SCR or Triac is in the off−state. Frequently referred to as leakage current in the forward off−state blocking mode. IGM FORWARD PEAK GATE CURRENT (SCR) The maximum peak gate current which may be safely PEAK GATE CURRENT (Triac) applied to the device to cause conduction. IGT GATE TRIGGER CURRENT The maximum value of gate current required to switch the device from the off−state to the on−state under specified conditions. The designer should consider the maximum gate trigger current as the minimum trigger current value that must be applied to the device in order to assure its proper triggering. IH HOLDING CURRENT The minimum current that must be flowing (MT1 & MT2; cathode and anode) to keep the device in a regenerative on−state condition. Below this holding current value the device will return to a blocking state, off condition. IL LATCHING CURRENT The minimum current that must be applied through the main terminals of a Triac (or cathode and anode of an SCR) in order to turn from the off−state to the on−state while its IGT is being correctly applied. IRRM PEAK REPETITIVE REVERSE BLOCKING The maximum value of current which will flow at VRRM and CURRENT specified temperature when the SCR or Triac is in the reverse mode, off−state. Frequently referred to as leakage current in the reverse off−state blocking mode. IT(AV) AVERAGE ON−STATE CURRENT (SCR) The maximum average on−state current the device may safely conduct under stated conditions without incurring damage. http://onsemi.com 5 THYRISTOR TERMINOLOGY (The following terms are used in SCR and TRIAC specifications.) Symbol Terminology Definition ITM PEAK REPETITIVE ON−STATE CURRENT (SCR) Peak discharge current capability of a thyristor useful (also called PEAK DISCHARGE CURRENT) when connected to discharge peak current usually from a capacitor. This is a rarely specified parameter. (See MCR68 and MCR69 data sheets, for examples where it is specified.) IT(RMS) ON−STATE RMS CURRENT The maximum value of on−state rms current that can be applied to the device through the two main terminals of a Triac (or cathode and anode if an SCR) on a continuous basis. ITSM PEAK NON−REPETITIVE SURGE CURRENT The maximum allowable non−repetitive surge current the device will withstand at a specified pulse width, usually specified at 60 Hz.
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