* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Lecture-5: Thyristors - Dr. Imtiaz Hussain
Cavity magnetron wikipedia , lookup
Power inverter wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Electrical ballast wikipedia , lookup
Three-phase electric power wikipedia , lookup
Electrical substation wikipedia , lookup
History of electric power transmission wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Voltage regulator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Semiconductor device wikipedia , lookup
Current source wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Distribution management system wikipedia , lookup
Voltage optimisation wikipedia , lookup
Power electronics wikipedia , lookup
Stray voltage wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Mains electricity wikipedia , lookup
Surge protector wikipedia , lookup
Opto-isolator wikipedia , lookup
Power Electronics Lecture-5 Thyristors Dr. Imtiaz Hussain Assistant Professor email: [email protected] URL :http://imtiazhussainkalwar.weebly.com/ 1 Introduction • One of the most important type semiconductor device. of power • Compared to transistors, thyristors have lower onstate conduction losses and higher power handling capability. • However, they have worse switching performances than transistors. • Name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR. 2 Introduction • Thyristors are four-layer semiconductor devices. pnpn power • These devices switch between conducting and nonconducting states in response to a control signal. • Thyristors are used in timing circuits, AC motor speed control and switching circuits. 3 Thyristors • Bell Laboratories were the first to fabricate a siliconbased thyristor. • Its first prototype was introduced by GE (USA) in 1957. • Later on many other devices having characteristics similar to of a thyristor were developed. • These semiconductor devices are SCR, SCS, Triac, Diac, PUT, GTO, e.t.c. • This whole family of semiconductor devices is given the name thyristors. 4 Thyristor/ SCR • SCR is a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N, • It can handle high currents and high voltages, with better switching speed and improved breakdown voltage . K A G 5 Thyristor/ SCR • Thyristor can handle high currents and high voltages. • Typical rating are 1.5kA & 10kV which responds to 15MW power handling capacity. • This power can be controlled by a gate current of about 1A only. • Thyristor acts as a bistable switch. – It conducts when gate receives a current pulse, and continue to conduct as long as forward biased (till device voltage is not reversed). – They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off. 6 Thyristor/ SCR Operation • When the anode voltage is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse biased. • The thyristor is said to be in the forward blocking or off-state condition. • A small leakage current flows from anode to cathode and is called the offstate current. 7 Thyristor/ SCR Operation • If the anode voltage VAK is increased to a sufficiently large value, the reverse biased junction J2 would breakdown. • This is known as avalanche breakdown and the corresponding voltage is called the forward breakdown voltage VBO. • Since the other two junctions J1 and J3 are already forward biased, there will be free movement of carriers across all three junctions. • This results in a large forward current and the device is now said to be in a conducting or onstate. • The voltage drop across the device in the onstate is due to the ohmic drop in the four layers and is very small (in the region of 1 V). 8 Thyristor/ SCR 9 Thyristor Operating modes Thyristors have three modes : • Forward blocking mode: Only leakage current flows, so thyristor is not conducting. • Forward conducting mode: large forward current flows through the thyristor. • Reverse blocking mode: When cathode voltage is increased to reverse breakdown voltage , Avalanche breakdown occurs and large current flows. 10 Important characteristics Latching Current IL • This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed. • If a gate current greater than the threshold gate current is applied until the anode current is greater than the latching current IL then the thyristor will be turned on or triggered. Holding Current IH • This is the minimum anode current required to maintain the thyristor in the on-state. • To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction. 11 Important characteristics Reverse Current IR • When the cathode voltage is positive with respect to the anode, the junction J2 is forward biased but junctions J1 and J3 are reverse biased. The thyristor is said to be in the reverse blocking state and a reverse leakage current known as reverse current IR will flow through the device. Forward Breakover Voltage VBO • If the forward voltage VAK is increased beyond VBO , the thyristor can be turned on. But such a turn-on could be destructive. In practice the forward voltage is maintained below VBO and the thyristor is turned on by applying a positive gate signal between gate and cathode. 12 Turn-on Characteristics ton td tr 13 13 Turn-off Characteristics VAK tC tq t IA Anode current begins to decrease Commutation di dt Recovery t1 t2 Recombination t3 t4 t5 t tq=device off time tc=circuit off time tgr trr tq tc 14 14 The SCR can be turned on at its gate terminal. With a dc source, the SCR stays on after it is gated. Load Anode Cathode Load current Gate Gate pulse occurs here Time 15 With an ac source, the SCR turns off at the zero-crossing. Load Anode Cathode Gate Load current off on Gate pulse occurs here Time Turns off here 16 The gate can be pulsed for each positive alternation. Load Anode Cathode Load current Gate Time 17 Load Anode Cathode Gate Load current The average load current can be decreased by gating the SCR later. Time 18 …. and later. Load Anode Cathode Load current Gate Time 19 …. or, not at all. Load Anode Cathode Load current Gate No gate pulses: ILoad = 0 0 Time 20 Thyristor turn-ON methods • Thyristor turning ON is also known as Triggering. • With anode is positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques : – – – – – Forward voltage triggering Gate triggering dv/dt triggering Temperature triggering Light triggering 21 Forward Voltage Triggering • When breakover voltage (VBO) across a thyristor is exceeded than the rated maximum voltage of the device, thyristor turns ON. • At the breakover voltage the value of the thyristor anode current is called the latching current (IL) . • Breakover voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence. • When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered). 22 Gate Triggering • Turning ON of thyristors by gate triggering is simple and efficient method of firing the forward biased SCRs. • In Gate Triggering, thyristor with forward breakover voltage (VBO), higher than the normal working voltage is chosen. • Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate and cathode is applied. • Forward voltage at which device switches to on-state depends upon the magnitude of gate current. – Higher the gate current, lower is the forward breakover voltage . 23 Gate Triggering • Turning ON of thyristors by gate triggering is simple and efficient method of firing the forward biased SCRs. 24 dv/dt triggering • With forward voltage across anode & cathode of a thyristor, two outer junctions (A & C) are forward biased but the inner junction (J2) is reverse biased. • The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. • As p-n junction has capacitance, so larger the junction area the larger the capacitance. • If a voltage ramp is applied across the anode-to-cathode, a current will flow in the device to charge the device capacitance according to the relation: • If the charging current becomes large enough, density of moving current carriers in the device induces switch-on. • This method of triggering is not desirable because high charging current (Ic) may damage the thyristor. 25 Temperature Triggering • During forward blocking, most of the applied voltage appears across reverse biased junction J2. • This voltage across junction J2 associated with leakage current may raise the temperature of this junction. • With increase in temperature, leakage current through junction J2 further increases. • This cumulative process may turn on the SCR at some high temperature. • High temperature triggering may cause Thermal runaway and is generally avoided. 26 Light Triggering • In this method light particles (photons) are made to strike the reverse biased junction, which causes an increase in the number of electron hole pairs and triggering of the thyristor. • For light-triggered SCRs, a slot (niche) is made in the inner p-layer. • When it is irradiated, free charge carriers are generated just like when gate signal is applied b/w gate and cathode. • Pulse light of appropriate wavelength is guided by optical fibers for irradiation. • If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR). • Light-triggered thyristors is mostly used in highvoltage direct current (HVDC) transmission systems. 27 Thyristor Gate Control Methods • An easy method to switch ON a SCR into conduction is to apply a proper positive signal to the gate. • This signal should be applied when the thyristor is forward biased and should be removed after the device has been switched ON. • Thyristor turn ON time should be in range of 1-4 micro seconds, while turn-OFF time must be between 8-50 micro seconds. • Thyristor gate signal can be of three varieties. – D.C Gate signal – A.C Gate Signal – Pulse 28 Thyristor Gate Control Methods D.C Gate signal: Application of a d.c gate signal causes the flow of gate current which triggers the SCR. – Disadvantage is that the gate signal has to be continuously applied, resulting in power loss. – Gate control circuit is also not isolated from the main power circuit. 29 Thyristor Gate Control Methods A.C Gate Signal: In this method a phase - shifted a.c voltage derived from the mains supplies the gate signal. – Instant of firing can be controlled by phase angle control of the gate signal. 30 Thyristor Gate Control Methods Pulse: Here the SCR is triggered by the application of a positive pulse of correct magnitude. – For Thyristors it is important to switched ON at proper instants in a certain sequence. – This can be done by train of the high frequency pulses at proper instants through a logic circuit. – A pulse transformer is used for circuit isolation. 31 Thyristor Commutation • Commutation: Process of turning off a conducting thyristor • SCR cannot be turned OFF via the gate terminal. • It will turn-off only after the anode current is negated either naturally or using forced commutation techniques. • Therefore, commutation can be classified as – Natural commutation – Forced commutation 32 Line Commutation (Natural Commutation) • Occurs only in AC circuits. • Natural Commutation of thyristor takes place in – AC Voltage Regulators – Phase controlled rectifiers – Cycloconverters 33 Thyristor Turn-Off: Line-Commutated Thyristor Circuit 34 Forced Commutation • Applied to d.c circuits. • If a thyristor is used in a DC circuit, when first turned on, it will stay on until the current goes to zero. To turn off the thyristor it is possible to use a Forced commutation circuit. The circuit creates a reverse voltage over the thyristor (and a small reverse current) for a short time, but long enough to turn off the thyristor. 35 To download this lecture visit http://imtiazhussainkalwar.weebly.com/ END OF LECTURE-5 36