application note AN302/0289 the holding current by e. leblanc thyristors and triacs, an important parameter : the purpose of this note is to familiarize the user of a triac (or a thyristor) with the parameter i h : hypo- static current or holding current. after a short definition, we will illustrate the impor- tance of this parameter by concreteexamples. then we will describe how to measure it and finally its va- riation with the conditions of use and the sensitivity of the components. in all cases we speak of triacs. however, the state- ments are also valid for thyristors. definition to keep an electromagnetic relay in the conducting state, it is necessary for a minimum current to circu- late in its coil. otherwise it would return to the blocked state. the same phenomenon can be ob- served for a triac. this minimum current which keeps the triac conducting is called the hypostaticor holding current (figure 1). figure 1 : controlled by gate pulse i g , the triac is fired and a current i t flows through it, fixed by the main circuit. when the current i t falls below the triac hypostatic current i h , it is reblocked. d89AN302-01 1/6
applications example 1 : light dimmer (figure 2). figure 2 : dimmer with interface suppression coil and capacitor (rfi filter). figure 3 : current in the dimmer triac : the interference suppression filter pro- duces oscillations. if i o >i h (as in the fig- ure) the triac remains fired. but if i o falls below i h , the triac will be blocked. figure 4 : control of a small motor by triac. if the coil is a poor quality one, the oscillation is in- sufficientlydamped. if the current in the triac falls be- low the hypostatic current, i h , this results in untimely blocking of the triac. it is fired at the next current pulse i g and is blocked again. the lamp flickers. this is known as the oflicker effecto. how can it be prevented ? by eliminating the cause, i.e. using an appropriate interference suppression filter which does not produce extensive oscillations, and then by choosing a triac with a lower hypostatic current i h . sgs-thomson microelectronics has developped a triac specially designed for applications where a low hypostaticcurrent i h is required, the bta 06 400 gp. this device is specified with a maximum holdingcur- rent i h of 13ma in both current flow directions. the designer wishes to control a small high-impe- dance motor (2500ohms for example) by triac. he obtains the parts and an operating manual and car- ries out tests. the circuit operates smoothly. after one year of production,the manufacturercomplains of low torque in his motor and blames the triac. what's happened ? example 2 : motor control (figure 4). d89AN302-02 d89AN302-03 d89AN302-04 application note 2/6
figure 5 : voltage accross the triac and current in the circuit of figure 4. the circuit was designed with a type of triac whose maximum specified holding current i h was 50ma. but the components used for the tests were more sensitive : i h + = 13ma and i h = 8ma and the desi- gner based his choice on these results. after a year of delivery, the component manufacturer continues to deliver parts which are in conformity with the spe- cification but less sensitive : i h +=40maandi h = 20ma. the conduction time decreases (figure 5) , the dissymmmetry is greater, a dc component ap- pears and the motor loses torque. to prevent this kind of difficulty, when designing the circuit it is thus necessary to take into account not the typical value of the sample used but the maximum value specified by the component manufacturer. example 3 : take the diagram of the previous example (fig- ure 4) , the control of a small high-impedance motor by triac. thistime, the designerselectsa triac with a lower maxi- mum specified holding current, i h . the motor seems to operate without problems. the motor is meant for mounting on out-door equipment. the equipmynt is installed in summer and works well. but in winter, the fault described above occurs. what has happened ? the designer studied the operation of his circuit at an ambient temperature of 25 c. but the holding current i h varies with the temperature : when the temperature decreases,the holding currentincreases (we will study this variation in paragraph 4.6) and the ph enomenon described in example 2 occurs. thus when designing a circuit which is to operate at low temperatures, it is essential to take into account the corrected value of the holding current and not its value at an ambient temperature of 25 c. these three examples illustrate the importance of this parameter and the different problems it can cause in a circuit if it is insufficiently known. if the device is to remain in the conducting state, it is imperative that the circuit in which it is used en- sures a current higher than the holding current i h of the device. in our data sheets, for all the types of triacs, the hypostatic current i h is specified as a maximum va- lue. a suitable triac should then be chosen whose holding current i h is lower than the minimum value of the current in the circuit, if the triac is to remain in the conducting state ; make the necessary correc- tions to compensate for temperature variations. measurement of the hypostatic current i h pushbutton p is used to fire the triac. the value of current i t is chosen much higher than the latching current. by increasing the value of the variable re- sistor r, current i t will decrease. the value of the hypostatic current i h is the value of i t read just be- fore the triac is blocked. the hypostatic current i h is always measured with the gate unconnected, i.e. disconnected from the trigger circuit. only sensitive thyristors (i gt 500 m a) are measured with a 1kohm resistor connected be- tween gate and cathode. for the measurementto be regularly repeatable,the triac should be suitably fired. the following rules should be observed : - before decreasing current it, it should be equal to at least 5 times the triac i l current. d89AN302-05 application note 3/6
example : bta 12 600c i l typ (qi and iii) = 20ma thus i t = 500ma. - if the i h current is measured by pulses (automatic testers, for example), the triac should consult for at least 500 m s before performing the measurement. for a triac, the ih current has two values : (i h +) when electrode a2 is positive with respect to elec- trode a1 and (i h ) when electrode a2 is negative with respect to electrode a1. in the documentation only one value is given for both quadrants. this va- lue is always the maximum value. example : bta 12 600c : i h max =25ma. depending on the production batch, i h can vary. however, the dispersion remains below the limits specified in the catalogue. to give an idea of this dispersion : - sensitive triacs : i gt (qi) 5ma (type t) : 2ma i h 8ma (specified i hmax : 15ma). - standard triacs : i gt (qi) 50ma (type b) : 8ma i h 40ma (specified i h max :50ma). note : the minimum value of the i h parameteris ne- ver specified in the data sheets. figure 6 : circuit for measurement of the holding current i h . d89AN302-06 variation of the holding current, i h a) variationof current i h withthe sensitivityof the devices and the direction of conduction (typical value) for low components (thyristors and triacs whose rated current is less than 60a), thehypostaticcurrent, i h , is related to the firing current, i gt (see figure 7). figure 7 : ratio between the holding current, i h (a2 +) and current i gt (qi) for sensitive and standard triacs. i h +/i gt (q1) example 1 : bta 06 600t : if i gt (qi) = 1.5ma then i h + = 4.5ma. bta 12 600c : if i gt (qi) = 10ma then i h = 15ma. 1.2 1.2 figure 8 : ratio between holding current i h + (a2 +) and holding current i h (a2 ) for sensitive and standard triacs. example 2 : bta 06 600t : if i h + = 4.3ma, i h = 3.8ma. bta 12 600b : if i h + = 15ma, i h = 12.5ma. sensitive triac 6 a rms (t type) standard triac 12 a rms (c type) i h +/i h - sensitive triac 6 a rms (t type) 3 1.5 standard triac 12 a rms (c type) in the case of the triac (as distinguished from the thy- ristor) it is important to note that current i h (elec- trode a2 negative with respect to a1) is generaly lower than i h + (see figure 8). application note 4/6
figure 9 : relative variation of the holding current i h , with the junction temperature, tj (typical values). b) variation of the hypostatic current, i h , with the junction temperature : the value of the hypostatic current is physically re- lated to that of the firing current, i gt . these two pa- rameters thus vary with the junction temperature in accordance with an analog law (see figure 9) . example : triac to 220, type bta 12 600c i h =20maattj=25 c, thus i h =14maattj=110 c. d89AN302-07 c) influence of the reapplied voltage : the rise time and the level of the reapplied reverse voltage across the triac after blocking have no in- fluence on the value of its holding current, i h . d) influence of the external gate cathode resistor, r gk the user can wire a resistor, r gk , between the gate and the cathodeof the component, eitherto improve its behaviour under voltage at high junction tempe- ratures (by-pass for leakage current) in the case of sensitive thyristors, or because it forms part of the fliring circuit. this resistor has an influence on the holding current, i h , in different proportions depen- ding on its resistive value and the sensitivity of the components : 1 - sensitive thyristors (i gt 500 m a) resistor r gk connected between the gate and the cathode (figure 10) has an important influence on the i h parameter of sensitive thyristors. for certain applications, the designer would be well-advised to define a high impedance control circuit. figure 10 : variation of the hypostatic current, i h , of a sensitive thyristor (e. g. tls 106-6) as a function of the gate-cathode resistor (typical values). d89AN302-08 note : the hypostati c current for sensitive thyristors is always specified for r gc = 1000ohms. application note 5/6
2 -standardthyristors, sensitive and standard triacs a resistor between gate and cathode on one of these components has no significant influence on the value of its holding current, i h (on condition that it is not too low, r gc > 20ohms). e) note : we have seen that the more sensitive the triac (low i g ), the lower the value of the holding current, i h . now, in certain applications, a sensitive triac (direct control by integratedcircuit) with a high i h (or i l )may be useful. in this case, the circuit of figure 11 could be used. the assembly is sensitive but has a higher hyposta- tic current. figure 11 : this component, a ?darlington triac? combines a high sensitivity with a high hypostatic current. t 1 : standard triac e. g. bta 12 600b i gt (qi) 50ma i h + 50ma t 2 : sensitive triac e. g. tlc 336t i gt (qi) 5ma i h + 15ma d89AN302-09 conclusion the choice of a thyristor or a triac does not depend only on the voltage, the rated current and the sen- sitivity. other parameters should be taken into ac- count. the hypostatic current, i h plays an important role in many circuits. the value of this parameter varies with : - dispersion of characteristics at manufacture, - temperature, - eventually the control circuit (in the case of sensitive thyristors), - the direction of current flow. taking into account these elements, the designer can obtain satisfactory operation of his circuit in in- dustrial real life applications. knowing the problems which could be created by this parameter, sgs-thomson microelectronics has introduced a new triac, bta 06 400 gp, now available for the designers. its low holding current, specified with a maximum value, enables it to be used in most applications. 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-tho mson 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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