Lecture Notes on Power Electronics VSSUT

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Disclaimer, This document does not claim any originality and cannot be used as a substitute for. prescribed textbooks The information presented here is merely a collection by the. committee members for their respective teaching assignments Various sources as. mentioned at the end of the document as well as freely available material from internet. were consulted for preparing this document The ownership of the information lies with the. respective authors or institutions Further this document is not intended to be used for. commercial purpose and the committee members are not accountable for any issues legal. or otherwise arising out of use of this document The committee members make no. representations or warranties with respect to the accuracy or completeness of the contents. of this document and specifically disclaim any implied warranties of merchantability or. fitness for a particular purpose The committee members shall be liable for any loss of profit. or any other commercial damages including but not limited to special incidental. consequential or other damages,6TH SEMESTER,POWER ELECTRONICS 3 1 0. MODULE I 10 HOURS, Thyristors Static V I Characteristics of SCR TRIAC GTO IGBT Turn On Turn OFF. Mechanism of SCR Gate Turnoff Thyristor GTO Power BJTs Power MOSFETs. Insulated Gate, Bipolar Transistors IGBTs Basic Structure and VI Characteristics Static dynamic and. characteristics Protection cooling and mounting techniques Series and Parallel operation of. Triggering and basics of driver circuits Different types of commutation schemes Natural and. commutation,MODULE II 10 HOURS, 1 Phase Half Full Wave Controlled Rectifier with various kinds of loads R R L E.
Midpoint and Bridge type converters Half Controlled and Fully Controlled Bridge circuits. waveforms Input Line Current Harmonics Power factor current distortion and displacement. Inverter Mode of Operation Continuous and discontinuous modes Effect of source. inductance, assuming constant load current Effect of freewheeling diode Three phase bridge converters. different types of load with constant load current different waveforms 180 and 120 degree. operations,MODULE III 10 HOURS, DC DC Converters Classification of types of choppers One Two and Four quadrant. operations, Step up and down choppers Analysis of Type A chopper Single and two quadrant operation. motor load, AC AC Converters Single phase mid point and bridge types of step up and step down. Cycloconverters, Single phase AC Voltage regulators and its basic analysis.
MODULE IV 10 HOURS, Single phase Half and Full bridge Inverter Pulse Width Modulated PWM technique for. control SPWM Technique 1 phase inverters Auxiliary Commutated Mc Murray and. Complementary Commutated Mc Murray Bedford Inverters Three phase Voltage Source. type of Inverters 120 and 180 Degree conduction modes Current Source Inverter. Applications UPS SMPS Induction Heating Electronic Ballast AC DC drives speed. POWER ELECTRONICS, The control of electric motor drives requires control of electric power Power electronics have. eased the concept of power control Power electronics signifies the word power electronics. and control or we can say the electronic that deal with power equipment for power control. Main power source,Ref signal,Control Digital Power Load. Circuit Circuit Electronic,Feedback Signal, Power electronics based on the switching of power semiconductor devices With the. development of power semiconductor technology the power handling capabilities and. switching speed of power devices have been improved tremendously. Power Semiconductor Devices, The first SCR was developed in late 1957 Power semiconductor devices are broadly.
categorized into 3 types,1 Power diodes 600V 4500A. 2 Transistors,3 Thyristors 10KV 300A 30MW, Thyristor is a four layer three junction pnpn semiconductor switching device It has 3. terminals these are anode cathode and gate SCRs are solid state device so they are compact. possess high reliability and have low loss, SCR is made up of silicon it act as a rectifier it has very low resistance in the forward. direction and high resistance in the reverse direction It is a unidirectional device. Static V I characteristics of a Thyristor, The circuit diagram for obtaining static V I characteristics is as shown. Anode and cathode are connected to main source voltage through the load The gate and. cathode are fed from source, A typical SCR V I characteristic is as shown below.
Forward breakover voltage,Reverse breakover voltage. Gate current,Anode voltage across the thyristor terminal A K. Anode current, It can be inferred from the static V I characteristic of SCR SCR have 3 modes of. 1 Reverse blocking mode,2 Forward blocking mode off state. 3 Forward conduction mode on state,1 Reverse Blocking Mode.
When cathode of the thyristor is made positive with respect to anode with switch open. thyristor is reverse biased Junctions 1 and 2 are reverse biased where junction 2 is. forward biased The device behaves as if two diodes are connected in series with reverse. voltage applied across them, A small leakage current of the order of few mA only flows As the thyristor is. reverse biased and in blocking mode It is called as acting in reverse blocking. mode of operation, Now if the reverse voltage is increased at a critical breakdown level called. reverse breakdown voltage an avalanche occurs at 1 and 3 and the reverse. current increases rapidly As a large current associated with and hence more. losses to the SCR, This results in Thyristor damage as junction temperature may exceed its maximum. temperature rise,2 Forward Blocking Mode, When anode is positive with respect to cathode with gate circuit open thyristor is said to. be forward biased, Thus junction 1 and 3 are forward biased and 2 is reverse biased As the forward.
voltage is increases junction 2 will have an avalanche breakdown at a voltage called. forward breakover voltage When forward voltage is less then thyristor offers high. impedance Thus a thyristor acts as an open switch in forward blocking mode. 3 Forward Conduction Mode, Here thyristor conducts current from anode to cathode with a very small voltage drop. across it So a thyristor can be brought from forward blocking mode to forward. conducting mode,1 By exceeding the forward breakover voltage. 2 By applying a gate pulse between gate and cathode. During forward conduction mode of operation thyristor is in on state and behave like a. close switch Voltage drop is of the order of 1 to 2mV This small voltage drop is due to. ohmic drop across the four layers of the device,Different turn ON methods for SCR. 1 Forward voltage triggering,2 Gate triggering,3 triggering. 4 Light triggering,5 Temperature triggering,1 Forward voltage triggering.
A forward voltage is applied between anode and cathode with gate circuit open. Junction 1 and 3 is forward biased,Juntion 2 is reverse biased. As the anode to cathode voltage is increased breakdown of the reverse biased junction. 2 occurs This is known as avalanche breakdown and the voltage at which this. phenomena occurs is called forward breakover voltage. The conduction of current continues even if the anode cathode voltage reduces below. till will not go below Where is the holding current for the thyristor. 2 Gate triggering, This is the simplest reliable and efficient method of firing the forward biased SCRs First. SCR is forward biased Then a positive gate voltage is applied between gate and cathode In. practice the transition from OFF state to ON state by exceeding is never employed as it. may destroy the device The magnitude of so forward breakover voltage is taken as final. voltage rating of the device during the design of SCR application. First step is to choose a thyristor with forward breakover voltage say 800V higher than the. normal working voltage The benefit is that the thyristor will be in blocking state with normal. working voltage applied across the anode and cathode with gate open When we require the. turning ON of a SCR a positive gate voltage between gate and cathode is applied The point. to be noted that cathode n layer is heavily doped as compared to gate p layer So when gate. supply is given between gate and cathode gate p layer is flooded with electron from cathode. n layer Now the thyristor is forward biased so some of these electron reach junction 2 As a. result width of 2 breaks down or conduction at 2 occur at a voltage less than As. increases reduces which decreases then turn ON time Another important point is. duration for which the gate current is applied should be more then turn ON time This means. that if the gate current is reduced to zero before the anode current reaches a minimum value. known as holding current SCR can t turn ON, In this process power loss is less and also low applied voltage is required for triggering. 3 dv dt triggering, This is a turning ON method but it may lead to destruction of SCR and so it must be avoided. When SCR is forward biased junction 1 and 3 are forward biased and junction 2 is. reversed biased so it behaves as if an insulator is place between two conducting plate Here 1. and 3 acts as a conducting plate and 2 acts as an insulator 2 is known as junction capacitor. So if we increase the rate of change of forward voltage instead of increasing the magnitude of. voltage Junction 2 breaks and starts conducting A high value of changing current may. damage the SCR So SCR may be protected from high,4 Temperature triggering.
During forward biased 2 is reverse biased so a leakage forward current always associated. with SCR Now as we know the leakage current is temperature dependant so if we increase. the temperature the leakage current will also increase and heat dissipitation of junction. 2 occurs When this heat reaches a sufficient value 2 will break and conduction starts. Disadvantages, This type of triggering causes local hot spot and may cause thermal run away of the device. This triggering cannot be controlled easily,It is very costly as protection is costly. 5 Light triggering, First a new recess niche is made in the inner p layer When this recess is irradiated then free. charge carriers electron and hole are generated Now if the intensity is increased above a. certain value then it leads to turn ON of SCR Such SCR are known as Light activated SCR. Some definitions,Latching current, The latching current may be defined as the minimum value of anode current which at must. attain during turn ON process to maintain conduction even if gate signal is removed. Holding current, It is the minimum value of anode current below which if it falls the SCR will turn OFF.
Switching characteristics of thyristors, The time variation of voltage across the thyristor and current through it during turn on. and turn off process gives the dynamic or switching characteristic of SCR. Switching characteristic during turn on,Turn on time. It is the time during which it changes from forward blocking state to ON state Total turn. on time is divided into 3 intervals,1 Delay time,2 Rise time. 3 Spread time,Delay time, If and represent the final value of gate current and anode current Then the delay time. can be explained as time during which the gate current attains 0 9 to the instant anode. current reaches 0 1 or the anode current rises from forward leakage current to 0 1. 1 Gate current 0 9 to 0 1,2 Anode voltage falls from to 0 9.
3 Anode current rises from forward leakage current to 0 1. Time during which,1 Anode current rises from 0 1 to 0 9. 2 Forward blocking voltage falls from 0 9 to 0 1 is the initial forward blocking. Spread time, 1 Time taken by the anode current to rise from 0 9 to. 2 Time for the forward voltage to fall from 0 1 to on state voltage drop of 1 to 1 5V. During turn on SCR is considered to be a charge controlled device A certain amount. of charge is injected in the gate region to begin conduction So higher the magnitude. of gate current it requires less time to inject the charges Thus turn on time is reduced. by using large magnitude of gate current,How the distribution of charge occurs. As the gate current begins to flow from gate to cathode with the application of gate. signal Gate current has a non uniform distribution of current density over the cathode. surface Distribution of current density is much higher near the gate The density decrease. as the distance from the gate increases So anode current flows in a narrow region near. gate where gate current densities are highest From the beginning of rise time the anode. current starts spreading itself The anode current spread at a rate of 0 1mm sec The. spreading anode current requires some time if the rise time is not sufficient then the anode. current cannot spread over the entire region of cathode Now a large anode current is. applied and also a large anode current flowing through the SCR As a result turn on losses. is high As these losses occur over a small conducting region so local hot spots may form. and it may damage the device,Switching Characteristics During Turn Off. Thyristor turn off means it changed from ON to OFF state Once thyristor is oON there is. no role of gate As we know thyristor can be made turn OFF by reducing the anode. current below the latching current Here we assume the latching current to be zero. Lecture Notes on Power Electronics Subject code BEE1602 6th Semester B Tech Electrical Engineering Disclaimer This document does not claim any originality and cannot be used as a substitute for prescribed textbooks The information presented here is merely a collection by the committee members for their respective teaching assignments Various sources as mentioned at the end of the

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