application note AN307/0289 use of triacs on inductive loads by j. bellin ? although triac circuits are now well known by desi- gners. the use of these components for inductive loads requires certain precautions which should not be neglected of optimum use is to be made of them. that is the purpose of this article which reviews the various triac control modesand recalls the principles which guarantee its correct operation. phenomena occuring when the circuit is closed. the triac is known as a component which is essen- tial in controlling power from an ac source (mains). in most cases, the circuit has an inductive compo- nent : either because of the nature of the load itself : motors, transformers, ballast inductance ; or be- cause of the source impedance : utilization of the secondaryof a transformer,lengthof the supply line, etc. on inductive loads, the operating conditions vary considerably, when closing the circuit, depend- ing on the control mode (gate current, polarity and width) and synchronization of the firing. in order to build an optimal control circuit it is indispensable to analyse the various possibilities. firing control signal the triac is fired by a gate current ig > igt whose du- ration should enable the main current to reach the triac holding current value (i l ). the width of the control signal is determined by the rate of increase of the main current (di/dt), limited by the load inductance and by the choice of the firing quadrant. the loading current, i l , is highest in the second quadrant (a 2 positive with respect to a 1 ,i g nega- tive) : ( figure 1-a ). figure 1 : width of control signal required as a function of the firing quadrant (a) ; width of control signal required as a function of the moment of firing (b). quadrants polarity i ii iii iv a2 + + - - g+--+ with respect to a1 b a d89AN307-01 1/6
the rate of rise of the main current, di/dt, is propor- tional to the amplitude of the power supply voltage at the moment of firing (di/dt = v/l). the width of the firing signal required is less when firing occurs near the peak of the mains voltage than when its occurs around zero of that voltage ( figure 1-b ). to fire the triac and to ensure conduction in continu- ous operation, we can compare various types of control circuits. gate current control by single pulse to ensure correct operation, the gate pulse should be synchronized with the triac current zero point and should be long enough to enable the main current to reach the latching current i l level ( figure 2-a ). in case the pulse occurs before the triac current reaches its zero point (incorrect synchronization) or if its duration is too short to allow the main current to exceed the latching current i l , the triac conducts only during alternate half-cycles. the high dc com- ponent thus introduced in the load can produce con- siderable overloads due to saturation of magnetic materials. figure 2 : gate control by a single pulse synchronized with zero current (a) ; in case of a single pulse whose duration is too short, the triac only conducts during alternate half-cycles (b). gate control by pulse train the control by gate pulse train eliminates problems of synchronization on the current. a recurrence fre- quency of several kilohertz guarantees correct op- eration of this type of control ( figure 3 ). this procedure, whose results are satisfactory, is often used for controlling triacs in inductive circuits. a variant of this principle consists in making use of a circuit which monitorsfiring and which delivers pulses to the gate as long as the voltage across the triac is higherthana threshold,usuallyfixed at about10 volts ( figure 4 ). this type of circuit enables delivering just the amount of gate current required for firing. gate control by dc current gate control by dc guaranteesideal firing but has the disadvantage of high consumption, specially when the control power supply is provided by the mains. in this case, it is preferable to use a negative current for the gate control (quadrants ii and iii). figure 3 : gate control by pulse train. d89AN307-02 d89AN307-03 ? application note 2/6
figure 4 : firing monitoring circuit : the control signal is repeated until firing. transient phenomena during triggering principles during continuous operation, the magnetic field h, proportionalto the current in the coil, varies with re- spect to the induction b, with a delay as shown by the hysteresis cycle in figure 5. in transient operation,the induction can follow a dif- ferent path and reach the saturation value b s for which the magneticfield h (according to the coil cur- rent) increases very rapidly ( figure 6 ). figure 5 : magnetic field h with respect to induc- tion b in continuous sinusoidal phase. figure 6 : induction bs versus field h variation. in the circuits controlled by a triac, opening occurs when the current is at zero. the induction thus has a remanent value br, corresponding to h = 0 ( figure 5 ). when the triac begins to conduct, the transients de- pend on the instant of synchronization of the control signal with respect to the mains voltage. d89AN307-04 ? application note 3/6
firing at zero mains voltage peak induction tends to the value : b max =2b n +b r , thus in most cases reaching saturation induction bs. the amplitude of the current proportional to the mag- neticfieldh becomesveryhigh ;this type ofcontrolpro- duces the highest transient overloads ( figure 7-a ). in order to limit the over current during firing at zero voltage, control must be done by complete periods. since the triac allows an integral number of half-cy- cles to pass, the polarity of the mains voltage at the moment of firing is the reverse of that at the moment the circuit is opened. peak induction thus reaches the value : b max =2b n b r , becauseb rises between p and q on the hysteresis cycle. the overload is lower than previously but still re- mains high ( figure 7-b ). firing at peak mains voltage peak induction takes the value : b max =b n +b r in general, the threshold of saturation bs is not reached and amplitude of the current remains within acceptable limits ( figure 7-c ). this type of synchronization is simple and efficient and should be adoptedwhenever possible on loads composed of materials which can be saturated. figure 7 : transient induction and current at beginning of conduction. d89AN307-07 ? application note 4/6
firing at inductance phase shift with conduction by complete periods firing at therealinductancephaseshift with conduction by complete periods places the magnetic field and the induction on the hysteresis cycle of conti nuous op- eration : consequently, transients are eliminated. however, the design of the control circuit for this fir- ing mode is complex and consequentlyit is reserved for special applications. firing by phase sweep the triac is first fired at the end of a half-cycle. then progressively the difference of phase between the voltage zero and the instant of firing decreases until total conduction. with a sufficiently low sweep speed, any transient overload is thus avoided ( figure 8 ). this procedure is widely used and gives very good results. figure 8 : firing by phase sweep. spurious firing the control circuit plays an important role in normal operation. however, in case of spurious firing, the triac may have to withstand an accidental overload. the peak amplitude of the current which could flow through the triac should be known to select its rat- ing : the maximum current which could flow through the circuit should not be higher than the accidental overload capacity of the triac (i tsm ). in this care the triac is oversized. d89AN307-08 conclusion we have seen the essential points guaranteeing correct operation of a triac. if the circuit is closed on an inductive load, you need to : fire the triac : with a sufficiently wide gate control signal, in the choosen quadrants (depending on whether higher sensitivity or a low latching current is required). avoid transient overloads : by synchronizing the control signal with respect to the mains at the moment of firing (firing of the triac at zero voltage should be avoided). keep the triac in conduction : by selectionof the type of control (avoid gate control by a single short pulse). ? application note 5/6
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifica- tions mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information pre- viously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - printed in italy - all rights reserved. sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. ? application note 6/6
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