AN303 APPLICATION NOTE LATCHING CURRENT INTRODUCTION An imporan problem concerning he uilizaion of componens such as hyrisors or riacs is he holding of he componen in he conducing sae afer he rigger curren has disappeared during firing. Very ofen, he firing problems supposedly due o he gae curren I G or o he firing ime GT are in realiy due o he laching curren I L. Afer a definiion we will illusrae he imporance of his parameer by concree examples. Then we will describe how o measure i and is variaion according o he uilizaion condiions of he componens. The sudy will be based on he riac. The poins reaed are valid for hyrisors (excep for he various conducion modes). DEFINITION The laching curren, I L, of a riac is he minimum value of he main curren (curren flowing beween elecrodes A 2 and A 1 ) which enables he componen o remain in he conducing sae afer he gae curren I G has ceased (see Figure 1). Figure 1. Conrolled by he gae pulse, I G, he riac is fired, and a curren I T flows hrough i, imposed by he main curren. If he gae curren I G is sopped before curren I T reaches he value of he laching curren I L, he riac is blocked (as shown in he figure). > T RL IT Gae curren A2 IT IL G A1 Main curren May 2004 REV. 2 1/13
APPLICATIONS Example 1: Conrol of a Low Power Signalling Lamp by Triac Figure 2. Conrol of a Low Power Signalling Lamp by Triac IL Lamp (10W) Main curren 220V IT A2 A1 G Gae curren Curren in he main circui of he riac and gae curren. The lamp power is oo low (e.g: P 10 W and he riac BTA 12.400 B) o impose a sufficien curren (shown in doed lines in he diagram) in he riac o keep i in he conducing sae afer inerrupion of he gae curren I G. The riac does no conduc. A BTA 12.400 B riac is used o conrol he flashing of a 10 W signalling ligh. The peak curren in he circui will herefore be 65 ma. This value is very close o ha of he ypical laching curren given in he daa book for his ype of riac: 50mA (quadran 1, 3 and 4). Thus he user s case could be ha described in Figure 2, ha is, a riac whose laching curren I L in he firs quadran is equal o 70mA. His riac will never be fired. For correc operaion, he user should hus employ a sensiive riac (e.g. Z0102MA I L : 8mA). 2/13
Example 2: Conrol of an inducive load by riac Figure 3. Volage Accross and Curren Through he Triac M IT VT A1 A2 G Conrol of an AC moor by riac Gae curren T1 T2 VT Volage across he riac I L Curren hrough he riac 3/13
In coninuous lines. Shor gae signal: he riac does no remain in he conducing sae because he main curren did no reach he value of he riac laching curren before suppression of he gae curren. In doed lines. Long gae signal: he riac is fired and remains in he conducing sae unil is curren falls below he holding curren I H afer suppression of he gae curren I G. On a highly inducive load, he inducance limis he curren rise ime o: di ------- T d (V A : power supply volage a he ime he gae signal is applied; L: load inducance). Consider he operaion on one full-wave of he power supply volage. If he duraion 1 of he gae curren pulse I G is very small compared wih a half-wave of he power supply volage, he riac curren canno reach he riac laching curren level in he firing mode considered (here he 1s quadran). Thus firing will no ake place and he volage across he riac increases. For riggering o be seady, he duraion of pulse 2 should be long when compared wih a half-wave of he power supply volage. The curren se up in he riac is imposed by he load impedance. The riac remains in he conducing sae unil he curren falls below he holding curren I H. I is blocked if he I G curren pulse has ended. Anoher mehod consiss of applying a rain of closely spaced pulses o he riac gae insead of a square wave. The STMicroelecronics applicaions laboraories have developed a number of riac conrol circuis, specially designed o work on inducive loads (see Bibliography, ref. N 1). Example 3: Conrol by riac of a load whose power varies considerably Figure 4. Conrol of an Arc Welding Se by Triac = V ------ A L A 2 C A 1 The designer of an arc welding se whose power is adjusable by riac, chooses a componen capable of conrolling high currens. For example, if he maximum curren o be conrolled is 40A RMS, he designer, for safey, will choose a riac raed a 60A RMS, hus a riac wih a high laching curren. Now, offload, he ransformer magneizing curren could be very low or even below he riac laching curren I L in one of he quadrans. This means ha he riac could fire correcly in he firs quadran and hen no fire if he nex firing is o ake place in he second quadran where he I L is much higher. A considerable unbalance hen occurs, generaing a DC curren heaing he ransformer and prevening he equipmen from operaing correcly. 4/13
Since he laching curren I L increases wih he size of componens, and hus wih heir raing, he user would hus be well advised no o selec an excessively high raing for his riac in order o have he lowes possible laching curren. A.N: For his ype of applicaion, he STMicroelecronics applicaions laboraories place a he disposal of designers a number of schemaics mean for his ype of circui (see bibliography, ref. N 1). These hree examples illusrae he imporance of he I L parameer and he problems ha i can cause in a circui. To ensure sable firing of a riac or a hyrisor, i is absoluely necessary for he circui which is conrolled o impose a curren which is higher han is laching curren. FAVORABLE EFFECT OF AN RC CIRCUIT ON THE FIRING OF A THYRISTOR OR A TRIAC In mos inducive load applicaions of riacs or hyrisors, he user connecs an RC nework beween he anode and cahode of he device o eliminae he risk of premaure firing by ransiens or sponaneous firing by (dv/d)c (case of riacs) (see Figure 5). Capaciance C and he load impedance aenuae seep volage ransiens ransmied by he mains or resuling from swiching inducive loads. Figure 5. Reducing he Risk of Unimely Firing on Inducive Loads: he RC Circui (called Snubber). I A 2 I T C L G A 1 R I C This RC nework has also a second advanage. In fac, he energy accumulaed in capacior C afer urning off is fed back o he riac when firing. The speed a which he curren increases in he riac during discharge of he capacior is hen limied only by he peak charge volage of he capacior and he inducance of he circui connecing he SNUBBER o he riac. The curren ampliude is he quoien of peak charge volage of he capacior by he series resisance R. This circui hus helps he curren o rise very quickly above he laching curren I L of he device (see Figure 6). 5/13
Figure 6. Favorable Effec of he RC Circui for Firing on a Highly Inducive Load I I T1 I T I L curren level I I : curren in he load I C : discharge curren of capacior C I T : I + IC : curren in he riac di T /d I C Noe: When using an RC circui, i is no advisable o work wih a series resisance R which is oo low. In fac, he combined effec during firing of I T1 (see Figure 6) (equal o he quoien of he capacior peak charge volage and resisance R) and he curren slope dl T /d (equal o he quoien of he capaciance charging volage by he inducance of he connecion beween he riac and he RC circui) could be dangerous for he riac. A value for R higher han 100Ω is recommended. LATCHING CURRENT (I L ) MEASUREMENT Figure 7. Laching Curren (I L ) Measuremen Circui R 1 R 1 33Ω V = ± 12V IT A A2 A1 a C R2 V = ± 12V The closing of conac C enables passage of he gae curren whose is seleced higher han ha of he riac firing curren, I GT o be measured. By gradually decreasing he value of resisance R1, while coninuing o ransmi pulses of gae curren I G, he main curren I T is increased. As long as he value of he I T curren is lower han ha of he device laching curren I L, he device does no remain in he conducing sae. The value of he laching curren I L is he value of he I T curren read as soon as he riac remains on, afer suppressing he gae curren I G. Only sensiive hyrisors (I GT 500µA) are measured wih a 1KΩ resisor beween gae and cahode. 6/13
Parameer I L varies wih he widh of he gae curren pulse I GT and is level. For he measuremen o be reproduced correcly, he following rules should hus be observed: Fix a sufficienly wide conrol pulse I G. The widh of he pulse should be a leas equal o 1ms. Impose a gae curren I G sufficienly high wih respec o ha of he riggering curren I GT of he device o be measured. An I G /I GT raio higher han or equal o 1.2 is advisable. Example: BTA 12.600 C I GT max (Q IV) = 50mA herefore I G = 60 ma In he case of a riac, here are four laching curren I L values ha correspond o he four quadrans of riac operaion: (I L + +) when he elecrodes A 2 and G are posiive wih respec o elecrode A 1. (I L + ) when elecrode A 2 is posiive wih respec o elecrode A 1 and elecrode G is negaive wih respec o elecrode A 1. (I L ) when elecrodes A 2 and G are negaive wih respec o elecrode A 1. (I L +) when elecrode A 2 is negaive wih respec o elecrode A 1 and elecrode G is posiive wih respec o elecrode A 1. VARIATIONS OF LATCHING CURRENT I L WITH THE UTILIZATION CONDITIONS a) Variaions of he I L curren wih sensiiviy of riacs and he various direcions of conducion (ypical values). For he low power componens (hyrisors and riacs whose raed curren is lower han 60A) he laching curren I L is dependen on he value of firing curren I GT (see Table 1). Table 1. Raio of he Laching Curren I L in he Differen Quadrans o he Triggering Curren I GT in he Firs Quadran, for Sensiive and Sandard Triacs (ypical values). I L (QI) I GT (QI) I L (QII) I GT (QI) I L (QIII) I GT (QI) 6 Arms Sensiive Triacs 3.5 15 5 3 I L (QIV) I GT (QI) 12 Arms Sandard Triacs 2 5 1.5 1.7 Example 1: BTA 06.600 T: if I GT (Ql) = 1mA hen: I L (Ql) 3.5mA; I L (QII) 15mA I L (QIII) 5mA; I L (QIV) 3mA and BTA 12.600 B: if I GT (QI) = 15mA hen: I L (QI) 30mA; I L (QII) 75mA I L (QIII) 22mA; I L (QIV) 25mA In he case of riacs, as opposed o ha of hyrisors, noe ha: as underlined in Table 1, he curren I L + (elecrode A 2 posiive wih respec o elecrode A 1 and elecrode G negaive wih respec o elecrode A 1 QII) is much higher han he I L curren in he hree oher quadrans. 7/13
In he daa shees wo values are specified: one value for quadrans I, III and IV and one value for quadran II. In general hese values are ypical. b) Relaion beween he laching curren I L and he holding curren I H The holding curren value I H (see bibliography, noe N 2) is linked o he laching curren value, I L. By definiion, he I L curren value will always be higher han he I H curren value. The I L / I H raio varies following he sensiiviy of he riacs and heir raings (see Table 2). Example 2: BTA 12.600 C: I L yp = 40 ma QI, III, IV I L yp = 70 ma QII Depending on he producion baches, parameer I L shows dispersion. Shown below are approximae values: sensiive riacs: I GT (QI) 5mA (ype T): QI, III, IV: 2mA I L 8mA; QII: 10mA I L 40mA sandard riacs: I GT (QI) 50mA (ype B): QI, III, IV: 15mA I L 50mA; QII: 50mA I L 120mA Table 2. Raio of he Laching Curren I L o he Holding Curren I H Depending on he Sensiiviy and Raings of he Devices (ypical values) Sensiive Triacs and Thyrisors I RMS 6A Noe: 1. 1 s quadran in he case of riacs. Medium Power Thyrisors and Triacs 6A I RMS 60A High Power Thyrisors and Triacs 60A I RMS 300A I L / I H (1) 1.1 o 1.5 1.5 o 2 2 o 5 c) Variaions of he laching curren I L wih he juncion emperaure. The value of he laching curren I L is physically linked ha of he riggering curren I GT. These wo parameers herefore way analogously wih he juncion emperaure (see Figure 8). Figure 8. Relaive variaions of he laching curren il versus he juncion emperaure j (yp. values). 1. Quadran 2 2. Quadrans 1, 3 and 4 2.5 I L (Tj) / I L (T j = 25 C) 2.0 1.5 1 2 1.0 0.5 0 j ( C) 40 20 0 20 40 60 80 100 120 8/13
Example 3: Triac TO 220, ype BTA 12.600 C If I L (QI) = 20mA a T j = 25 C, hen I L (QI) = 30mA a T j = 40 C d) Influence of he exernal gae-cahode resisor RGC When using sensiive hyrisors, he designer could wire a resisor RGC beween cahode and gae o improve heir volage capabiliy a high emperaures (shuning of leakage currens). This resisor affecs he value of he laching curren I L in differen proporions depending on is resisive value and he sensiiviy of he componen. 1. Sensiive hyrisors (I GT < 500 ma). Resisor RGC conneced beween gae and cahode (see Figure 9) has an imporan influence on he laching curren I L of sensiive hyrisors. For some applicaions, he designer would be well advised o define a high impedance riggering circui. Figure 9. Variaion of he laching curren I L of a sensiive hyrisor (e.g. TLS106-6) as a funcion of he gae-cahode resisance RGC (yp. values) 0 9 8 7 6 5 4 3 2 1 0 1 10 100 1000 10000 RGK (Ω) A K G Sensiive hyrisor RGK Noe: The laching curren of sensiive hyrisors is always specified wih a 1000Ω gae-cahode resisor. 2. Sandard hyrisors, sensiive and sandard riacs. A resisor conneced beween he gae and cahode of one of hese componens does no have a significaive influence on he value of is laching curren I L (on condiion ha is value is no oo low RGC > 20Ω). e) Variaion of he laching curren I L wih he conrol condiions The laching curren I L of a riac or a hyrisor raed a less han 60A RMS varies wih he ampliude and he widh of he riggering pulse I G. Wih a consan pulse widh (< 50µs), an increase in he ampliude of I G will lead o an increase in he laching curren I L and vice versa, if he ampliude of I G is kep consan, a decrease in he widh of he riggering pulse will lead o an increase in he laching curren I L ha can even lead o an absence of firing of he device (see Figure 10). 9/13
Figure 10. Variaion of he Laching Curren I L versus he Widh p and he Level of he Gae Curren (represened here as a muliple of he riggering curren I GT of he riac under consideraion) Triac BTB 16.600 B (quadran 1) (ypical values) IL (ma) 100 90 80 = 10 T 70 = 5 T 60 50 = 2 T 40 30 20 10 0 T P (µs) 0 10 20 30 40 50 Negaive biasing of he gae circui (example: shape of he pulse in Figure 11) increases he laching curren I L in considerable proporions. If he decreasing speed dl G /d of he gae curren is low (example: pulse shape of Figure 12) (less han 0.5A/µs) he value of he laching curren approaches he holding curren I H. Figure 11. Gae Curren Pulse wih Negaive Curren a he end of he Pulse: Increase of he Laching Curren I L Figure 12. Gae Curren Pulse (diac conrolled ype) wih ailing and wihou Negaive Curren: decrease of he Laching Curren I L 10/13
In order o obain he lowes possible values for he laching curren I L, and hus ensure correc firing of he device, i is advisable o work wih an ampliude of I G equal o 1.2 I GT and a widh of he conrol curren as high as possible. The firing echnique using rains of closely spaced pulses ensures sable firing in oal securiy. Conrol pulses wih smooh ailing edges and wihou reverse curren allo reducing he laching curren. CONCLUSION The choice of a hyrisor or of a riac does no depend only on he raed curren, volage and sensiiviy. Oher parameers also play an imporan par in he correc operaion of a circui and should be aken ino accoun. The laching curren I L is one of hese. Is value varies wih: he way in which he device is conrolled (shape of he gae pulse) he emperaure he rigger circui (case of sensiive hyrisors) he direcion of he curren Triac and hyrisor applicaions involving highly inducive loads or loads wih considerable variaions of conrolled power are he main applicaions where he laching curren I L plays a deermining role. Taking hese elemens ino accoun will enable he designer o obain saisfacory operaion of his circui in indusrial applicaions. BIBLIOGRAPHIE 1. "Conrol of riacs for inducive loads": Technical Informaion TI36 / STMicroelecronics. 2. "Hyposaic curren or holding curren". 11/13
REVISION HISTORY Table 3. Revision Hisory Dae Revision Descripion of Changes Apr-1995 1 Firs Issue 15-May-2004 2 Syleshee updae. No conen change. 12/13
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