Page 1 : 2.2 SCR mounting & Cooling :, 2.2.1 SCR Mounting :, Mounting of SCR : When the current passes through SCR is greater than the rated, value, the thermal stress produced in it which generates mechanical force., If this mechanical force does not control, the SCR may damaged., The protection of the SCR in such a condition is done by proper mounting of it. The, mounting method depends upon the rating of the SCR., Lead mounting :, This method is used when load current is of small value., The SCR does not require cooling device or heat sink in this method because most, of the heat is dissipated by radiation and convection., Bolt mounting :, There is one hole is provided in the SCR. The heat sink and SCR are joined by the, bolt. The mica or fibre insulation is kept in between the heat sink and SCR., This type of mounting is used in the small and medium rating SCR., Stud Mounting :There are two molybdenum plates kept on both sides of SCR., The anode is soldered with aluminium resulting one stud is created., The SCR is joined to heat sink by this stud., If there is not necessary electrical isolation, mica or fibre type washers are used., The conduction of heat is done easily though mica or fibre type washers. It will, also works as electrical insulator.

Page 2 : 2.2 SCR mounting & Cooling :, 2.2.1 SCR Cooling :, Different methods of cooling a power devices as follows :, 1.Natural air cooling :, -Here the heat is transferred from the heat sink to the atmosphere., -For effective cooling the heat sink should be mounted vertically., -If it is mounted horizonatally , then it must be derated by 15% to 20 %., 2. Force air cooling :, - The effectiveness of the heat sink can be increased by using forced air, cooling , with the help of blower fans., - This increses the volume of air flowing over the heat sink in given time, period & therefore takes out more heat., - The size of the heat can be reduced can be reduced if air cooling is used., 3.Forced liquid cooling :, - Water cooled heat sink use to remove the generated heat within the, device., - This type of cooling is used when high power rectifiers are used., - Water as coolant has high thermal conductivity and low viscosity., -However it has poor insulating properties & freezing point at 00 c . for long, term operation deionized distilled water should be used., -This allows the water to be used as coolant in system requiring 100kv, isolation ., - This has further advantage that it prevents corrosionof aluminium & copper, pipes.

Page 4 : Construction of a Triac : As mentioned above, triac is a three terminal, four layer, bilateral semiconductor device. It incorporates two SCRs connected in inverse, parallel with a comÂ-mon gate terminal in a single chip device. The arrangement of, the triac is shown in figure. As seen, it has six doped regions. The gate terminal G, makes ohmic contacts with both the N and P materials. This permits trigger pulse of, either polarity to start conduction. Electrical equivalent circuit and schematic, symbol are shown in figure.b and figure.c respectively. Since the triac is a bilateral, device, the term “anode” and “cathode” has no meaning, and therefore, terminals, are designated as main terminal 1. (MT1), main terminal 2 (MT2) and gate G. To, avoid confusion, it has become common practice to specify all voltages and currents, using MT1 as the reference., Operation and Working of a Triac :, Though the triac can be turned on without any gate current provided the supply, voltage becomes equal to the breakover voltage of the triac but the normal way to, turn on the triac is by applying a proper gate current. As in case of SCR, here too,, the larger the gate current, the smaller the supply voltage at which the triac is turned, on. Triac can conduct current irrespective of the voltage polarity of terminals MT1, and MT2 with respect to each other and that of gate and terminal MT2., Consequently four different possibilities of operation of triac exists. They are:

Page 5 : 1. Terminal MT2 and gate are positive with respect to terminal MT1 When terminal, MT2 is positive with respect to terminal MT1 current flows through path P1-N1P2-N2. The two junctions P1-N1 and P2-N2 are forward biased whereas junction, N1 P2 is blocked. The triac is now said to be positively biased., A positive gate with respect to terminal MT1 forward biases the junction P2-N2 and, the breakÂ-down occurs as in a normal SCR., 2. Terminal MT2 is positive but gate is negative with respect to terminal MT1, Though the flow path of current remains the same as in mode 1 but now junction, P2-N3 is forward biased and current carriers injected into P2 turn on the triac., 3.Terminal MT2 and gate are negative with respect to terminal MT1, When terminal MT2 is negative with respect to terminal MT1, the current flow path, is P2-N1-P1-N4. The two junctions P2-N1 and P1 – N4 are forward biased whereas, junction N1-P1 is blocked. The triac is now said to be negatively biased., SCR A negative gate with respect to terminal MT1 injects current carriers by, forward biasing juncÂ-tion P2-N3 and thus initiates the conduction., 4. Terminal MT2 is negative but gate is positive with respect to terminal MT1, Though the flow path of current remains the same as in mode 3 but now junction, P2-N2 is forward biased, current carriers are injected and therefore, the triac is, turned on., Generally, trigger mode 4 should be avoided especially in circuits where high di/dt, Â may occur. The sensitivity of triggering modes 2 and 3 is high and in case of, marginal triggerÂ-ing capability negative gate pulses should be used. Though the, triggering mode 1 is more sensitive compared to modes 2 and 3, it requires a

Page 6 : positive gate trigger. However, for bidirectional control and uniform gate trigger, modes 2 and 3 are preferred., , V-I characteristics of triac :, , Because the triac essentially consists of two SCRs of opposite orientation fabricated, in the same crystal, its operating characteristics in the first and third quadrants are, the same except for the direction of applied voltage and current flow., The following points may be noted from the triac characteristics:, The V-I characteristics for triac in the Ist and IIIrd quadrants are essentially, identical to those of an SCR in the Ist quadrant., The triac can be operated with either positive or negative gate control voltage but in, normal operation usually the gate voltage is positive in Ist quadrant and negative in, IIIrd quadrant., The supply voltage at which the triac is turned ON depends upon the gate current., The greater the gate current, the smaller the supply voltage at which the triac is, turned on. This permits to use a triac to control a.c. power in a load from zero to full, power in a smooth and continuous manner with no loss in the controlling device.

Page 7 : EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 2. LASCR :, , Construction of LASCR :, The basic construction of LASCR (light activated silicon controlled rectifier) is, shown in above fig., LASCR is a semiconductor opto electronic switch which has a lens that focuses, light on its gate. It is triggered into conduction by light. If sufficient light does not, fall on the LASCR then it remains in off state., Working :, When light is focused on LASCR (light activated silicon controlled rectifier), the, incident photons will generate electron hole pairs. The number of optically, generated electron hole pairs is proportional to the intensity of light. These electrons, will constitute a gate current for LASCR and due to the internal current, multiplication. The LASCR is latched into ON state. The LASCR is most sensitive, to light when its gate terminal is left open. Its sensitivity can be reduced and

Page 8 : controlled to some extent by inserting a resistor between its gate and cathode, terminals. It offers complete electrical isolation between the triggering source and, the switching device. The forward breakdown voltage decreases with increase in, light intensity., VI Characteristics :, , The VI characteristics of LASCR is similar to that of an SCR ., The forward breakdown voltage decrease with increase in light intensity., Applications :, o, , The light activated silicon control rectifier (LASCR) is used in high voltage and, high current applications., , o, , The LASCR is used in HVDC transmission and VAR compensation., , o, , It is used in light activated flash units, logic circuits and etc., , o, , It is used in alarm circuits., , o, , The LASCR is used in large computer applications., , o, , It is used in optical light control., , o, , It is used in phase control., , o, , It is used in motor control.

Page 9 : EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 3. GTO (Gate turn off SCR) :, , Construction of a Gate Turn-Off Thyristor, Consider the below structure of GTO, which is almost similar to the thyristor. It is, also a four layer, three junction P-N-P-N device like a standard thyristor. In this, the, n+ layer at the cathode end is highly doped to obtain high emitter efficiency. This, result the breakdown voltage of the junction J3 is low which is typically in the, range of 20 to 40 volts., The doping level of the p type gate is highly graded because the doping level should, be low to maintain high emitter efficiency, whereas for having a good turn OFF, properties, doping of this region should be high. In addition, gate and cathodes

Page 10 : should be highly interdigited with various geometric forms to optimize the current, turn off capability., The junction between the P+ anode and N base is called anode junction. A heavily, doped P+ anode region is required to obtain the higher efficiency anode junction so, that a good turn ON properties is achieved. However, the turn OFF capabilities are, affected with such GTOs., This problem can be solved by introducing heavily doped N+ layers at regular, intervals in P+ anode layer as shown in figure. So this N+ layer makes a direct, contact with N layer at junction J1. This cause the electrons to travel from base N, region directly to anode metal contact without causing hole injection from P+, anode. This is called as a anode shorted GTO structure., Due to these anode shorts, the reverse blocking capacity of the GTO is reduced to, the reverse breakdown voltage of junction j3 and hence speeds up the turn OFF, mechanism., However, with a large number of anode shorts, the efficiency of the anode junction, reduces and hence the turn ON performance of the GTO degrades. Therefore,, careful considerations have to be taken about the density of these anode shorts for a, good turn ON and OFF performance., Gate Turn-Off Thyristor Operation Principles :, The turn ON operation of GTO is similar to a conventional thyristor. When the, anode terminal is made positive with respect to cathode by applying a positive gate, current, the hole current injection from gate forward bias the cathode p-base, junction.

Page 11 : This results in the emission of electrons from the cathode towards the anode, terminal. This induces the hole injection from the anode terminal into the base, region. This injection of holes and electrons continuous till the GTO comes into the, conduction state., In case of thyristor, the conduction starts initially by turning ON the area of cathode, adjacent to the gate terminal. And thus, by plasma spreading the remaining area, comes into the conduction., Unlike a thyristor, GTO consists of narrow cathode elements which are heavily, interdigitated with gate terminal, thereby initial turned ON area is very large and, plasma spreading is small. Hence the GTO comes into the conduction state very, quickly., To turn OFF a conducting GTO, a reverse bias is applied at the gate by making the, gate negative with respect to cathode. A part of the holes from the P base layer is, extracted through the gate which suppress the injection of electrons from the, cathode., In response to this, more hole current is extracted through the gate results more, suppression of electrons from the cathode. Eventually, the voltage drop across the p, base junction causes to reverse bias the gate cathode junction and hence the GTO is, turned OFF., During the hole extraction process, the p-base region is gradually depleted so that, the conduction area squeezed. As this process continuous, the anode current flows, through remote areas forming high current density filaments. This causes local hot, spots which can damage the device unless these filaments are extinguished quickly.

Page 12 : By the application of high negative gate voltage these filaments are extinguished, rapidly. Due to the N base region stored charge, the anode to gate current continues, to flow even though the cathode current is ceased. This is called a tail current which, decays exponentially as the excess charge carriers are reduced by the recombination, process. Once the tail current reduced to a leakage current level, the device retains, its forward blocking characteristics., V-I Characteristics of Gate Turn-Off Thyristor :, , During the turn ON, GTO is similar to thyristor in its operates.So the first quadrant, characteristics are similar to the thyristor. When the anode is made positive with, respect to cathode, the device operates in forward blocking mode. By the, application of positive gate signal triggers the GTO into conduction state., The latching current and forward leakage currents are considerably higher in GTO, compared to the thyristor as shown in figure. The gate drive can be removed if the, anode current is above the holding current level., But it is recommended not to remove the positive gate drive during conduction and, to hold at value more than the maximum critical gate current. This is because the

Page 13 : cathode is subdivided into small finger elements as discussed above to assist the, turn OFF process., This causes the anode current dips below the holding current level transiently,, which forces a high anode current at a high rate back into the GTO. This can be, potentially destructive. Therefore, some manufacturers recommend the continuous, gate signal during the conduction state., The GTO can be turned OFF by the application of reverse gate current which can be, either step or ramp drive. The GTO can be turned OFF without reversing anode, voltage. The dashed line in the figure shows i-v trajectory during the turn OFF for, an inductive load. It should be noted that during the turn OFF, GTO can block a, rated forward voltage only., To avoid dv/dt triggering and protect the device during turn OFF, either a, recommended value of resistance must be connected between the gate and cathode, or a small reverse bias voltage (typically -2V) must be maintained on the gate, terminal. This prevents the gate cathode junction to become forward biased and, hence the GTO sustains during the turn OFF state., In reverse biased condition of GTO, the blocking capability is depends on the type, of GTO. A symmetric GTO has a high reverse blocking capability while, asymmetric GTO has a small reverse blocking capability as shown in figure., It is observed that, during reverse biased condition, after a small reverse voltage (20, to 30 V) GTO starts conducting in reverse direction due to the anode short structure., This mode of operation does not destroy the device provided that the gate is, negatively biased and the time of this operation should be small.

Page 14 : EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 4.DIAC :, , Construction and Operation of DIAC :, Basically, the DIAC is a two terminal device; it is a combination of parallel, semiconductor layers that allows activating in one direction.This device is, used to activating device for the triac. The basic construction of diac consist, of two terminals namely MT1 and MT2. When the MT1 terminal is designed, +Ve with respect to the terminal MT2, the transmission will take place to the, p-n-p-n structure that is another four layer diaode. The diac can be, performing for both the direction. Then symbol of the diac look like a, transistor., The DIAC is bosicaly a diode that conducts after a ‘break-over’ voltage,, selected VBO, and is exceeded. When the diode surpasses the break-over, voltage, then it goes into the negative dynamic resistance of region. This, causes in a reduce in the voltage drop across the diode with rising voltage.

Page 15 : So there is a quick increase in the current level that is mannered by the, device., The diode leftovers in its transmission state until the current through it falls, below, what is termed the holding current, which is usually chosen by the, letters IH. The holding current, the DIAC reverts to its non-conducting state., Its behavior is bidirectional and thus its function takes place on both halves, of an alternating cycle., , Characteristics of DIAC :, , Volt-ampere characteristic of a diac is shown in figure. Its looks like a letter, Z due to symmetrical switching characteristics for each polarity of the, applied voltage., The diac performs like an open-circuit until its switching is exceeded. At that, position the diac performs until its current decreases toward zero. Because

Page 16 : of its abnormal construction, doesn’t switch sharply into a low voltage, condition at a low current level like the triac or SCR, once it goes into, transmission, the diac preserves an almost continuous –Ve resistance, characteristic, that means, voltage reduces with the enlarge in current. This, means that, unlike the triac and the SCR, the diac cannot be estimated to, maintain a low voltage drop until its current falls below the level of holding, current., EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 5. SCS (silicon controlled switch) :, , It is four-layer, 3 junction, two gate PNPN device with end p-terminal forming the, anode, and n-terminal forming the cathode, a cathode gate at the p layer next to the, cathode and an anode gate at the n-layer next to the anode., The working of SCS may be studied on considering the SCS to be formed of two, transistor Q1 and Q2 placed back-to-back as shown in figure 2. The transistor Q1 is, of npn type. Figure 3 gives the circuit representation using circuit symbols for the, transistors. In the circuit of figure 3, the base current IB of npn transistor Q2 gets, multiplied by the current gain to constitute the collector current of Q2 and this

Page 17 : current form the base current of transistor Q1. This current after multiplication by, the current gain of transistor Q1 constitutes the base current of transistor Q2. If, this base current exceeds the initial base current IB i.e. if then both the transistors, Q1 and Q2 are driven to saturation and the device operates in the ON state. The, quantity decides the ON condition of the device. Hence, any factor with influences, either or will also influenced the operation of the device. Reduction in either or, reduced the product and in case reduces below unity, SCS gets switched off., , Volt-Ampere Characteristic of Silicon Controlled Switch (SCS) :, , Figure 4 gives the volt-ampere characteristic of SCS for specific value of gate, voltages. It is similar to that of SCR. Thus, as the applied forward voltage V, increases, the current initially is extremely small and increases extremely slowly

Page 18 : upto point A on the characteristic. However, beyond point A, current increases, rapidly. At voltage corresponding to point B, the product exceeds unity and, suddenly the device switches to ON state. In the ON state, the current becomes vary, layer limited only by the extremely series resistor., In the ON state, as the applied forward voltage is reduced, reduces, current reduces, and conduction stops if current is reduced below the holding current IH., Corresponding voltage is the holding voltage VH., In similarity with SCR, the silicon-controlled switch also exhibits negative, differential resistance in the region BH shown dotted in figure 4. If the anode, voltage gets applied suddenly, the SCS gets switched on accidentally due to the rate, effect caused by transition capacitance at reverse biased junction J2., EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 5. UJT (Unijuction transistor) :, , Construction of UJT:, UJT is a three-terminal, single-junction, two-layered device, and it is, similar to a thyristor compare to a transistors. It has a high-impedance off, state and low-impedance on state quite similar to a thyristor. From off

Page 19 : state to an on state, switching is caused by conductivity modulation and, not by a bipolar transistor action., the silicon bar has two Ohmic contacts designated as base1 and base2, as, shown in the fig. The function of the base and the emitter are different, from the base and emitter of a bipolar transistor., The emitter is of P-type, and it is heavily doped. The resistance between, B1 and B2 when the emitter is open-circuited is called an inter-base, resistance. The emitter junction is usually situated closer to the base B2, than the base B1. So the device is not symmetrical, because symmetrical, unit does not provide electrical characteristics to most of the applications., The symbol for uni-junction transistor is shown in the fig. When the, device is forward-biased, it is active or is in the conducting state. The, emitter is drawn at an angle to the vertical line which represents the Ntype material slab and the arrow head points in the direction of, conventional current., VI characteristics :

Page 20 : The static emitter charÂ-acteristic (a curve showing the relation between, emitter voltage VE and emitter current IE) of a UJT at a given inter base, voltage VBB is shown in figure. Â From figure it is noted that for emitter, potentials to the left of peak point, emitter current IE never exceeds IEo ., The current IEo corresponds very closely to the reverse leakage current, ICo of the conventional BJT. This region, as shown in the figure, is called, the cut-off region. Once conÂ-duction is established at VE = VP the, emitter poÂ-tential VE starts decreasing with the increase in emitter, current IE. This Corresponds exactly with the decrease in resistance RB, for increasing curÂ-rent IE. This device, therefore, has a negative, resistance region which is stable enough to be used with a great deal of, reliability in the areas of applications listed earlier. Eventually, the valley, point reaches, and any further increase in emitter current IE places the, device in the saturation region, as shown in the figure. Three other

Page 21 : important parameters for the UJT are IP, VV and IV and are defined, below:, Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It, repreÂ-sents the rnimrnum current that is required to trigger the device, (UJT). It is inversely proportional to the interbase voltage VBB., Valley Point Voltage VV The valley point voltage is the emitter voltage at, the valley point. The valley voltage increases with the increase in interbase, voltage VBB., Valley Point Current IV The valley point current is the emitter current at, the valley point. It increases with the increase in inter-base voltage VBB., Special Features of UJT. The special features of a UJT are :, A stable triggering voltage (VP)— a fixed fraction of applied inter base, voltage VBB., A very low value of triggering current., A high pulse current capability., A negative resistance characteristic., Low cost., EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 7. PUT : Prpogrammable Unijuction transistor

Page 22 : Construction & Working :, Programmable unijunction transistor or PUT is a close relative of the thyristor, family. Its has a four layered construction just like the thyristors and have three, terminals named anode(A), cathode(K) and gate(G) again like the thyristors. Yet, some authors call it a programmable UJT just because its characteristics and, parameters have much similarity to that of the unijunction transistor. It is called, programmable because the parameters like intrinsic standoff ratio (η), peak, voltage(Vp) etc can be programmed with the help of two external resistors. In a, UJT, the parameters like Vp, η etc are fixed and we cannot change it. The main, application of programmable UJT are relaxation oscillators , thyristor firing, pulse, circuits and timing circuits. ON Semiconductor® is the only manufacturer of PUT, now.  2N6027 is the most common type number and it is available in the TO-92, plastic package.  The internal block diagram and circuit symbol of PUT are shown, below., , From the above figure, you can see that the PUT has a four layered construction., Topmost P-layer is called the anode (A). The N-layer next to the anode is called the, gate (G). The P-layer next to the gate is left alone. The bottom most N-layer is, called cathode (K). Ohmic contacts are made on the anode, cathode and gate layers, for external connection.

Page 23 : PUT characteristics., PUT characteristics is essentially a plot between the anode voltage Va and anode, current Ia of the PUT. The typical biasing diagram and characteristics plot of a PUT, is shown below., , Typically the anode of the PUT is connected to a positive voltage and the cathode is, connected to the ground. The gate is connected to the junction of the two external, resistor R1 and R2 which forms a voltage divider network. It is the value of these, two resistors that determines the intrinsic standoff ratio(η) and peak voltage (Vp) of, the PUT., When the anode to cathode voltage (Va)is increased the anode current will also get, increased and the  junction behaves like a typical P-N junction. But the Va cannot, be increased beyond a particular point. At this point sufficient number of charges, are injected and the junction starts to saturate. Beyond this point the anode current, (Ia) increases and the anode voltage (Va) decreases. This is equal to a negative, resistance scenario and this negative resistance region in the PUT characteristic is, used in relaxation oscillators. When the anode voltage (Va) is reduced to a, particular level called “Valley Point”, the device becomes fully saturated and no, more decrease in Va is possible. There after the device behaves like a fully, saturated P-N junction.

Page 24 : Peak voltage (Vp): It is the anode to cathode voltage after which the PUT jumps, into the negative resistance region. The peak voltage Vp will be usually one diode, drop (0.7V) plus the gate to cathode voltage (Vg). Peak voltage can be expressed, using the equation:, Vp = 0.7V + Vg = 0.7V + VR1 = 0.7V + ηVbb . Where  η is the intrinsic, standoff ratio and Vbb is the total voltage across the external resistor network., Intrinsic standoff ratio ( η) : Intrinsic standoff ratio of a PUT is the ratio of the, external resistor R1 to the sum of R1 and R2.  It helps us to predict how much, voltage will be dropped across the gate and cathode for a given Vbb. The intrinsic, standoff ratio can be expressed using the equation:, η = R1/(R1+R2)., EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 2.4 Protection Circuits :, Thyristor Protection or SCR Protection :, Protection of a device is an important aspect for its reliable and efficient operation., Silicon Controlled Rectifier (SCR) are a very delicate semiconductor device. So, we have to use it in its specified ratings to get desired output. SCR may face, different types of threats during its operation due to over voltages, over currents etc., There are different types of thyristor protection schemes available for satisfactory, operation of the device like, Over voltage protection.

Page 25 : Over current protection., High dv/dt protection., High di/dt protection., Thermal protection., Over Voltage Protection :, It is the most important protection scheme w. r. t. others as thyristors are very, sensitive to over voltages. Maximum time thyristor failures happen due to overvoltage transients., A thyristor may be subjected to internal or external over-voltages., Internal Over-Voltages : After commutation of a thyristor reverse recovery current, decays abruptly with high di/dt which causes a high reverse voltage [as, V =, L(di/dt) so if di/dt is high then V will be large] that can exceed the rated break-over, voltage and the device may be damaged., External Over-Voltages : These are caused due to various reasons in the supply line, like lightning, surge conditions (abnormal voltage spike) etc. External over voltage, may cause different types of problem in thyristor operation like increase in leakage, current, permanent breakdown of junctions, unwanted turn-on of devices etc. So,, we have to suppress the over-voltages., Protection Against Over voltages

Page 26 : To protect the SCR against the transient over voltages, a parallel R-C snubber, network is provided for each SCR in a converter circuit. This snubber network, protects the SCR against internal over voltages that are caused during the reverse, recovery process. After the SCR is turned OFF or commutated, the reverse recover, current is diverted to the snubber circuit which consists of energy storing elements., The lightning and switching surges at the input side may damage the converter or, the transformer. And the effect of these voltages is minimised by using voltage, clamping devices across the SCR. Therefore, voltage clamping devices like metal, oxide varistors, selenium thyrector diodes and avalanche diode suppressors are most, commonly employed., These devices have falling resistance characteristics with an increase in voltage., Therefore, these devices provide a low resistance path across the SCR when a surge, voltage appears across the device. The figure below shows the protection of SCR, against over voltages using thyrector diode and snubber network., EE3I/FPE/UNIT2

Page 27 : Unit 2- Thyristor family Devices :, 2.Overcurrent :, During the short circuit conditions, over current flows through the SCR. These short, circuits are either internal or external. The internal short circuits are caused by the, reasons like failure of SCRs to block forward or reverse voltages, misalignment of, firing pulses, short circuit of converter output terminals due to fault in connecting, cables or the load, etc. The external short circuits are caused by sustained overloads, and short circuit in the load., In the event of a short circuit, the fault current depends on the source impedance. If, the source impedance is sufficient during the short circuit, then the fault current is, limited below the multi-cycle surge rating of the SCR. In case of AC circuits, the, fault occurs at the instant of peak voltages if the source resistance is neglected., In case of DC circuits, fault current is limited by the source resistance. Therefore,, the fault current is very large if the source impedance is very low. The rapid rise of, this current increase the junction temperature and hence the SCR may get damaged., Hence the fault must be cleared before occurrence of its first peak in other words, fault current must be interrupted before the current zero position., Protection Against Overcurrent, The SCRs can be protected against the over currents using conventional over, current protection devices like ordinary fuses (HRC fuse, rewirable fuse,, semiconductor fuse, etc,), contractors, relays and circuit breakers. Generally for, continuous overloads and surge currents of long duration, a circuit breaker is, employed to protect the SCR due to its long tripping time.

Page 28 : For an effective tripping of the circuit breaker, tripping time must be properly, coordinated with SCR rating. Also, the large surge currents with short duration (are, also called as sub-cycle surge currents) are limited by connecting the fast acting, fuse in series with an SCR., So the proper coordination of fusing time and the sub-cycle rating must be selected, for a reliable protection against over currents. Therefore, the proper coordination of, fuse and circuit breaker is essential with the rating of the SCR., , The selection of fuse for protecting the SCR must satisfy the following conditions., , , Fuse must be rated to carry the full load current continuously plus a marginal, overload current for a small period., , , , I2t rating of the fuse must be less than the I2t rating of the SCR, , , , During arcing period, fuse voltage must be high in order to force down the, current value., , , , After interrupting the current, fuse must withstand for any restricted voltage.

Page 29 : di/dt Protection of SCR, The anode current starts flowing through the SCR when it is turned ON by the, application of gate signal. This anode current takes some finite time to spread, across the junctions of an SCR. For a good working of SCR, this current must, spread uniformly over the surface of the junction., If the rate of rise of anode current (di/dt) is high results a non-uniform spreading of, current over the junction. Due to the high current density, this further leads to form, local hot spots near the gate-cathode junction. This effect may damage the SCR due, to overheating. Hence, during turn ON process of SCR, the di/dt must be kept, below the specified limits., To prevent the high rate of change of current, an inductor is connected in series, with thyristor. Typical SCR di/dt ratings are in range between 20- 500 ampere per, microseconds., dv/dt Protection of SCR, When the SCR is forward biased, junctions J1 and J3 forward biased and junction, J2 is reverse biased. This reverse biased junction J2 exhibits the characteristics of a, capacitor. Therefore, if the rate of forward voltage applied is very high across the, SCR, charging current flows through the junction J2 is high enough to turn ON the, SCR even without any gate signal., This is called as dv/dt triggering of the SCR which is generally not employed as it is, false triggering process. Hence, the rate of rise of anode to cathode voltage, dv/dt

Page 30 : must be in specified limit to protect the SCR against false triggering. This can be, achieved by using RC snubber network across the SCR., EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :, 3. Snubber :, The main purpose of Snubber Circuit is to prevent the unwanted triggering of SCR, or thyristor due to high rate of rise of voltage i.e. dv/dt. We already know that if the, rate of rise of anode to cathode voltage of SCR is high then it may lead to false, triggering. This is commonly known asdv/dt triggering. Thus we need to have some, arrangement to protect SCR from such undesirable turning. Application of Snubber, Circuit prevents from such spurious triggering of SCR. Thus it is basically dv/dt, protection of SCR., Working of Snubber Circuit, As we discussed above, the protection against high voltage reverse recovery, transients and dv/dt is achieved by using an RC snubber circuit. This snubber circuit, consists of a series combination of capacitor and resistor which is connected across, the SCR. This also consist an inductance in series with the SCR to prevent the high, di/dt. The resistance value is of few hundred ohms. The snubber network used for, the protection of SCR is shown below.

Page 31 : When the switch closed, a sudden voltage appears across the SCR which is, bypassed to the RC network. This is because the capacitor acts as a short circuit, which reduces the voltage across the SCR to zero. As the time increases, voltage, across the capacitor builds up at slow rate such that dv/dt across the capacitor is too, small to turn ON the SCR. Therefore, the dv/dt across the SCR and the capacitor is, less than the maximum dv/dt rating of the SCR., Normally, the capacitor is charged to a voltage equal the maximum supply voltage, which is the forward blocking voltage of the SCR. If the SCR is turned ON, the, capacitor starts discharging which causes a high current to flow through the SCR., This produces a high di/dt that leads to damage the SCR. And hence, to limit the, high di/dt and peak discharge current, a small resistance is placed in series with the, capacitor as shown in above. These snubber circuits can also be connected to any, switching circuit to limit the high surge or transient voltages., , EE3I/FPE/UNIT2, , Unit 2- Thyristor family Devices :

Page 32 : 3. Crowbar :, Thyristor / SCR overvoltage crowbar circuit :, The thyristor crowbar circuit shown is very simple, only using a few components. It, can be used within many power supplies, and could even be retro-fitted in situations, where no over-voltage protection may be incorporated., It uses just four components: a silicon controlled rectifier or SCR, a zener diode, a, resistor and a capacitor., , The SCR over voltage crowbar or protection circuit is connected between the output, of the power supply and ground. The Zener diode voltage is chosen to be slightly, above that of the output rail. Typically a 5 volt rail may run with a 6.2 volt Zener, diode. When the Zener diode voltage is reached, current will flow through the Zener, and trigger the silicon controlled rectifier or thyristor. This will then provide a short, circuit to ground, thereby protecting the circuitry that is being supplied form any

Page 33 : damage and also blowing the fuse that will then remove the voltage from the series, regulator., As a silicon controlled rectifier, SCR, or thyristor is able to carry a relatively high, current - even quite average devices can conduct five amps and short current peaks, of may be 50 and more amps, cheap devices can provide a very good level of, protection for small cost. Also voltage across the SCR will be low, typically only a, volt when it has fired and as a result the heat sinking is not a problem., The small resistor, often around 100 ohms from the gate of the thyristor or SCR to, ground is required so that the Zener can supply a reasonable current when it turns, on. It also clamps the gate voltage at ground potential until the Zener turns on. The, capacitor C1 is present to ensure that short spikes to not trigger the circuit. Some, optimisation may be required in choosing the correct value although 0.1, microfarads is a good starting point., If the power supply is to be used with radio transmitters, the filtering on the input to, the gate may need to be a little more sophisticated, otherwise RF from the, transmitter may get onto the gate and cause false triggering. The capacitor C1 will, need to be present, but a small amount of inductance may also help. A ferrite bead, may even be sufficient. Experimentation to ensure that the time delay for the, thyristor to trigger is not too long against removing the RF. Filtering on the power, line to / from the transmitter can also help