application note AN303/0495 latching current by e. leblanc ? an important problem concerning the utilization of components such as thyristors or triacs is the hol- ding of the component in the conducting state after the trigger current has disappeared during firing. very often, the firing problems supposedly due to the gate current i g or to the firing time t gt are in rea- lity due to the latching current i l . after a definition we will illustrate the importance of this parameter by concrete examples. then we will describe how to measure it and its variation accor- ding to the utilization conditions of the components. the study will be based on the triac. the points trea- ted are valid for thyristors (except for the various conduction modes). definition the latching current, i l , ofa triac is the minimum va- lue of the main current (current flowing between electrodesa 2 and a 1 ) which enables the component to remain in the conducting state after the gate cur- rent i g has ceased ( figure 1 ). figure 1 : controlled by the gate pulse, i g , the triac is fired, and a current i t flows through it, imposed by the main current. if the gate current i g is stopped before current i t reaches the value of the latching current i l , the triac is blocked (as shown in the figure). 1/10
applications example 1 : control of a low power signalling lamp by triac. figure 2 : control of a low power signalling lamp by triac. current in the main circuit of the triac and gate cur- rent. the lamp power is too low (eg. : p 10 w and the triac bta12.400b) to impose a sufficient current (shown in dotted lines in the diagram) in the triac to keep it in the conducting state after interruption of the gate current i g . the triac does not conduct. a bta 12.400 b triac is used to control the flashing of a 10 w signalling light. the peak current in the circuit will therefore be 65 ma. this value is very close to that of the typical latching current given in the data book for this type of triac : 50 ma (quadrant 1, 3 and 4). thus the user's case could be that described in figure 2, that is, a triac whose latching current i l in the first quadrant is equal to 70 ma. his triac will never be fired. for cor- rect operation,the user should thusemploy a sensitive triac (e.g. z0102ma i l :8ma). d89AN303-02 ? application note 2/10
example 2 : control of an inductive load by triac. in continuous lines : short gate signal : the triac does not remain in the conducting state because the main current did not reach the value of the triac lat- ching current before suppression of the gate current. in dotted lines : long gate signal : the triac is fired and remains in the conducting state until its current falls below the holding current i h after suppression of the gate current i g . figure 3 : voltage accross and current through the triac. d89AN303-03 ? application note 3/10
on a highly inductive load, the inductance limits the current rise time to : (va : power supplyvoltage at the time the gate signal is applied ; l : load inductance). consider the operation on one full-wave of the po- wer supply voltage. if the duration t 1 of the gate cur- rent pulse i g is very small compared with a half-wave of the power supply voltage, the triac cur- rent cannot reach the triac latching current level in the firing mode considered (here the 1st quadrant). thus firing will not take place and the voltage across the triac increases. for triggering to be steady, the duration of pulse t 2 should be long when compared with a half-wave of the power supply voltage. the current set up in the triac is imposed by the load im- pedance. the triac remains in the conducting state until the current falls below the holding current i h .it is blocked if the i g current pulse has ended. another method consists of applying a train of clo- sely spaced pulses to the triac gate instead of a square wave. the sgs-thomson microelectronics applications laboratories have developed a number of triac control circuits, specially designed to work on induc- tive loads (see bibliography, ref. n 1). example 3 : control by triac of a load whose power varies considerably. figure 4 : control of an arc welding set by triac. the designer of an arc welding set whose power is adjustable by triac, chooses a component capable of controlling high currents. for example, if the maxi- mum current to be controlled is 40 a rms , the desi- gner, for safety, will choose a triac rated at 60 a rms , thus a triac with a high latching current. now, off- load, the transformer magnetizing current could be very low or even below the triac latching current i l in one of the quadrants. this means that the triac could fire correctly in the first quadrantand then not fire if the next firing is to take place in the second quadrant where the i l is much higher. a consider- able unbalance then occurs, generating a dc cur- rent heating the transformer and preventing the equipment from operating correctly. since the latching current i l increases with the size of components, and thus with their rating, the user would thus be well advised not to select an exces- sively high rating for his triac in order to have the lo- west possible latching current. a.n : for this type of application, the sgs-thom- son microelectronics applications laboratories place at the disposal of designersa number of sche- matics meant for this type of circuit (see bibliog- raphy, ref. n 1). these three examples illustrate the importance of the i l parameter and the problems that it can cause in a circuit. to ensure stable firing of a triac or a thy- ristor, it is absolutely necessary for the circuit which is controlled to impose a current which is higher than its latching current. d89AN303-04 dl t d t va l = ? application note 4/10
figure 5 : reducing the risk of untimely firing on inductive loads : the rc circuit (called ? snubber ?). favorable effect of an rc circuit on the firing of a thyristor or a triac in most inductive load applications of triacs or thy- ristors, the user connects an rc network between the anode and cathode of the device to eliminate the risk of premature firing by transients or spontaneous firing by (dv/dt)c (case of triacs) ( see figure 5 ). ca- pacitance c and the load impedance attenuate steep voltage transients transmitted by the mains or resulting from switching inductive loads. figure 6 : favorable effect of the rc circuit for firing on a highly inductive load. i : current in the load i c : discharge current of capacitor c i t :i+ i c : current in the triac this rc network has also a second advantage. in fact, the energy accumulated in capacitorc aftertur- ning off is fed back to the triac when firing. the speed at which the current increases in the triac during dis- charge of the capacitor is then limited only by the peak charge voltage of the capacitor and the induc- tance of the circuit connecting the snubber to the triac. the current amplitude is the quotient of peak charge voltage of the capacitor by the series resis- tance r. this circuit thus helps the current to rise very quickly above the latching current i l of the de- vice ( see figure 6 ). note : when using an rc circuit, it is not advisable to work with a series resistance r which is too low. in fact, the combined effect during firing of i t1 ( figure 6 ) (equal to the quotient of the capacitor peak charge voltage and resistance r) and the current slope dl t /dt (equal to the quotient of the capacitance charging voltage by the inductance of the connection between the triac and the rc circuit) could be dangerous for the triac. a value for r higher than 100 ohms is recommended. d89AN303-05 d89AN303-06 ? application note 5/10
latching current ( i l ) measurement figure 7 : latching current (il) measurement circuit. the closing of contact c enables passage of the gate current whose is selected higher than that of the triac firing current, i gt to be measured. by gra- dually decreasing the value of resistance r 1 , while continuing to transmit pulses of gate current i g ,the main current i t is increased. as long as the value of the i t current is lower than that of the device latching current i l , the device does not remain in the conduc- ting state. the value of the latching current i l is the value of the i t current read as soon as the triac re- mains on, after suppressing the gate current i g . only sensitive thyristors (i gt 500 m a) are measured with a 1 k w resistor between gate and cathode. parameteri l varieswith the width of the gate current pulse i gt and its level. for the measurement to be reproducedcorrectly, the following rules should thus be observed : fix a sufficiently wide control pulse i g . the width of the pulse should be at least equal to 1 ms. impose a gate current i g sufficiently high with res- pect to that of the triggering current i gt of the device to be measured. an ratio higher than or equal to 1.2 is advisable. example : bta 12.600 c i gt max (q iv) = 50 ma therefore i g =60ma in the case of a triac, there are four latching current i l values that correspond to the four quadrants of triac operation : -(i l + +) when the electrodes a 2 and g are positive with respect to electrode a 1 . -(i l + ) when electrode a 2 is positive with respect to electrode a 1 and electrode g is negative with res- pect to electrode a 1 . -(i l ) when electrodes a 2 and g are negative with respect to electrode a 1 . -(i l +) when electrode a 2 is negative with respect to electrode a 1 and electrode g is positive with res- pect to electrode a 1 . d89AN303-07 i g i gt ? application note 6/10
variations of latching current i l with the utilization conditions a) variations of the i l current with sensitivity of triacs and the various directions of conduction (typical values) . for the low power components (thyristors and triacs whose rated current is lower than 60a) the latching current i l is dependenton the value of firing current i gt ( see figure 8 ). figure 8 : ratio of the latching current il in the different quadrants to the triggering current igt in the first quadrant, for sensitive and standard triacs (typical values). i l (qi) i l (qii) i l (qiii) i l (qiv) i gt (qi) i gt (qi) i gt (qi) i gt (qi) 6a rms sensitive triacs 3.5 15 5 3 12 a rms standard triacs 2 5 1.5 1.7 example 1 : bta 06.600 t : if i gt (ql) = 1 ma then : i l (ql) 3.5 ma ; i l (qii) 15 ma i l (qiii) 5 ma; i l (qiv) 3ma and bta 12.600 bif i gt (qi) = 15 ma then : i l (qi) 30 ma; i l (qii) 75 ma i l (qiii) 22 ma ; i l (qiv) 25 ma in the case of triacs, as opposed to that of thyristors, note that : as underlined in the table of figure 8 ,the current i l + (electrode a 2 positive with respect to electrode a 1 and electrode g negative with respect to electrode a 1 qii) is much higher than the i l cur- rent in the three other quadrants. in the data sheets two values are specified : one va- lue for quadrants i, iii and iv and one value for qua- drant ii. in general these values are typical. b) relation between the latching current i l and the holding current i h the holding current value i h (see bibliography, note n 2) is linked to the latchingcurrent value, i l .byde- finition, the i l current value will always be higher than the i h current value. the i l /i h ratio varies following the sensitivity of the triacs and their ratings (see figure 9). example 2 : bta 12.600 c :i l typ = 40 ma qi, iii, iv i l typ = 70 ma qii depending on the production batches, parameter i l shows dispersion. shown below are approximate values : - sensitive triacs : i gt (qi) 5 ma (type t): qi, iii, iv : 2 ma i l 8ma qii : 10 ma i l 40 ma - standard triacs : i gt (qi) 50 ma (type b): qi, iii, iv : 15 ma i l 50 ma qii : 50 ma i l 120 ma figure 9 : ratio of the latching current i l to the holding current i h depending on the sensitivity and ratings of the devices (typical values). sensitive triacs and thyristors i rms 6a medium power thyristors and triacs 6a i rms 60 a high power thyristors and triacs 60 a i rms 300 a il / ih (1) 1.1 to 1.5 1.5 to 2 2 to 5 (1) 1 st quadrant in the case of triacs. ? application note 7/10
c) variations of the latching current i l with the junction temperature. the value of the latching current i l is physically lin- ked that of the triggering current i gt . these two pa- rameters therefore way analogously with the junc- tion temperature ( see figure 10 ). figure 10 : relative variations of the latching current i l versus the junction temperature t j (typ. values). 1. quadrant 2 2. quadrants 1, 3 and 4. example 3 : triac to 220, type bta 12.600 c if i l (qi) = 20 ma at tj = 25 c, then i l (qi) = 30 ma at tj = 40 c d) influence of the external gate-cathode resistor r gc when using sensitive thyristors, the designer could wire a resistor r gc betweencathode and gate to im- prove their voltage capability at high temperatures (shunting of leakage currents). this resistor affects the value of the latching current i l in different proportions depending on its resistive value and the sensitivity of the component. 1. sensitive thyristors (i gt < 500 m a) resistor r gc connectedbetween gate and cathode ( figure 11 ) hasan important influenceon the latching current i l of sensitive thyristors. for some applica- tions, the designer would be well advised to define a high impedance triggering circuit. figure 11 : variation of the latching current i l of a sensitive thyristor (e. g. tls106-6) as a function of the gate-cathode resistance r gc (typ. values). note : the latching current of sensitive thyristors is always specified with a 1000-ohm gate-cathode resistor. d89AN303-08 d89AN303-09 ? application note 8/10
2. standard thyristors, sensitive and standard triacs a resistor connected between the gate and cathode of one of these componentsdoesnot have a significative influenceon the valueof itslatching currenti l (oncondi- tion that its value is not too low r gc > 20 ohms). e) variation of the latching current i l with the control conditions the latching current i l of a triac or a thyristor rated at less than 60 a rms varies with the amplitude and the width of the triggering pulse i g . with a constant pulse width(< 50 m s), an increase in the amplitude of i g will lead to an increase in the latching current i l and vice versa, if the amplitude of i g is kept constant, a decrease in the width of the triggering pulse will lead to an increase in the latching current i l that can even lead to an absence of firing of the device ( figure 12 ). figure 12 : variation of the latching current i l versus the width tp and the level of the gate current i g (represented here as a multiple of the triggering current i gt of the triac under consideration) triac btb 16.600 b (quadrant 1) (typical values). negative biasing of the gate circuit (example : shape of the pulse in figure 13a ) increases the latching current i l in considerable proportions. if the de- creasing speed dl g /dt of the gate current is low (example: pulse shape of figure 13 b ) (less than 0.5 a/ m s) the valueof the latching currentapproachesthe holding current i h . figure 13a : gate current pulse with negative current at the end of the pulse : increase of the latching current il. figure 13b : gate current pulse (diac controlled type) with tailing and without nega- tive current : decrease of the latching current il. d89AN303-10 d89AN303-11 d89AN303-12 ? application note 9/10
in order to obtain the lowest possible values for the lat- ching currenti l , andthusensurecorrectfiring of the de- vice,it is advisableto workwith anamplitude ofi g equal to 1.2 i gt and a width of the control current as high as possible. the firing technique using trains of closely spaced pulses ensures stable firing in total security. control pulses with smooth tailing edges and wit hout reverse current allo reducing the latching current. conclusion the choice of a thyristor or ofa triac does not depend only on the rated current, voltage and sensitivity. o- ther parameters also play an important part in the correct operation of a circuit and should be taken into account. the latching current i l is one of these. its value varies with : - the way in which the device is controlled (shape of the gate pulse) - the temperature - the trigger circuit (case of sensitive thyristors) - the direction of the current. triac and thyristor applications involving highly in- ductive loads or loads with considerable variations of controlled power are the main applications where the latching current i l plays a determining role. taking theseelementsinto account will enablethedes- igner to obtain satisfactoryoperation of his circuit in in- dustrial applications. bibliographie 1 - ocontrol of triacs for inductive loadso : technical information ti 36 / sgs thomson microelectronics by x. durbecq. 2 - ohypostatic current or holding currento by e. le- blanc. information furnished is believed to be accurate and reliable. however, sgs-thom son 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 10/10
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