Subject Name: POWER ELECTRONICS Subject Code: 10EC73 Prepared By: D.KAVITHA,M.PRIYADHARSINI Department: ECE Date: 8/12/2014 9/16/2018

INTRODUCTION TO POWER ELECTRONICS UNIT 1 INTRODUCTION TO POWER ELECTRONICS 9/16/2018

Applications of power electronics Power Semiconductor devices Topic details Introduction Applications of power electronics Power Semiconductor devices Control characteristics Types of power electronics circuits Peripheral effects 9/16/2018

Relationship of Power Electronics INTRODUCTION Relationship of Power Electronics The electrical engineering field may be divided into three areas of specialization : Electronics , Power and Control 9/16/2018

Stability and response characteristics of closed loop system Electronics Processing of information at low power levels Power Generation,transmission,distribution & utilization of vast quantities of electrical power Control Stability and response characteristics of closed loop system 9/16/2018

Electrical energy in one form Power electronic system Example: Input is AC and output is DC. This is conversion is performed by a rectifier ,so it is a power electronic system Electrical energy in one form Power electronic system Electrical energy in the other form 9/16/2018

Applications of power electronics 1 Applications of power electronics 1. UPS and stand by power supplies for critical loads – Computers 2. Power control in resistance welding induction heating, electrolysis and process control 3. Power conversion for HVDC and HVAC transmission systems 4. Speed control of motors in traction drives, textile mills, cranes ,lifts etc 5. Solid state power compensators ,static contactors, transformer tap changers 6. High voltage supplies for electrostatic precipitators and x-ray generators 7. Power supplies for communication system,stelephone exchanges ,satellite system 9/16/2018

Power Controller or Converter Load Power Source Power Controller or Converter Load Control Unit Sensing and feedback circuit Reference settings and commands 9/16/2018

Power source : It can be a AC mains, generators or batteries Power Controller : Converts input power and it may be of SCR,GTO,MOSFET or IGBT Sensing & Feedback Circuit : Monitor the load condition & the feedback is given to control unit Control Unit : Drive switches and it is adjusted whenever there is a difference between actual speed and reference speed Reference Settings & Commands: Sends command to the control unit for proper functioning of power electronic system and load 9/16/2018

TYPES OF SEMICONDUCTOR DEVICES Power devices Diodes Thyristors Transistors BJT MOSFET IGBT SIT General purpose High speed Schottky SCR GTO RCT SITH GATT LASCR MCT TRIAC 9/16/2018

DIODES General Purpose Diodes: High reverse recovery time of 25μs Used in low speed applications such as rectifiers & converters Operate up to 1khz Ratings from 1A/50v to 1000A/500V Fast Recovery Diodes: RRT is less than 5μs Used for high speed applications such as chopper & inverters Ratings from 1A/50V to 100A/3KV Schottky Diodes: Pn junction is eliminated Metal is placed directly on semiconductor Storage time is absent Turn-off time is very small High Switching frequencys Large reverse leakage currents Breakdown voltage is less than 100v 9/16/2018

SCR SCR or thyristor, is a multi-junction device with latching switch behavior It is similar to a diode, but with one important difference. Two conditions have to be met for forward current to flow: 1. Forward-biased voltage 2. A pulse of current on the gate terminal 3. This allows the thyristor to control very large currents very easily A thyristor (or silicon-controlled rectifier) is a pnpn device with three terminals: 1. Anode (A) 2. Cathode (K) 3. Gate (G) 9/16/2018

Anode is positive with respect to cathode ,so is said to be forward bias Small positive voltage is applied between gate and cathode to turn on SCR When SCR is on, internal drop is negligible and current flows from anode to cathode Once SCR turns on , gate has no control over conduction Even if gate drive is removed , SCR does not turn off SCR turns off when current falls to zero. 9/16/2018

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GTO (Gate Turn-off Thyristor) Similar to SCR But can be turned off by gate terminals Gate has full control over the conduction of thyristor Do not require any commutation circuit Used for low power applications Gate drive required to turn off is very large VI-CHARACTERISTICS SYMBOL 9/16/2018

SITH (Static Induction Thyristor) It can be turned off by negative gate pulse Gate has full control over the conduction Do not require any commutation circuit Characteristics is similar to GTO Used for medium power converter circuit SYMBOL VI CHARACTERISTICS 9/16/2018

MCT (MOS-CONTROLLED THYRISTOR) Turn on by giving negative voltage to gate terminal Turn off by giving positive voltage to gate terminal Has low power ratings SYMBOL VI-CHARACTERISTICS 9/16/2018

LASCR (LIGHT ACTIVATED SCR) Turned on by light signal Gate is photo sensitive and SCR turn on when light falls on it Isolation between gate and drive circuit Used in HVDC in transmission 9/16/2018

RCT (Reverse Conducting SCR) It has an SCR antiparallel with the diode High current during forward direction Less current during reverse direction Suitable for inductive loads GATT (Gate Assisted Turn-off Thyristor) Suitable for high speed switching & Mediun power applications Require force to turn off the circuits TRIAC (Bidirectional Triode Thyristor) Considered as antiparallel SCR Conducts in both the direction It has a single gate Used for heat control, light control and motor control Turned off by line commutation of AC supply Not suitable for inductive loads 9/16/2018

TRIAC SYMBOL AND CHARACTERISTICS VI-CHARACTERISTICS SYMBOL 9/16/2018

CONTROL CHARACTERISTICS SCR Vs is applied through load across anode-cathode Vg is applied across gate-cathode Positive Vg turns on SCR, hence complete Vs appears across the load Gate has no control over the conduction of SCR Even if we apply negative gate pulse does not turn-off SCR 9/16/2018

CONTROL CHARACTERISTICS OF GTO &SITH 9/16/2018

GTO & SITH has the similar control characteristics Vs applied through load across anode-cathode Whenever device turns - on , voltage applied across the load Whenever device turns - off, voltage is zero Positive gate turns-on GTO & SITH Negative gate turns-off GTO & SITH 9/16/2018

CONTROL CHARACTERISTICS OF MCT Negative voltage between gate – cathode turn –on the device Positive voltage between gate – cathode turn –off the device 9/16/2018

CONTROL CHARACTERISTICS OF BJT Positive voltage is applied between collector and emitter Control voltage is applied between base and emitter When base-emitter voltage is positive , BJT turns on and Vs appears across the load When base-emitter voltage is negative , BJT turns off and Vo is zero Circuit diagram of BJT 9/16/2018

CONTROL CHARACTERISTICS OF MOSFET Positive gate-source voltage turn-on the device,Vs appears across the load Negative gate-source voltage turn-off the device, load voltage is zero 9/16/2018

CONTROL CHARACTERISTICS OF IGBT Positive gate-emitter voltage turn-on the device,Vs appears across the load Negative gate-emitter voltage turn-off the device, load voltage is zero 9/16/2018

TYPES OF POWER ELECTRONICS CONVERTERS: It is classified into 5 categories depends upon input /output AC to DC converters DC to AC converters DC to DC converters AC to AC converters AC Regulators 9/16/2018

AC to DC converters 9/16/2018

1φ or 3φ AC supply available from mains It converts AC into controlled DC voltage & current It includes diode rectifiers as well as controlled rectifiers Controlled rectifiers use SCR, since input is AC supply SCR is turned off by natural commutation External commutation are not required AC to DC converters are also called line commutated converters Used for DC drives , UPS & HVDC systems 9/16/2018

Converters are commonly called as inverters DC to AC Converters Converters are commonly called as inverters Input to inverters is fixed DC voltage The DC voltage is from batteries Output of the inverter is fixed or variable frequency AC voltage magnitude is also variable Inverters are mainly used whenever mains are not available Applications: Used for speed control of induction motors Used in standby & emergency power supplies 9/16/2018

Input waveform Vdc t Output waveform Vo t 9/16/2018

It takes the input from the fixed voltage DC supply DC to DC Converters It takes the input from the fixed voltage DC supply Output is fixed or variable DC voltage It is used in DC drives Speed of motor can be controlled in forward & reverse directions It is also used in SMPS 9/16/2018

Input waveform Vdc t Output waveform Vo t 9/16/2018

AC to AC Converters (Cycloconverters) Input to the cycloconverters is 1Ф or 3ф AC Input is fixed voltage & fixed frequency Output has variable voltage & variable frequency Output frequency is lower than input frequency Cycloconverters are used mainly for AC traction drives 9/16/2018

Input is fixed voltage AC mains AC Regulators Input is fixed voltage AC mains Output frequency is same as input frequency It does not change the frequency , whereas cycloconverters change the frequency Used for speed control of large fans & pumps 9/16/2018

It takes input power from power source & delivers to the load. Peripheral Effects It takes input power from power source & delivers to the load. Normally uses switches to convert input power in required form. Because of switching action following things takes place: Switching voltage/current pulses are induced in power supply Harmonics are induced in power supply due to improper waveforms Load contains voltage/current spikes & harmonics Interference is radiated due to switching of drives These effects certainly affect other loads which are connected to same power supply Similarly output power converter is also distorted Hence, it affects the performance of the load connected to it. These problems can be reduced by putting filters before & after the power converters.These filters attenuate the harmonics & noise spikes. 9/16/2018