Lecture notes on Power Electronics and drives

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING COURSE: POWER ELECTRONICS BRANCH: EEE CLASS: III/I Sem. YEAR: 2013-14 LECTURE NOTES SHRI VISHNU ENGINEERING COLLEGE FOR WOMEN VISNUPUR, BHIMAVARAM - 534202 INDEX S. NO. CONTENT PAGE NO. 1 UNIT I: POWER SEMICONDUCTOR 3 - 32 DEVICES 2 UNIT II: FIRING AND COMMUTATION 33 - 50 CIRCUITS OF SCR 3 UNIT III: SINGLE PHASE HALF 51 - 60 CONTROLLED CONVERTERS 4 UNIT IV: SINGLE PHASE FULLY 61 - 70 CONTROLLED CONVERTERS 5 UNIT V: THREE PHASE LINE 71 – 87 COMMUTATED CONVERTER 6 UNIT VI: AC VOLTAGE CONTROLLERS 88 – 99 & CYCLO CONVERTERS DEPARTMENT OF EEE – SVECW Page 2 Unit-1 Power Semi Conductor Devices DEPARTMENT OF EEE – SVECW Page 3 1.1 INTRODUCTION TO POWER ELECTRONICS: Power Electronics is a field which combines Power (electric power), Electronics and Control systems. Power engineering deals with the static and rotating power equipment for the generation, transmission and distribution of electric power. Electronics deals with the study of solid state semiconductor power devices and circuits for Power conversion to meet the desired control objectives (to control the output voltage and output power). Power electronics may be defined as the subject of applications of solid state power semiconductor devices (Thyristors) for the control and conversion of electric power. Power electronics deals with the study and design of Thyristorised power controllers for variety of application like Heat control, Light/Illumination control, Motor control - AC/DC motor drives used in industries, High voltage power supplies, Vehicle propulsion systems, High voltage direct current (HVDC) transmission. 1.2 BRIEF HISTORY OF POWER ELECTRONICS The first Power Electronic Device developed was the Mercury Arc Rectifier during the year 1900. Then the other Power devices like metal tank rectifier, grid controlled vacuum tube rectifier, ignitron, phanotron, thyratron and magnetic amplifier, were developed & used gradually for power control applications until 1950. The first SCR (silicon controlled rectifier) or Thyristor was invented and developed by Bell Lab's in 1956 which was the first PNPN triggering transistor. The second electronic revolution began in the year 1958 with the development of the commercial grade Thyristor by the General Electric Company (GE). Thus the new era of power electronics was born. After that many different types of power semiconductor devices & power conversion techniques have been introduced.The power electronics revolution is giving us the ability to convert, shape and control large amounts of power. 1.3 SOME APPLICATIONS OF POWER ELECTRONICS Advertising, air conditioning, aircraft power supplies, alarms, appliances - (domestic and industrial), audio amplifiers, battery chargers, blenders, blowers, boilers, burglar alarms, cement kiln, chemical processing, clothes dryers, computers, conveyors, cranes and hoists, dimmers (light dimmers), displays, electric door openers, electric dryers, electric fans, electric vehicles, electromagnets, electro mechanical electro plating, electronic ignition, electrostatic precipitators, elevators, fans, flashers, food mixers, food warmer trays, fork lift trucks, furnaces, games, garage door openers, gas turbine starting, generator exciters, grinders, hand power tools, heat controls, high frequency lighting, HVDC transmission, induction heating, laser power supplies, latching relays, light flashers, linear induction motor controls, locomotives, machine tools, magnetic recording, magnets, mass transit railway system, mercury arc lamp ballasts, mining, model trains, motor controls, motor drives, movie projectors, nuclear reactor control rod, oil well drilling, oven controls, paper mills, particle accelerators, phonographs, photo copiers, power suppliers, printing press, pumps and compressors, radar/sonar power supplies, refrigerators, regulators, RF amplifiers, security systems, servo systems, sewing DEPARTMENT OF EEE – SVECW Page 4 machines, solar power supplies, solid-state contactors, solid-state relays, static circuit breakers, static relays, steel mills, synchronous motor starting, TV circuits, temperature controls, timers and toys, traffic signal controls, trains, TV deflection circuits, ultrasonic generators, UPS, vacuum cleaners, VAR compensation, vending machines, VLF transmitters, voltage regulators, washing machines, welding equipment. 1.4 POWER ELECTRONIC APPLICATIONS COMMERCIAL APPLICATIONS Heating Systems Ventilating, Air Conditioners, Central Refrigeration, Lighting, Computers and Office equipments, Uninterruptible Power Supplies (UPS), Elevators, and Emergency Lamps. DOMESTIC APPLICATIONS Cooking Equipments, Lighting, Heating, Air Conditioners, Refrigerators & Freezers, Personal Computers, Entertainment Equipments, UPS. INDUSTRIAL APPLICATIONS Pumps, compressors, blowers and fans. Machine tools, arc furnaces, induction furnaces, lighting control circuits, industrial lasers, induction heating, welding equipments. AEROSPACE APPLICATIONS Space shuttle power supply systems, satellite power systems, aircraft power systems. TELECOMMUNICATIONS Battery chargers, power supplies (DC and UPS), mobile cell phone battery chargers. TRANSPORTATION Traction control of electric vehicles, battery chargers for electric vehicles, electric locomotives, street cars, trolley buses, automobile electronics including engine controls. UTILITY SYSTEMS High voltage DC transmission (HVDC), static VAR compensation (SVC), Alternative energy sources (wind, photovoltaic), fuel cells, energy storage systems, induced draft fans and boiler feed water pumps. DEPARTMENT OF EEE – SVECW Page 5 1.5 POWER SEMICONDUCTOR DEVICES Power Diodes. Power transistors (BJT's). Power MOSFETS. IGBT's. Thyristors Thyristors are a family of p-n-p-n structured power semiconductor switching devices 1.6 SCR's (Silicon Controlled Rectifier) The silicon controlled rectifier is the most commonly and widely used member of the thyristor family. The family of thyristor devices include SCR's, Diacs, Triacs, SCS, SUS, LASCR's and so on. 1.7 POWER SEMICONDUCTOR DEVICES USED IN POWER ELECTRONICS The first thyristor or the SCR was developed in 1957. The conventional Thyristors (SCR's) were exclusively used for power control in industrial applications until 1970. After 1970, various types of power semiconductor devices were developed and became commercially available. The power semiconductor devices can be divided broadly into five types Power Diodes. Thyristors. Power BJT's. Power MOSFET's. Insulated Gate Bipolar Transistors (IGBT's). Static Induction Transistors (SIT's). The Thyristors can be subdivided into different types Forced-commutated Thyristors (Inverter grade Thyristors) Line-commutated Thyristors (converter-grade Thyristors) Gate-turn off Thyristors (GTO). Reverse conducting Thyristors (RCT's). Static Induction Thyristors (SITH). Gate assisted turn-off Thyristors (GATT). Light activated silicon controlled rectifier (LASCR) or Photo SCR's. MOS-Controlled Thyristors (MCT's). 1.8 POWER DIODES Power diodes are made of silicon p-n junction with two terminals, anode and cathode. P-N junction is formed by alloying, diffusion and epitaxial growth. Modern techniques in diffusion and epitaxial processes permit desired device characteristics. DEPARTMENT OF EEE – SVECW Page 6 The diodes have the following advantages High mechanical and thermal reliability High peak inverse voltage Low reverse current Low forward voltage drop High efficiency Compactness. 1.9 POWER TRANSISTORS Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices are used a switching devices and are operated in the saturation region resulting in low on-state voltage drop. They are turned on when a current signal is given to base or control terminal. The transistor remains on so long as the control signal is present. The switching speed of modern transistors is much higher than that of thyristors and are used extensively in dc-dc and dc-ac converters. However their voltage and current ratings are lower than those of thyristors and are therefore used in low to medium power applications. Power transistors are classified as follows o Bipolar junction transistors(BJTs) o Metal-oxide semiconductor filed-effect transistors(MOSFETs) o Static Induction transistors(SITs) o Insulated-gate bipolar transistors(IGBTs) 1.9.1 BIPOLAR JUNCTION TRANSISTORS The need for a large blocking voltage in the off state and a high current carrying capability in the on state means that a power BJT must have substantially different structure than its small signal equivalent. The modified structure leads to significant differences in the I-V characteristics and switching behavior between power transistors and its logic level counterpart. 1.9.2 POWER TRANSISTOR STRUCTURE If we recall the structure of conventional transistor we see a thin p-layer is sandwiched between two n-layers or vice versa to form a three terminal device with the terminals named as Emitter, Base and Collector. The difference in the two structures is obvious. A power transistor is a vertically oriented four layer structure of alternating p-type and n-type. The vertical structure is preferred because it maximizes the cross sectional area and through which the current in the device is flowing. This also minimizes on-state resistance and thus power dissipation in the transistor. 19 -3 The doping of emitter layer and collector layer is quite large typically 10 cm . A - 14 special layer called the collector drift region (n ) has a light doping level of 10 . The thickness of the drift region determines the breakdown voltage of the transistor. The base thickness is made as small as possible in order to have good amplification capabilities, however if the base thickness is small the breakdown voltage capability of the transistor is compromised. DEPARTMENT OF EEE – SVECW Page 7 Practical power transistors have their emitters and bases interleaved as narrow fingers as shown. The purpose of this arrangement is to reduce the effects of current crowding. This multiple emitter layout also reduces parasitic ohmic resistance in the base current path which reduces power dissipation in the transistor. Fig. 2 1.9.3 STEADY STATE CHARACTERISTICS Figure 3(a) shows the circuit to obtain the steady state characteristics. Fig 3(b) shows the input characteristics of the transistor which is a plot of I versus V . Fig 3(c) B BE shows the output characteristics of the transistor which is a plot I versus V . The C CE characteristics shown are that for a signal level transistor. The power transistor has steady state characteristics almost similar to signal level transistors except that the V-I characteristics has a region of quasi saturation as shown by figure 4. DEPARTMENT OF EEE – SVECW Page 8 Fig. 3: Characteristics of NPN Transistors There are four regions clearly shown: Cutoff region, Active region, quasi saturation and hard saturation. The cutoff region is the area where base current is almost zero. Hence no collector current flows and transistor is off. In the quasi saturation and hard saturation, the base drive is applied and transistor is said to be on. Hence collector current flows depending upon the load. The power BJT is never operated in the active region (i.e. as an amplifier) it is always operated between cutoff and saturation. The BV is the maximum collector to SUS emitter voltage that can be sustained when BJT is carrying substantial collector current. The BV is the maximum collector to emitter breakdown voltage that can be sustained when CEO base current is zero and BV is the collector base breakdown voltage when the emitter is CBO open circuited DEPARTMENT OF EEE – SVECW Page 9 Quasi-saturation Hard - 1/R d Saturation Second breakdown iC I I ,etc. B5 B4 I B5 IB4 Active region IB3 Primary breakdown I B2 IB1 I 0 B I =0 B I =0 B 0 vC BV CEO E BV SUS BVCBO Fig. 4: Characteristics of NPN Power Transistors The primary breakdown shown takes place because of avalanche breakdown of collector base junction. Large power dissipation normally leads to primary breakdown. The second breakdown shown is due to localized thermal runaway. This is explained in detail later. 1.9.4 TRANSFER CHARACTERISTICS Fig. 5: Transfer Characteristics DEPARTMENT OF EEE – SVECW Page 10 1.10 TRANSISTOR AS A SWITCH The transistor is used as a switch therefore it is used only between saturation and cutoff. From fig. 5 we can write the following equations Fig. 6: Transistor Switch If the base current is increased above I ,V increases, the collector current BM BE increases and V falls belowV . This continues until the CBJ is forward biased with CE BE V BC of about 0.4 to 0.5V, the transistor than goes into saturation. The transistor saturation may be defined as the point above which any increase in the base current does not increase the collector current significantly. In saturation, the collector current remains almost constant. If the collector emitter voltage is V the collector current is CE sat V increases due to increased base current resulting in increased power loss. Once the BE transistor is saturated, the CE voltage is not reduced in relation to increase in base current. However the power is increased at a high value of ODF, the transistor may be damaged I I B BS may operate in active region, V increases resulting in increased power loss. CE 1.11 SWITCHING CHARACTERISTICS A forward biased p-n junction exhibits two parallel capacitances; a depletion layer capacitance and a diffusion capacitance. On the other hand, a reverse biased p-n junction has only depletion capacitance. Under steady state the capacitances do not play any role. However under transient conditions, they influence turn-on and turn-off behavior of the transistor. 1.12 TRANSIENT MODEL OF BJT Fig. 7: Transient Model of BJT DEPARTMENT OF EEE – SVECW Page 11 Fig. 8: Switching Times of BJT Due to internal capacitances, the transistor does not turn on instantly. As the voltage V rises from zero to V and the base current rises to I , the collector current B 1 B1 does not respond immediately. There is a delay known as delay time td, before any collector current flows. The delay is due to the time required to charge up the BEJ to the forward bias voltage V (0.7V). The collector current rises to the steady value of I and BE CS this time is called rise time t . r The base current is normally more than that required to saturate the transistor. As a result excess minority carrier charge is stored in the base region. The higher the ODF, the greater is the amount of extra charge stored in the base. This extra charge which is called the saturating charge is proportional to the excess base drive. This extra charge which is called the saturating charge, is proportional to the excess base drive and the corresponding current I . e When the input voltage is reversed from V to -V , the reverse current –I helps 1 2 B2 to discharge the base. Without –I the saturating charge has to be removed entirely due B2 to recombination and the storage time t would be longer. s Once the extra charge is removed, BEJ charges to the input voltage –V and the base 2 current falls to zero. t depends on the time constant which is determined by the reverse f biased BEJ capacitance. DEPARTMENT OF EEE – SVECW Page 12 Turn-on time t : The turn-on time can be decreased by increasing the base drive for a on fixed value of collector current. t is dependent on input capacitance does not change d significantly with I . However t increases with increase in I . C r C Turn off time t : The storage time t is dependent on over drive factor and does not off s change significantly with I . t is a function of capacitance and increases with I . C f C t & t can be reduced by providing negative base drive during turn-off. t is less s f f sensitive to negative base drive. Cross-over t : The crossover time t is defined as the interval during which the collector C C voltage V rises from 10% of its peak off state value and collector current. I falls to CE C 10% of its on-state value. t is a function of collector current negative base drive. C 1.13 POWER DERATING Fig. 11: Thermal Equivalent Circuit of Transistor 1.14 BREAK DOWN VOLTAGES A break down voltage is defined as the absolute maximum voltage between two terminals with the third terminal open, shorted or biased in either forward or reverse direction. BV : The maximum voltage between the collector and emitter that can be sustained SUS across the transistor when it is carrying substantial collector current. BV : The maximum voltage between the collector and emitter terminal with base CEO open circuited. BV : This is the collector to base break down voltage when emitter is open circuited. CBO 1.15 BASE DRIVE CONTROL This is required to optimize the base drive of transistor. Optimization is required to increase switching speeds. t can be reduced by allowing base current peaking during on can be increased to a sufficiently high value to maintain the transistor in quasi-saturation region. t can be reduced by reversing base current and allowing base current peaking off during turn off since increasing I decreases storage time. B 2 DEPARTMENT OF EEE – SVECW Page 13 A typical waveform for base current is shown. IB I B1 IBS 0 t -I B2 Fig. 12: Base Drive Current Waveform Some common types of optimizing base drive of transistor are Turn-on Control. Turn-off Control. Proportional Base Control. Antisaturation Control 1.16 TURN-ON CONTROL Fig. 13: Base current peaking during turn-on When input voltage is turned on, the base current is limited by resistor R and C 1 1 discharges through R . The discharging time constant is R C . To allow sufficient 2 2 2 1 charging and discharging time, the width of base pulse must be t and off 1 1 1.17 TURN-OFF CONTROL If the input voltage is changed to during turn-off the capacitor voltage V is added C to V as reverse voltage across the transistor. There will be base current peaking during 2 turn off. As the capacitor C discharges, the reverse voltage will be reduced to a steady 1 state value, V . If different turn-on and turn-off characteristics are required, a turn-off 2 C , R & R D isolates the forward base drive 2 3 4 1 circuit from the reverse base drive circuit during turn off. DEPARTMENT OF EEE – SVECW Page 14 Fig: 14. Base current peaking during turn-on and turn-off 1.18 PROPORTIONAL BASE CONTROL This type of control has advantages over the constant drive circuit. If the collector current changes due to change in load demand, the base drive current is changed in proportion to collector current. When switch S is turned on a pulse current of short duration would flow through 1 the base of transistor Q and Q is turned on into saturation. Once the collector current 1 1 starts to flow, a corresponding base current is induced due to transformer action. The transistor would latch on itself and S can be turned off. For proper operation of the 1 circuit, the magnetizing current which must be much smaller than the collector current should be as small as possible. The switch S can be implemented by a small signal 1 transistor and additional arrangement is necessary to discharge capacitor C and reset the 1 transformer core during turn-off of the power transistor. 1.19 ANTISATURATION CONTROL Fig: 16: Collector Clamping Circuit If a transistor is driven hard, the storage time which is proportional to the base current increases and the switching speed is reduced. The storage time can be reduced by DEPARTMENT OF EEE – SVECW Page 15 operating the transistor in soft saturation rather than hard saturation. This can be accomplished by clamping CE voltage to a pre-determined level and the collector current VCC VCM is given by I . C R C Where V is the clamping voltage and V V . CM CM CE sat This means that the CE voltage is raised above saturation level and there are no excess carriers in the base and storage time is reduced. The clamping action thus results a reduced collector current and almost elimination of the storage time. At the same time, a fast turn-on is accomplished. However, due to increased V , the on-state power dissipation in the transistor is CE increased, whereas the switching power loss is decreased. ADVANTAGES OF BJT’S BJT’s have high switching frequencies since their turn-on and turn-off time are low. The turn-on losses of a BJT are small. BJT has controlled turn-on and turn-off characteristics since base drive control is possible. BJT does not require commutation circuits. DEMERITS OF BJT Drive circuit of BJT is complex. It has the problem of charge storage which sets a limit on switching frequencies. It cannot be used in parallel operation due to problems of negative temperature coefficient. 1.20. POWER MOSFETS 1.20.1 INTRODUCTION TO FET’S FET’s use field effect for their operation. FET is manufactured by diffusing two areas of p-type into the n-type semiconductor as shown. Each p-region is connected to a gate terminal; the gate is a p-region while source and drain are n-region. Since it is similar to two diodes one is a gate source diode and the other is a gate drain diode. Fig:1: Schematic symbol of JFET DEPARTMENT OF EEE – SVECW Page 16 Fig. 2: Structure of FET with biasing In BJT’s we forward bias the B-E diode but in a JFET, we always reverse bias the gate-source diode. Since only a small reverse current can exist in the gate lead. Therefore I 0 , therefore R ideal G in The term field effect is related to the depletion layers around each p-region as shown. When the supply voltage V is applied as shown it forces free electrons to flow DD from source to drain. With gate reverse biased, the electrons need to flow from source to drain, they must pass through the narrow channel between the two depletion layers. The more the negative gate voltage is the tighter the channel becomes. Therefore JFET acts as a voltage controlled device rather than a current controlled device. JFET has almost infinite input impedance but the price paid for this is loss of control over the output current, since JFET is less sensitive to changes in the output voltage than a BJT. JFET CHARACTERISTICS DEPARTMENT OF EEE – SVECW Page 17 The maximum drain current out of a JFET occurs when V V is increased for GS DS 0 to a few volts, the current will increase as determined by ohms law. As V approaches V the DS P depletion region will widen, carrying a noticeable reduction in channel width. If V is increased DS to a level where the two depletion region would touch a pinch-off will result. I now maintains a D saturation level I . Between 0 volts and pinch off voltage V is the ohmic region. After V , the DSS P P regions constant current or active region. If negative voltage is applied between gate and source the depletion region similar to those obtained with V V . Therefore GS DS saturation level is reached earlier. 1.20.2 Classification of MOSFET MOSFET stands for metal oxide semiconductor field effect transistor. There are two types of MOSFET Depletion type MOSFET Enhancement type MOSFET 1.20.3 DEPLETION TYPE MOSFET CONSTRUCTION Symbol of n-channel depletion type MOSFET DEPARTMENT OF EEE – SVECW Page 18 It consists of a highly doped p-type substrate into which two blocks of heavily doped n- type material are diffused to form a source and drain. A n-channel is formed by diffusing between source and drain. A thin layer of SiO is grown over the entire surface 2 and holes are cut in SiO to make contact with n-type blocks. The gate is also connected to a 2 metal contact surface but remains insulated from the n-channel by the SiO layer. SiO layer 2 2 10 15 results in an extremely high input impedance of the order of 10 to 10 fig. 4: Structure of n-channel depletion type MOSFET OPERATION When V V and V is applied and current flows from drain to source similar to GS DS JFET. When V V , the negative potential will tend to pressure electrons towards GS the p-type substrate and attracts hole from p-type substrate. Therefore recombination occurs and will reduce the number of free electrons in the n-channel for conduction. Therefore with increased negative gate voltage I reduces. D For positive values,V , additional electrons from p-substrate will flow into the gs channel and establish new carriers which will result in an increase in drain current with positive gate voltage. 1.20.4 DRAIN CHARACTERISTICS DEPARTMENT OF EEE – SVECW Page 19 1.20.5 TRANSFER CHARACTERISTICS 1.21 ENHANCEMENT TYPE MOSFET Here current control in an n-channel device is now affected by positive gate to source voltage rather than the range of negative voltages of JFET’s and depletion type MOSFET. 1.21.1 BASIC CONSTRUCTION A slab of p-type material is formed and two n-regions are formed in the substrate. The source and drain terminals are connected through metallic contacts to n-doped regions, but the absence of a channel between the doped n-regions. The SiO layer is still 2 present to isolate the gate metallic platform from the region between drain and source, but now it is separated by a section of p-type material. Fig. 5: Structure of n-channel enhancement type MOSFET DEPARTMENT OF EEE – SVECW Page 20

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