IE-COMPLETE-LEC-SIR-DING.pdf - ECE 208 INDUSTRIAL ELECTRONICS Prepared by Armando V Barretto References \u2022 \u2022 Modern Industrial Electronics by Timothy

IE-COMPLETE-LEC-SIR-DING.pdf - ECE 208 INDUSTRIAL...

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Unformatted text preview: ECE 208 INDUSTRIAL ELECTRONICS Prepared by: Armando V. Barretto References • • Modern Industrial Electronics by Timothy J. Maloney 4th edition Electronic Devices and Circuit Theory by Boylestad and Nashelsky Grading System • Passing Grade >= (Q1 + Q2 + Q3…..+ Qn + 2 P.E + 2 F.E + assignment + reports )/ (m+4) • • • • Q = Quiz PE = Preliminary exam FE= Final exam m= number of quiz + 1 (if there are assignments) + 1 (if there are reports) Course Outline • • • • • • • • • • SCR UJT PUT Triac Diac Light Activated Thyristors Feedback Systems Input Transducers Optoelectronics Final Correcting Devices Industrial Electronics • Electronic technology in industrial applications. – Relays to control application of power to devices – Logic circuits to control operation of a process – Electronic circuits/devices to control application of power to other devices or circuits – Robotics – Power amplifiers – Voltage converters – Computers / microcontrollers used in industrial applications – Optoelectronics in industrial applications – Other applications Thyristors • • • • A Thyristor is a semiconductor device which shows inherent on-off behavior, as opposed to allowing gradual change in conduction. Thyristors are regenerative devices, and they cannot operate in a linear manner. Some thyristors can be gated into the “on” state (one terminal is used to turn on the device), as in the case of SCR. Some thryristors cannot be gated “on”, but they turn on when the applied voltage reaches a certain breakover value, such as in the case of diacs and four-layer diodes. Silicon Controlled Rectifier (SCR) Prepared by: Armando V. Barretto Ways Of Controlling The Electrical Power To A Load • • Some ways to control the electrical power to the load are: – Transformer – Rheostat in series with the load (large, expensive, need maintenance, wastes energy) – Thyristors (electronic devices such as SCRs and Triacs, which are small, cheaper, more power efficient, and have no maintenance requirements) An SCR can be used to control the current to the load by placing it in series with the load. Silicon Controlled Rectifier (SCR) • • Three terminal semiconductor device used to control the flow of current to a load, which can flow in only one direction (has rectification capability). Schematic symbol is: Anode (A) Cathode (K) Gate (G) • The construction of an SCR is shown below: Anode (A) P N Gate (G) Current flow when SCR is on P Gate-cathode junction Gate current flow N Cathode (K) Silicon Controlled Rectifier (SCR) • The equivalent circuit of an SCR is shown below: Anode (A) Anode (A) Q1 Gate (G) Gate current flow IE1 Q1 P N N P P Current flow when SCR is on IB1 = IC2 IC1 Gate (G) N Q2 Q2 Cathode (K) Gate current flow IB2 Q2 IE2 Cathode (K) When the gate potential is 0 volt or negative with respect to the cathode, Q2 is off and practically no current flows from the collector of Q2. Thus, there is also no current at the base of Q1, and Q1 is also off. When the gate voltage is positive enough to turn Q2 on, Q2 will have collector current, Q1 will have base current, and both Q1 and Q2 will be on. The arrows at the equivalent circuit indicate the current flow when Q1 and Q2 are on (SCR is on). Silicon Controlled Rectifier (SCR) • • • • • • Acts like a switch. – When turned “on”, it has low resistance and current could flow from anode to cathode but not from cathode to anode. – When “on”, voltage drop across anode and cathode is approximately zero volt. – When turned “off”, it has very high resistance and no current could flow from anode to cathode, or cathode to anode. Switching action is very fast (no mechanical delay). Small and relatively inexpensive. For current to flow from anode to cathode, the following conditions must exist: – Gate must be around + 0.6 to + 0.8 volt with respect to cathode. (Gate- cathode junction must be forward bias) – Gate current must be equal to or greater than gate current required to turn on the SCR (IGT) – Anode must be positive with respect to cathode, and minimum anode to cathode current to keep the SCR on (IHO). must be present. IGT is typically between 0.1 to 50 mA. SCR’s can also be turned on by increasing its operating temperature, or by increasing the anode-cathode voltage to its breakover value. However, these are not usually done. Silicon Controlled Rectifier (SCR) • • • • • • • • Once triggered on, SCR will remain on even if IGT is no longer present as long as the anode to cathode current (IAK) does not fall below some minimum value, called holding current (IHO). SCR’s are normally triggered off when anode to cathode voltage passes through zero into its negative region. Two ways of turning off an SCR are: – Anode current interruption – anode current is cut off. – Forced commutation – anode current is forced to be in the reverse (cathode to anode) direction. SCR can control currents of several hundred to 2000 amperes in circuits operating at voltages higher than 1000 to 1800 volts. – Current can be controlled by resistance in series with anode and cathode. Some SCRs can operate at frequencies as high as 50 Khz, permitting applications involving ultrasonic cleaning and induction heating. Time during which current could flow from anode to cathode can be controlled using the gate terminal. Average current from anode to cathode can be controlled by controlling the application of gate to cathode voltage. Gate current flows from gate to the cathode. Silicon Controlled Rectifier (SCR) Characteristics IA (anode current) Reverse breakdown voltage Forward conduction region IG2 IG1 IG=0 IHO (Holding current) Forward breakover voltage VF Reverse blocking region VF3 VF2 VF1 V(BR)F* Forward blocking region • • • For IG = 0, VF must reach the largest required breakover voltage (VBRF*) before the collapsing effect results and the SCR can enter the conduction region corresponding to the on state. If gate current is increased to IG1 by applying a bias voltage to the gate, the value of VF required for conduction (VF1) is considerably less. If gate current is increased to IG2, the SCR will fire at low values of voltage (VF3) Silicon Controlled Rectifier (SCR) Characteristics • Forward breakover voltage – voltage across anode and cathode above which the SCR enters the conduction region. The asterisk (*) denotes the letter to be added, which is dependent on the condition of the gate terminal. As follows: O = open circuit from G to K S = short circuit from G to K R = resistor from G to K V = fixed bias voltage from G to K • • • Holding current (IHO) – value of anode to cathode current below which the SCR switches from conduction state to forward blocking region under stated conditions. Forward and reverse blocking regions – regions corresponding to the opencircuit condition for the controlled rectifier that block the flow of charge (current) from anode to cathode. Reverse breakdown voltage – voltage between anode and cathode which is equivalent to the zener or avalanche region of the fundamental two-layer semiconductor diode. When this is reached, huge amount of current from cathode to anode could flow. Silicon Controlled Rectifier (SCR) • • • Two terms used to describe SCR operation are conduction angle and firing delay angle. – Conduction angle is the number of degrees of an ac cycle during which an SCR is turned on. – Firing delay angle is the number of degrees of an ac cycle that elapses before the SCR is turned on. Sum of conduction angle and firing delay angle is always 1800. The specifications usually given for SCRs are: – Breakover voltage – minimum anode to cathode forward voltage at which the SCR is turned on when the gate is left open. – Holding current – minimum anode to cathode current required to keep SCR “on” even if there is no gate current. – Forward current rating – maximum anode current that an SCR is capable of handling without being damaged. – Gate trigger voltage – gate to cathode voltage needed to trigger the SCR. Silicon Controlled Rectifier (SCR) R1 Control circuit determines when the SCR will be turned on. Control circuit Anode to cathode voltage drops to approximately zero volt (around 1 to 2 volts for some), when SCR is on. R2 Ac signal source A VAK G K VGK RL (Load) VAK – voltage between anode and cathode VGK – voltage between gate and cathode VRL Supply voltage is applied to the load when SCR is on. Supply voltage appears across anode and cathode when SCR is off. Silicon Controlled Rectifier (SCR) Supply voltage Supply voltage t VAK SCR is on SCR is off SCR is off t VAK SCR is on SCR is off SCR is off t VRL t VRL 600 1800 t Firing Delay Angle = 600 Conduction Angle = 1200 (1800-600) 1200 1800 t Firing Delay Angle = 1200 Conduction Angle = 600 (1800-1200) Silicon Controlled Rectifier (SCR) • Example: Given the circuit below, determine the input voltage required to fire the SCR if the gate current needed to fire the SCR (ITH) is 30 mA and the voltage needed to make the gate – cathode junction forward bias is 0.7 volt. A ITH Input voltage 200 ohms G K Solution: Input voltage = (200)(30 mA) + 0.7 = 6.7 volts (or higher) Typical SCR Circuit (#1) •When switch S1 is open, SCR is not triggered on. R1 Ac signal source R2 S1 G •When S1 is closed, SCR is triggered on as long as gate current is equal or above IGT and anode to cathode current is equal to or greater than IHO. A VAK K VGK RL (Load) VAK – voltage between anode and cathode VGK – voltage between gate and cathode VRL •Firing delay angle is determined by R2. •If R2 is low, gate current will be sufficiently large to fire the SCR when the supply voltage is relatively low. •R1 is used to protect the gate from overcurrents, and it also determines the minimum firing delay angle. •Firing delay angle can only be between 00 and 900, which is the biggest disadvantage of the circuit. Typical SCR Circuit (#1) VAK SCR is off • SCR is on SCR is off t IGT SCR is on SCR is off t SCR is off • Gate current flows during positive portion of input signal, then it stops when the SCR is turned on because the anode to cathode voltage drops to around 0 volt when the SCR is on. There is no gate current during the negative portion of the input signal because the gate-cathode junction is reverse biased. Typical SCR Circuit (#1) • Example: Given the typical SCR circuit (#1) with the following parameters: Input voltage = 220 volts rms (ac), IGT = 20 mA, R1 = 5 kohm, RL = 100 ohms, forward bias voltage across gate and cathode = 0.7 volt. The firing delay desired is 900. What should be the value of R2? Solution: At 900, the supply voltage is 220 / 0.707 = 311.17 volts Voltage across R2 = 311.17 – (20mA)(5 kohm) – 0.7 – (20mA)(100) = 208.47 volts R2 = 208.47 / 20 mA = 10,423.5 ohms Typical SCR Circuit (#1) • Example: A 220 volts source is connected to an SCR in series with the load. The resistance of the load is 50 ohms, the voltage across the SCR when it is on is 1.5 volts. Determine the average power dissipated by the SCR when it is on. Since the SCR only conducts during the positive half of the input signal, it only dissipates power during the positive half (assuming leakage current is negligible). Vdc half (0.318)(220 / 0.707) 98.95 v average voltage of positive half of input voltage 98.95 - 1.5 Iave RL Iave SCR 1.949 A average current flowing through SCR and load 50 Pave SCR (Iave SCR)(Vave SCR) (1.949)(1.5) 2.9235 watts Typical SCR Circuit (#1.1) R1 Ac signal source R2 S1 G A VAK K VGK RL (Load) VAK – voltage between anode and cathode VGK – voltage between gate and cathode VRL •The circuit is similar to that of the previous slide except for the diode in series with the gate. The diode is used to protect the gate – cathode junction from large reverse bias voltage. •Because of the diode, firing delay angle will be higher because there will be a voltage drop across the diode. Typical SCR Circuit (#2) •Firing delay angle can be adjusted past 900 VRL RL (Load) •When ac supply is negative, the capacitor is charged positive on the bottom plate and negative on the top plate. R1 R2 A Ac signal source VAK G K VGK C1 •When ac supply is positive, C1 is charged in the opposite direction. Voltage buildup across C1 is delayed until the negative charge is removed from C1. Delay can be extended beyond the 900 point. • For a certain value of C1, R1 determines the minimum firing delay angle. VAK – voltage between anode and cathode VGK – voltage between gate and cathode •The higher R1 and R2 are, the longer is the charging time of C. Typical SCR Circuit (#2.1) VRL RL (Load) •Circuit operation is similar to SCR circuit #2. R2 •R3 and D1 are added to further delay the triggering of the SCR. R1 R2 A Ac signal source D1 R3 C1 VAK G IGT VAK – voltage between anode and cathode VGK – voltage between gate and cathode •Because of R3 and D1, capacitor voltage must reach a higher value (above gate to cathode trigger voltage) to produce IGT and trigger the SCR K •In general terms, when the gate control circuit is used with a 60 hz ac supply, the RC time constant (R x C) should fall in the range 1 to 30 ms. That is: 1 ms < (R1 + R2) C < 30 ms Typical SCR Circuit (#2.2) VRL RL (Load) R1 R2 A Ac signal source D1 R3 C1 C2 VAK G IGT VAK – voltage between anode and cathode VGK – voltage between gate and cathode K Typical SCR Circuit (#2.2) •Circuit operation is similar to SCR circuit #2.1. •C2 is added to further delay the triggering of the SCR. •Because it takes time to charge C2, triggering of the SCR is further delayed. •Minimum firing delay angle is set by fixed resistors R1 and R3. •Maximum firing delay angle is mostly set by variable resistor R2. •Typical values for C1 and C2 are from 0.01 to 1 microfarad. •In general terms, when the gate control circuit is used with a 60 hz ac supply, the RC time constant (R x C) should fall in the range 1 ms to 30 ms. That is: 1 ms < (R1 + R2) C1 < 30 ms 1 ms < (R3) (C2) < 30 ms Typical SCR Circuit (#2.2) • Example: Given the typical SCR circuit (#2.2) with the following parameters: C1 = 0.07 microfarad and C2 = 0.33 microfarad. Determine approximate values for R1, R2 and R3. 1 ms < (R1 + R2) C1 < 30 ms 1 ms < (R3) (C2) < 30 ms When R2 is 0 ohms, the minimum time constant for R1, R2 and C1 is achieved. (R1 0)(C1) 1x10 3 sec minimum time constant 1 x 10 3 14,285 ohms R1 6 0.07 x 10 Choose nearest higher available value for R1 which is 15 Kohms. The maximum time constant for R1, R2 and C1 is achieved when R2 is maximum. (R1 R2)(C1) 30 x10 3 sec maximum time constant 30 x10 3 - R1C1 30 x10 3 - (15,000)(0.07 x 10 6 ) R2 413,571 ohms 6 C1 0.07 x 10 Choose nearest lower available value for R2. Choose 400 kohm potentiometer. Typical SCR Circuit (#2.2) For R3 and C2 combination, time constant typically should fall at the lower end of time constant range. Choose time constant 2 ms. (R3)(0.33x 10 6 ) 2x10 3 sec minimum time constant R3 6060 ohms Choose nearest higher value for R3. Choose 6,200 ohms The chosen values are approximates only. If low firing delay angle could not be achieved, R1 and R3 could be made smaller, so that C1 and C2 could charge faster. If firing delay beyond 90 0 could not be achieved, R1 , R2 or R3 could be made larger The time constants used in the computation could also be adjusted towards the middle of the range. Typical SCR Circuits • • • The SCR circuits in the preceding slides have two disadvantages, namely: – The firing delay angle is temperature dependent. Once temperature increases, the SCR could be triggered using lower gate currents. – SCRs having the same part numbers could have inconsistent firing behavior because of inconsistencies in SCR parameters. SCRs with the same part number could have different parameters because of variance in manufacturing processes and conditions. Other electronic devices such as four layer diodes, silicon unilateral switch, silicon bilateral switch, diac and unijunction transistors could be used for triggering SCRs. Because the breakover voltage of four layer diodes and other breakover devices is relatively independent with temperature, they can be used to improve the triggering of SCRs. Triggering SCR Using Four Layer Diode • VRL R2 • RL (Load) R1 R2 K A (cathode) (anode) Ac signal source G C1 IGT VAK – voltage between anode and cathode VGK – voltage between gate and cathode • A VAK K • • A four layer diode is a two terminal unilateral breakover device. It conducts when the forward breakover voltage is reached, or when the reverse breakdown voltage is reached (not normally done). The forward breakover voltage is much smaller than the reverse breakdown voltage. When the capacitor voltage rises to the diode breakover voltage, the four layer diode fires, thus triggering the SCR. The forward breakover voltage is relatively independent of temperature. SCR Circuit For Unidirectional Full Wave Control VAK1 I+ I+ Vp1/2sec AC Input voltage (sine wave) Trigger circuit I- V1/2sec I+ SCR1 is off t VRL = output voltage (Pulsating DC voltage) RL I+ Io - VRL SCR1 is on SCR2 is on II- V1/2sec + I- IVp1/2sec SCR1 is on SCR1 is off SCR1 Trigger circuit SCRs are off Io= output current I+ = current during positive half I- = current during negative half SCR2 ISCR Circuit For Unidirectional Full Wave Control t Input Signal SCR Circuit For Unidirectional Full Wave Control • • • • The circuit provides full wave power control. The circuit’s operation is similar to a center tapped half wave rectifier, except that the voltage across RL can be controlled using the trigger circuits. The two SCRs can conduct alternately. SCR1 can conduct during the positive half of the input signal, while SCR2 can conduct during the negative half of the input signal. Firing delay angle for each SCR is controlled by the trigger circuit connected to each SCR. SCR Circuit For Bidirectional Full Wave Control I+ SCR1 VAK1 SCR1 is on SCR1 is off SCR1 is off Trigger circuit I+ t IAC Input voltage (sine wave) I- I+ SCR2 VRL = output voltage (Bidirectional) RL VRL SCR1 is on SCR2 is on II- Io= output current I+ = current during positive half I- = current during negative half SCRs are off SCR 1 conducts during positive half and SCR2 conducts during negative half. Output voltage has positive and negative values. t SCR Circuit For Bidirectional Full Wave Control • • • • The circuit provides full wave power control. The two SCRs conduct alternately. SCR1 can conduct during the positive half of the input signal, while SCR2 can conduct during the negative half of the input signal. The flow of current at the load is bidirectional as indicated in the diagram. Firing delay angle for each SCR is controlled by the trigger circuit. SCR Bridge Circuits • • D1 SCR1 D2 • Trigger circuit D3 220 volt ac signal IRL source RL D4 • VRL = output voltage (Bidirectional) • • The circuit provides full wave power control. During the positive half of the input signal, D2, D3, and the SCR can conduct. During the negative half of the input signal, D4, D1, and the SCR can conduct. The voltage across the load RL is bidirectional. The flow of current at the load is bidirectional as indicated in the diagram. Firing delay angle for SCR is controlled by trigger circuit. SCR Bridge Circuits • D1 D2 IRL • SCR1 • RL Trigger circuit D3 220 volt ac signal source D4 • • • The circuit provides full wave power control. During the positive half of the input signal, D2, D3, and the SCR can conduct. During the negative half of the input signal, D4, D1, and the SCR can conduct. The voltage across the load RL is unidirectional. The flow of current at the load is unidirectional as indicated in the diagram. Firing delay angle for SCR is controlled by trigger circuit. SCR In DC Circuit (#1) RL SCR1 • • Trigger circuit IB Q1 Because the supply voltage is DC, SCR does not turn off automatically because the supply voltage does not go down to 0 volt. To turn off the SCR, Q1 is driven to saturation by the trigger circuit making the anode to cathode voltage of the SCR almost 0 volt (0.2 volt typically). The anode to cathode current falls bel...
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