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ON Semiconductor Is ON Semiconductor Is Now To learn more about onsemi™, please visit our website at www.onsemi.com onsemi and and other names, marks, and brands are registered and/or common law trademarks of Semiconductor Components Industries, LLC dba “onsemi” or its affiliates and/or subsidiaries in the United States and/or other countries. onsemi owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of onsemi product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. onsemi reserves the right to make changes at any time to any products or information herein, without notice. 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AN1045/D Series Triacs In AC High Voltage Switching Circuits By George Templeton http://onsemi.com Thyristor Applications Engineer APPLICATION NOTE INTRODUCTION Edited and Updated This paper describes the series connection of triacs to winding, and start capacitor voltage. This voltage increases create a high voltage switch suitable for operation at volt- when triac turn-off occurs at higher rpm. ages up to 2000 Volts. They can replace electromechanical contactors or extend their current rating and lifetime. Motor TRIGGERING starters and controllers operating at line voltages of 240 Figure 1 illustrates a series thyristor switching circuit. In Volts or more require high-voltage switches. Transformer this circuit, the top triac triggers in Quadrant 1 when the action and resonant snubber charging result in voltages bottom triac triggers in Quadrant 3. When the optocoupler much greater than the peak of the line. Triacs can be sub- turns on, gate current flows until the triacs latch. At that jected to both commutating and static dV/dt when multiple time, the voltage between the gate terminals drops to about switching devices are present in the circuit. Snubber 0.6 Volts stopping the gate current. This process repeats designs to prevent static dV/dt turn-on result in higher volt- each half cycle. The power rating of the gate resistor can be ages at turn-off. Variable load impedances also raise volt- small because of the short duration of the gate current. age requirements. Optocoupler surge or triac gate ratings determine the mini- The benefits of series operation include: higher blocking mum resistance value. For example, when the maximum voltage, reduced leakage, better thermal stability, higher optocoupler ITSM rating is 1 A: dV/dt capability, reduced snubber costs, possible snubber- R u+ V ńI (1.0) less operation, and greater latitude in snubber design. The g peak max advantages of triacs as replacements for relays include: Rg + 750 Vń1A+ 750 Ohm • Small size and light weight The triacs retrigger every half cycle as soon as the line • Safety — freedom from arcing and spark initiated voltage rises to the value necessary to force the trigger cur- explosions rent. The instantaneous line voltage V is • Long lifespan — contact bounce and burning eliminated + ) ) V IGT Rg 2VGT 2VTM (1.1) • Fast operation — turn-on in microseconds and turn-off in milliseconds where VGT, IGT are data book specifications for the triac and • Quiet operation VTM is the on-voltage specification for the optocoupler. The phase delay angle is Triacs can be used to replace the centrifugal switch in capacitor start motors. The blocking voltage required of the q + *1ƪ V ƫ triac can be much greater than the line voltage would sug- d SIN Ǹ (1.2) 2 V gest. It must block the vector sum of the line, auxiliary LINE Semiconductor Components Industries, LLC, 1999 1 Publication Order Number: August, 1999 – Rev. 2 AN1045/D AN1045/D IG IL G MEAN MT1 RG DESIGN ∆I CAPABILITY MT2 6 σ 6 σ 3 σ 3 σ MT2 PROCESS WIDTH G MT1 Figure 6.1. Series Switch Figure 6.2. Designing for Probable Leakage STATIC VOLTAGE SHARING turn leads to greater leakage. If the rate of heat release at the Maximum blocking voltage capability results when the junction exceeds the rate of removal as temperature triacs share voltage equally. The blocking voltage can be dc increases, this process repeats until the leakage current is or ac. A combination of both results when the triac switches sufficient to trigger the thyristor on. the start winding in capacitor start motors. In the simple DC blocking simplifies analysis. A design providing series connection, both triacs operate with an identical leak- stable dc operation guarantees ac performance. AC opera- age current which is less than that of either part operated tion allows smaller heatsinks. alone at the same voltage. The voltages across the devices The last term in the stability equation is the applied volt- are the same only when their leakage resistances are identi- age when the load resistance is low and the leakage causes cal. Dividing the voltage by the leakage current gives the negligible voltage drop across it. The second term is the leakage resistance. It can range from 200 kohm to 2000 thermal resistance from junction to ambient. The first term megohm depending on device characteristics, temperature, describes the behavior of leakage at the operating condi- and applied voltage. tions. For example, if leakage doubles every 10°C, a triac Drawing a line corresponding to the measured series operating with 2 mA of leakage at 800 Vdc with a 6°C/W leakage on each device’s characteristic curve locates its thermal resistance is stable because operating point. Figure 3a shows the highest and lowest ° 2mA@ 6 C @ 800 V + 0.96 leakage units from a sample of 100 units. At room tempera- 10°C W ture, a leakage of 350 nA results at 920 Volts. The lowest Operating two triacs in series improves thermal stability. leakage unit blocks at the maximum specified value of 600 When two devices have matched leakages, each device sees Volts, while the highest blocks 320 Volts. A 50 percent half the voltage and current or 1/4 of the power in a single boost results. triac. The total leakage dissipation will approach half that Figure 3b shows the same two triacs at rated TJmax. The of a single device operated at the same voltage. The addi- magnitude of their leakage increased by a factor of about tional voltage margin resulting from the higher total block- 1000. Matching between the devices improved, allowing ing voltage reduces the chance that either device will oper- operation to 1100 Volts without exceeding the 600 Volt rat- ate near its breakdown voltage where the leakage current ing of either device. increases rapidly with small increments in voltage. Higher Identical case temperatures are necessary to achieve voltage devices have lower leakage currents when operated good matching. Mounting the devices closely together on a near breakdown. Consequently, the highest breakover volt- common heatsink helps. age unit in the pair will carry the greatest proportion of the A stable blocking condition for operation of a single triac burden. If the leakage current is large enough to cause sig- with no other components on the heatsink results when nificant changes in junction temperature, (∆TJ = φJC PD), the effect will tend to balance the voltage division between dIMT dTJ dPJ @ @ t 1 (2.0) the two by lowering the leakage resistance of the hotter dT dP dI J J MT unit. If the leakage mismatch between the two is large, Thermal run-away is a regenerative process which occurs nearly all the voltage will drop across one device. As a whenever the loop gain in the thermal feedback circuit result there will be little benefit connecting two in series. reaches unity. An increase in junction temperature causes Series blocking voltage depends on leakage matching. increased leakage current and higher power dissipation. Blocking stability depends on predictable changes in leak- Higher power causes higher junction temperature which in age with temperature.
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