Thyristor Family Devices PDF

2. Thyristor Family Devices Q. Draw the structural diagram and symbol of SCR. Features- Unidirectional device Latching device Current controlled device Positive gate current used to turn on SCR Operation of SCR- ▪ Case1- Operation without gate  With the polarity of V, the...

2. Thyristor Family Devices Q. Draw the structural diagram and symbol of SCR. Features- Unidirectional device Latching device Current controlled device Positive gate current used to turn on SCR Operation of SCR- ▪ Case1- Operation without gate  With the polarity of V, the junctions J1 and J3 become forward biased whereas J2 is reverse- biased. Hence, no current (except leakage current) can flow through the SCR. The current flow is blocked due to reverse-biased junction J2. However, when anode voltage is increased, a certain critical value called forward break over voltage (VBO) is reached, when J2 breaks down and SCR switches suddenly to a highly conducting state. The applied voltage at which SCR conducts heavily without gate voltage is called Breakover voltage. Case II- Operation with positive gate current Fig. 20.3 The SCR can be made to conduct heavily at smaller applied voltage by applying a small positive potential to the gate as shown in Fig. 20.3. Now junction J3 is forward biased and junction J2 is reverse biased. The electrons from n-type material start moving across junction J3 towards left whereas holes from p-type towards the right. Consequently, the electrons from junction J3 are attracted across junction J2 and gate current starts flowing. As soon as the gate current flows, anode current increases. The increased anode current in turn makes more electrons available at junction J2. This process continues and in an extremely small time, junction J2 breaks down and the SCR starts conducting heavily. Once SCR starts conducting, the gate loses all control. Even if gate voltage is removed, the anode current does not decrease at all. The only way to stop conduction (i.e. bring SCR in off condition) is to reduce the applied voltage to zero means reduce anode current below holding current or apply reverse supply voltage As gate current increases, VBO value decreases. Case III- Operation with reverse bias SCR With supply connection as in Fig, the current through the SCR is blocked by the two reverse biased junctions J1 and J3. When V is increased, a stage comes when which may destroy the SCR is known Reverse Breakdown Voltage (VBR) VI characteristics of SCR Q. Draw and explain the VI characteristics of SCR.OR Draw the VI characteristics of SCR. State the effects of gate current on the break over voltage. Explanation- Forward Blocking mode: When anode is at a higher potential than cathode, thyristor is said to be forward biased, It is seen from the figure that when the gate circuit is open J1 and J3 are forward biased and junction J2 is reverse bias. In this mode a small current, called forward leakage current flows from anode to cathode. Forward Conduction mode: When anode to cathode forward voltage is increased with gate circuit open, reverse biased junction J2 will have an avalanche breakdown at a voltage called forward breakover voltage VBO. After this breakdown, thyristor gets turned ON. Reverse Blocking mode: When cathode is made high potential with respect to anode with gate open, then the SCR is said to be reverse biased. J1 and J3 are reverse biased and J2 is forward biased. A small current flows through the SCR this is called as reverse leakage current. This is reverse blocking mode, called the OFF state of the SCR. If the reverse voltage increased, then at reverse breakdown voltage VBR, an avalanche breakdown occurs at J1 and J3 and the reverse current increases rapidly VFBO = Forward Break over voltage at Zero gate. It is maximum forward voltage across the SCR at which SCR will turn ON at zero gate current VRBO = Reverse Break down voltage. It is maximum reverse voltage across the SCR at which SCR will damage permanently Effects of Gate current: As the value of gate current (Figure) increases, the value of Forward Break over voltage decreases. Q.Define holding and latching current. Holding current(IH): Holding current may be defined as the minimum value of anode to cathode current below which the SCR stops conducting and returns to its OFF- state. Latching current(IL): latching current may be defined as the minimum ON – state anode to cathode current required to keep the SCR in the ON- state after the triggering pulse has been removed Two transistor analogy of SCR Q.Draw and explain the two transistor analogy of SCR. Working: Fig. Thus, the equivalent circuit of SCR is composed of pnp transistor and npn transistor connected as shown in fig. It is clear that collector of each transistor is coupled to the base of the other, thereby making a positive feedback loop. The working of SCR can be easily explained from its equivalent circuit. Fig. shows the equivalent circuit of SCR with supply voltage V and load resistance RL. Assume the supply voltage V is less than breakover voltage as is usually the case. With gate open (i.e. switch S open), there is no base current in transistor T2. Therefore, no current flows in the collector of T2 and hence that of T1. Under such conditions, the SCR is open. However, if switch S is closed, a small gate current will flow through the base of T2 which means its collector current will increase. The collector current of T2 is the base current of T1. Therefore, collector current of T1 increases. But collector current of T1 is the base current of T2 This action is cumulative since an increase of current in one transistor causes increase in current of another transistor. As a result of this action both the transistor driven into saturation and heavy current flows through the load R. Under this condition, the SCR closes. Applications- ▪ Controlled Rectifier ▪ Choppers ▪ Inverters ▪ As static switch ▪ Speed control of DC and AC motors TRIAC Q. Draw symbol of TRIAC. It can be operated in how many modes? Which mode is sensitive? Features- Bidirectional device so two SCR connected anti-parallel to each other. Latching device Current controlled device Positive as well as negative gate current used to turn on TRIAC Q. Explain four operating modes of TRIAC TRIAC can operate in following four modes: i) Mode 1(I+): MT2 positive w.r.t.MT1 and gate current positive Here terminal MT2 is positive with respect to terminal MT1 current flows through path P1-N1-P2-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 breakdown occurs as in a normal SCR. The device is more sensitive in this mode than other modes of operation. The sensitivity is defined as the amount of gate current required to turn on the device. Lower the gate current is higher is the sensitivity. ii)Mode 2 (I-): MT2 positive w.r.t.MT1 and gate current negative  In this mode, terminal MT2 is positive and MT1is negative and TRIAC is turned on by applying a negative gate voltage with respect to MT1. The initial conduction of current is through the structure P1N1P2N3 which acts like a pilot /auxiliary SCR. Final conduction takes place through the structure P1N1P2N2 which is main SCR The anode current of the pilot SCR acts as gate current for the main SCR. The device is less sensitive to the gate current in this mode than I + mode and needs more gate current to turn on the TRIAC. iii)Mode 3(III+): MT2 negative w.r.t.MT1 and gate current negative When terminal MT2 is negative with respect to terminal MT1, the current flow path is P2-N1-P1-N4. The two junctions P2-N1 andP1 – N4 are forward biased whereas junction N1-P1 is blocked. The TRIAC is now said to be negatively biased. The two transistors P2N1P1 and N1P1N4 form the main P2N1P1N4 SCR whereas the remote N2 along with P2N1 forms an additional transistor see in fig. As the gate P2 is positive w.r.t MT1(N2 layer), the collector current of Q1 acts as base current for Q2 transistor. This will initiate the regenerative action in the main SCR P2N1P1N4 and TRIAC starts conducting. The gate current flows from P2(gate)to N2(MT1) terminal. The gate P2 is not part of main SCR so this mode of TRIAC is switched by the ‘remote gate’ operation. The TRIAC is less sensitive in this mode. iv)Mode 4(III-): MT2 negative w.r.t.MT1 and gate current positive 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-P1is blocked. The TRIAC is now said to be negatively biased. The two transistors P2N1P1 and N1P1N4 form the main P2N1P1N4 SCR whereas the remote N3 along with P2N1 forms an additional transistor see in fig. As the gate P2 is positive w.r.t MT1(N3 layer), the collector current of Q1 acts as base current for Q2 transistor. This will initiate the regenerative action in the main SCR P2N1P1N4 and TRIAC starts conducting. The gate current flows from P2(gate)to N3(MT1) terminal. The gate P2 is not part of main SCR so this mode of TRIAC is switched by the ‘remote gate’ operation. The TRIAC is less sensitive than III+ mode. Out of these all modes TRIAC is more sensitive in mode 1 i.e. MT2 positive w.r.t.MT1 and gate current positive and mode 3 is less sensitive MT2 negative w.r.t MT1 gate negative. Q. Draw V-I characteristics of TRIAC. Give two applications of TRIAC Applications- ▪ Light dimmer using TRIAC-DIAC ▪ Fan speed control using TRIAC –DIAC ▪ As a static switch Q. Differentiate between SCR and TRIAC on the basis of: Symbol Layered diagram Operating quadrant and Application Parameter SCR TRIAC Symbol Layered Diagram Operating Only 1st quadrant Depending upon supply either quadrant Controlled Rectifies, in inverters, 1st quadrant or 3rd quadrant Applicatio Battery charger, speed control of DC As a static switch fans n and AC motors Regulator, lamp dimmer, in AC voltage stabilizer Gate Only positive gate current Positive as well as Negative Current Gate Current GTO Q. Draw the structural diagram and symbol of GTO. Describe its working. OR Draw constructional diagram of GTO and “It is a special type of SCR which can be turn on as well as turn off by using gate terminal” Working : Operating Principle:- Basic operation of GTO is same as that of the conventional SCR but the major difference between is that the conducting GTO can be turned off by applying a negative gate current to it. Thus positive gate current turns it on and negative gate current turns it off. From two transistor model of GTO both transistor Q1 and Q2 are in saturation when the GTO is in it’s on state. In GTO, regenerative action is reduced by using shorted anode structure in which N+ layer touches to base N layer as shown in fig. If the base current of Q2 could be made less than the value needed for maintaining it in saturation, then Q2 will come out of saturation and will be in active state, this will reduce the regeneration and GTO will begin to turn off.  In order to reduce the base current of Q2 & –ve gate current must flow in the direction as shown in diagram. It can be proved that the negative gate current required for turning off a conducting GTO. Q. State two differences between GTO and SCR. GTO SCR 2. GTO can turn off by application of SCR cannot turn off by application of negative pulse of gate terminal. pulse at gate input 3. Gate cathode structure of GTO is Gate cathode structure SCR is not interdigitated. interdigitated. 4. Reverse blocking capacity is less than Reverse blocking capacity of SCR is SCR. more than GTO. 5. Turn off time is less than SCR turn off Turn off time is More than GTO turn off time. time Q.State the advantages and applications of GTO. Advantages of GTO:( any 2) 1. No commutation circuit required, reducing the cost, size and weight of the circuit. 2. As commutation choke is not used, the associated acoustic and electromagnetic disturbances are absent. 3. Less turn off time. Hence high frequency. Applications of GTO:( any 2) 1. Inverter 2. UPS 3. DC motor drives Triggering devices Q. Name any two triggering devices used triggering TRIAC Triggering devices for TRIACs: 1. UJT 2. PUT 3. SUS 4. SBS 5. DIAC LASCR Q. Describe LASCR. Give its industrial applications. The light activated SCR is a three terminal, four layer device that can be turned on by direct radiation of light on the silicon wafer. Operating Principle: When forward voltage is applied between anode and cathode terminals, junction J1 & J3 becomes forward biased. Junction J2 is reversed biased therefore it blocks forward current when the pulse of appropriate wavelength is full on to the special sensitive area of the wafer, and if the intensity of light exceeds a certain value ,excess electron hole pairs are generated due to radiation and forward Biased thyristor gets turned on. As light intensity increases forward break over voltage goes on decreasing Applications: 1) light coupling 2) Triggering circuits 3) Photoelectric control 4) Relays 5) Motor speed control 6) Used in computer 7) Used in high voltage dc transmission (HVDC) 8) Static reactive power or volt ampere reactive (VAR) compensation. DIAC Q Draw and explain the VI characteristics of DIAC OR Draw V-I characteristics of DIAC, Is DIAC equally sensitive in both the directions? Give two application of DIAC. The term DIAC stands for the DIode for Alternating Current (DIAC), it is a bidirectional semiconductor switch that can be turned ON in both forward and reverse direction Constructional details- ▪ Each layer having equal width and equal doping concentration so it delivers symmetrical switching properties in both the polarities of the applied voltage. Working- The above image shows the clear operation of the DIAC with respective to the polarities. Consider the MT1 terminal to be positive, then the P1 layer near MT1 will be activated, so the conduction will be taking place in the order of P1-N2-P2-N3. When the current is flowing from MT1 to MT2 the junction between P1-N2 and P2-N3 are Forward Biased and the junction between N2-P2 is reverse biased. Similarly, if we consider MT2 terminal to be positive, then the P2 layer near MT2 will be activated and the conduction will be taking place in the order of P2-N2-P1-N1. The current will be flowing from MT2 to MT1 and the junctions between P2-N2 and P1-N1 are forward biased and the junction Between N2- P1 is reverse biased. Hence the conduction will be possible in both the directions. VI characteristics of DIAC Q. Draw the constructional details of DIAC. draw the VI characteristics of DIAC Description: ▪ The figure below shows the V-I characteristics of DIAC which indicates the current flow through the DIAC with respect to the voltage across it. ▪ As long as the voltage across the DIAC is within its break-over limits that is from –VBO to +VBO, the resistance offered by the DIAC is very high and only a small leakage current flows through the device ( portion OA & OA') as shown in figure. Under these conditions DIAC operates as an open switch. ▪ The voltages +VBO and –VBO are the breakdown voltages which are generally in the range of 30 to 50 volts.  ▪ Once the positive or negative applied voltage is more than the respective breakdown voltages DIAC start conducting. During the positive half cycle, at point A in the figure the DIAC begins to conduct and the voltage drop across the device becomes a few volts. The portion AB represents the conduction of DIAC. Conduction continuous until the device current falls below its holding current level. The holding current and break-over voltage values are identical for reverse and forward region of operation. The first and third quadrant characteristics represent the forward and reverse bias conditions of the DIAC Application of DIAC: ( any 2) a) Used to trigger TRIAC b) Heat control circuit c) Triac light dimmer circuit d) Triac fan speed control circuit e) Low power triggering circuit UJT (Unijunction Transistor)  It is a three-terminal silicon diode. As its name indicates, it has only one P-N junction. A slab of lightly doped (increased resistance characteristic) n-type silicon material has two base contacts attached to both ends of one surface and an aluminum rod alloyed to the opposite surface. The p-n junction of the device is formed at the boundary of the aluminum rod and the n-type silicon slab. Operation of UJT INTER-BASE RESISTANCE (RBB) It is the resistance between B2and B1 i.e. it is the total resistance of the silicon bar from one end to the other with emitter terminal open. It should also be noted that RB1 > RB2. Usually, RB1 = 60% of RB1 The resistance RB1 has been shown as a variable resistor because its value varies inversely with IE. Intrinsic Standoff Ratio It may be noted that part of V BB is dropped over RB2 and part on RB1. Let us call the voltage drop across RB1 as VA. Using simple voltage divider relationship  When emitter voltage is greater than V RB1 by VD (0.7v) then diode will forward bias and UJT will turn on. Peak Voltage (Vp)-It is emitter voltage at which UJT will turn on is called as Peak voltage Vp =VE=VRB1 +VD (0.7) VI characteristics of UJT Q.Draw and explain the VI characteristics of UJT.OR Draw static characteristics of UJT and define peak point voltage. V-I characteristic of UJT. There are two important points on the characteristic curve namely the peak-point and the valley- point. These points divide the curve into three important regions i.e , cut off region ,negative resistance region and saturation region. 1) Cut-off region: The region, to the left peak-point, is called cut-off region. In the region, the emitter voltage is below the peak-point voltage (Vp) and the emitter current is approximately zero. The UJT is in its OFF position in this region. 2) Negative resistance region: The region, between the peak –point and the valley point called negative – resistance region. In this region, the emitter voltage decreases from Vp to Vv and the emitter current increases from Ip to Iv. The increase in emitter current is due to the decrease in resistance RB1. It is because of this fact that this region is called negative- resistance region. It is the most important region from the application point of view. 3) Saturation region: the region, beyond the valley point, is called saturation region. In this region, the device is in its ON position. The emitter voltage (V E) remains almost constant with the increasing emitter current. Peak Point Voltage: The maximum voltage across base to emitter at which current starts flowing is called peak point voltage Appilcations 1.As a triggering device for SCR and TRIAC 2. In the relaxation oscillator PUT (Programmable Unijunction Transistor) Q. Explain the working of “PUT” with relevant diagrams. Why it is called programmable? ▪ The only difference is the position of gate terminal. As the gate is closer to anode than cathode, it is called as anode gate. ▪ The major advantages of PUT over UJT is that a intrinsic stand off ratio of PUT can be modified/changed using external resistors. Operation of PUT  ▪ 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. ▪ The gate voltage (V G) is decided by voltage across R1 so according to voltage- divider rule VR1= R1/(R1+R2)×Vbb Where η=R1/(R1+R2) is intrinsic standoff ratio of PUT means value η depends on external resistor R1 and R2. VR1= η ×Vbb ▪ When anode voltage (V A) is greater than gate voltage (VG) by 0.7 then PUT will turn on and carry current from anode to cathode. ▪ The anode voltage at which PUT will turn on is called as Peak Voltage Vp = 0.7V + VG =0.7V + VR1 = 0.7V + ηVbb PUT is called programmable : The PUT is called as programmable because its intrinsic standoff ratio Ƞ and triggering voltage Vp can be changed by external voltage divider. VI characteristics of PUT Q.Draw VI characteristics of PUT & describe the role of its operating regions Fig shows the VI characteristic of PUT. There are two important points on the characteristic curve namely the Peak -point and the valley point. These points divide the curve into three important regions i.e , cut off region , negative resistance region and saturation region. these regions are explained below  Cut off region: The region, to the left peak point, is called cutoff region. In the region, the emitter voltage is below the peak point voltage (Vp) and the emitter current is approximately zero. The PUT is in its OFF position in this region. Negative resistance region: The region, between the peak point and the valleypoint called negative resistance region. In this region, the emitter voltage decreases from Vp to Vv and the emitter current increases from Ip to Iv. The increase in emitter current is due to the decrease in resistance rb1. It is because of this fact that this region is called negative resistance region. It is the most important region from the application point of view. Saturation region: the region, beyond the valley point, is called saturation region. In this region, the device is in its ON position. The emitter voltage (Ve) remains almost constant with the increasing emitter current. Applications- In the triggering circuits for SCR and TRIAC In the relaxation oscillator Protection circuit of SCR Q.Give the types of protection circuit for overvoltage 1. Snubber circuits 2. Crowbar circuits 3. Nonlinear surge suppressor Snubber circuits Q.For the Snubber circuit (dv/dt Suppression), answer the following i)Give the importance in SCR. ii) Justify with circuit diagram. OR Q.Explain the operation of snubber protection circuit with diagram Operation of Snubber protection circuit: 1)The snubber circuit is used to provide protection against high dv/dt. During turn-on of thyristor, it is switched from high impedance state to low impedance state and current is suddenly increased. During turnoff, the forward current is first reduced to zero, then due to storage charges at the junctions, reverse current flows. This reverse recovery current reaches to peak value IRR and then it is abruptly reduced to zero. When current is reduced abruptly (very high rate), the circuit inductance which includes load inductance, stray inductance and di/dt inductance cause high emf (Ldi/dt) that appears at high rate (dv/dt) across the thyristor. 2)A snubber circuit having series RC combination is used to limit this high dv/dt as well as peak reverse voltage appearing across thyristor. As prior to turn-off, thyristor was conducting, voltage across it and also across snubber R-C circuit was negligibly small. The capacitor voltage at this instant therefore can be assumed to be zero. Thus when thyristor turns-off abruptly, the reverse recovery current IRR is transferred through snubber R-C circuit. The uncharged capacitor initially acts as short-circuit and hence at this instant the voltage appearing across thyristor is only because of drop in resistance RS i.e. RS IRR. 3)Once the thyristor is turned-off, the snubber circuit R-C and circuit inductance with load impedance forms a series RLC circuit. After turn-off, the capacitor charges slowly and limits dv/dt across thyristor. The reverse voltage to which the capacitor be charged and the rate(dv/dt) at which it is charged, is determined by the circuit parameters R, L and C. Crowbar protection Q.Explain the operation of crowbar protection circuit with diagram 1)A crowbar can be used for over voltage or over current protection on both AC and DC circuits 2)Fig shows how an SCR can be used to provide fault protection for sensitive dc power electronic circuits and loads. Whenever a fault condition occurs the crowbar SCR is triggered, shorting the supply. The resultant high supply current flowing blows the fuse therefore isolating the load from the supply. Diode D f provides a current path for inductive load energy 3) Load current is measured by the voltage across the sense resistor R. When this voltage reaches a preset limit, that is the load current has reached the fault level, the SCR is triggered. The load voltage is measured from the resistor divider R2-R3.When this voltage exceeds the predetermined limit, the SCR is triggered and the fuse is blown by the crowbar short circuit current, isolating the sensitive load from the supply.