Thyristor Theory and Design Considerations

1 Theory and Design ConsiderationsHandbookHBD855/DRev. 1, Nov 2006 SCILLC, 2005 Previous Edition 2005 as Excerpted from DL137/D All Rights Reserved 2 ABOUT THYRISTORST hyristors can take many forms, but they have certainthings in common. All of them are solid state switcheswhich act as open circuits capable of withstanding therated voltage until triggered. When they are triggered,thyristors become low impedance current paths andremain in that condition until the current either stops ordrops below a minimum value called the holding a Thyristor has been triggered, the trigger current canbe removed without turning off the controlled rectifiers (SCRs) and triacs are bothmembers of the Thyristor family.

2 SCRs are unidirectionaldevices where triacs are bidirectional. An SCR isdesigned to switch load current in one direction, while atriac is designed to conduct load current in , all thyristors consist of several alternatinglayers of opposite P and N silicon, with the exact structurevarying with the particular kind of device. The load isapplied across the multiple junctions and the triggercurrent is injected at one of them. The trigger currentallows the load current to flow through the device, settingup a regenerative action which keeps the current flowingeven after the trigger is characteristics make thyristors extremely usefulin control applications. Compared to a mechanical switch,a Thyristor has a very long service life and very fast turnon and turn off times.

3 Because of their fast reaction times,regenerative action and low resistance once triggered,thyristors are useful as power controllers and transientovervoltage protectors, as well as simply turning deviceson and off. Thyristors are used in motor controls,incandescent lights, home appliances, cameras, officeequipment, programmable logic controls, ground faultinterrupters, dimmer switches, power tools,telecommunication equipment, power supplies, timers,capacitor discharge ignitors, engine ignition systems, andmany other kinds of thyristors of all sorts are generally rugged,there are several points to keep in mind when designingcircuits using them. One of the most important is torespect the devices rated limits on rate of change ofvoltage and current (dv/dt and di/dt).

4 If these areexceeded, the Thyristor may be damaged or destroyed. Onthe other hand, it is important to provide a trigger pulselarge enough and fast enough to turn the gate on quicklyand completely. Usually the gate trigger current should beat least 50 percent greater than the maximum rated gatetrigger current. Thyristors may be driven in manydifferent ways, including directly from transistors or logicfamilies, power control integrated circuits, byoptoisolated triac drivers, programmable unijunctiontransistors (PUTs) and SIDACs. These and other designconsiderations are covered in this interest too, is a new line of Thyristor SurgeSuppressors in the surface mount SMB package coveringsurge currents of 50, 80 and 100 amps, with breakovervoltages from 77 to 400 volts.

5 NP Series Thyristor SurgeProtector Devices (TSPD) protect telecommunicationcircuits such as central office, access, and customerpremises equipment from overvoltage conditions. Theseare bidirectional devices so they are able to havefunctionality of 2 devices in one package, saving valuablespace on board layout. These devices will act as a crowbarwhen overvoltage occurs and will divert the energy awayfrom circuit or device that is being protected. Use of theNP Series in equipment will help meet various regulatoryrequirements including: GR 1089 CORE, IEC61000 4 5, ITU , IEC 60950, TIA 968 A,FCC Part 68, EN 60950, UL 1950. See ONSemiconductor application note AND8022/D foradditional 3 Sections 1 thru 9 CHAPTER 1 Theory and ApplicationsPageSection 1: Symbols and Terminology 4.

6 Section 2: Theory of Thyristor Operation 10.. Basic Behavior 10.. Switching Characteristics 13.. False Triggering 15.. Theory of SCR Power Control 16.. Triac Theory 22.. Methods of Control 24.. Zero Point Switching Techniques 25.. Section 3: Thyristor Drivers and Triggering 29.. Pulse Triggering of SCRs 29.. Effect of Temperature, Voltage and Loads 33.. Using Negative Bias and Shunting 35.. Snubbing Thyristors 38.. Using Sensitive Gate SCRs 40.. Drivers: Programmable UnijunctionTransistors 44.. Section 4: The SIDAC, A New High VoltageBilateral Trigger 49.. Section 5: SCR Characteristics 60.. SCR Turn Off Characteristics 60.. SCR Turn Off Mechanism 60.. SCR Turn Off Time tq 60.. Parameters Affecting tq 65.. Characterizing SCRs for Crowbar Applications 71.. Switches as Line Type Modulators 79.

7 Parallel Connected SCRs 85.. RFI Suppression in Thyristor Circuits 89.. Section 6: Applications 93.. Phase Control with Thyristors 93.. Motor Control 94.. Phase Control with Trigger Devices 102.. Cycle Control with Optically IsolatedTriac Drivers 105.. AC Power Control with Solid State Relays 110.. Triacs and Inductive Loads 114.. Inverse Parallel SCRs for Power Control 117.. PageInterfacing Digital Circuits to ThyristorControlled AC Loads 118.. DC Motor Control with Thyristors 127.. Programmable Unijunction Transistor (PUT)Applications 132.. Triac Zero Point Switch Applications 136.. AN1045 Series Triacs in AC High VoltageSwitching Circuits141.. AN1048 RC Snubber Networks for ThyristorPower Control and Transient Suppression152.. AND8005 Automatic AC Line VoltageSelector174.

8 AND8006 Electronic Starter for FlourescentLamps177.. AND8007 Momentary Solid State Switchfor Split Phase Motors181.. AND8008 Solid State Control Solutionsfor Three Phase 1 HP Motor186.. AND8015 Long Life Incandescent Lampsusing SIDACs194.. AND8017 Solid State Control forBi Directional Motors198.. Section 7: Mounting Techniques for Thyristors 201.. Mounting Surface Considerations 202.. Thermal Interface 203.. Insulation Considerations 204.. Fastening Techniques 209.. Insulated Packages 210.. Surface Mount Devices 212.. Thermal System Evaluation 214.. Section 8: Reliability and Quality 218.. Using Transient Thermal Resistance Data inHigh Power Pulsed Thyristor Applications 218.. Thyristor Construction 230.. In Process Controls and Inspections 230.. Reliability Tests 231.

9 Stress Testing 233.. Environmental Testing 233.. Section 9: Appendices 234.. 4 SECTION 1 SYMBOLS AND TERMINOLOGYSYMBOLSThe following are the most commonly used schematic symbols for Thyristors:Name of DeviceSymbolSilicon ControlledRectifier (SCR)TriacProgrammable UnijunctionTransistor (PUT) Thyristor Surge ProtectiveDevices & SidacMT1MT2MT1 GMT2 5 Thyristor TERMINOLOGY (The following terms are used in SCR and TRIAC specifications.)SymbolTerminologyDefinit iondi/dtCRITICAL RATE OF RISE OF ON STATECURRENTThe maximum rate of change of current the device willwithstand after switching from an off state to an on statewhen using recommended gate drive. In other words, themaximum value of the rate of rise of on state currentwhich a Triac or SCR can withstand without damage.

10 (di/dt)cRATE OF CHANGE OF COMMUTATINGCURRENT (Triacs)Is the ability of a Triac to turn off itself when it is driving aninductive load and a resultant commutating dv/dt condi-tion associated with the nature of the RATE OF RISE OF OFF STATEVOLTAGEAlso, commonly called static dv/dt. It is the minimumvalue of the rate of rise of forward voltage which willcause switching from the off state to the on state withgate REPETITIVE BLOCKING CURRENTThe maximum value of current which will flow at VDRMand specified temperature when the SCR or Triac is in theoff state. Frequently referred to as leakage current in theforward off state blocking PEAK GATE CURRENT (SCR)PEAK GATE CURRENT (Triac)The maximum peak gate current which may be safelyapplied to the device to cause TRIGGER CURRENTThe maximum value of gate current required to switch thedevice from the off state to the on state under specifiedconditions.