I.INTRODUCTION TO POWER ELECTRONICS

POWER CONVERTER AND ITS TOPOLOGIES 1 I.INTRODUCTION TO POWER ELECTRONICS

A.What is Power Electronics?Electric Power Supply(Electric Utility or Electric Energy SourcesPower Electronics Converters or systemElectrical Energy Utilization System( Consumers LoadFig 1. A General System for Power Electronics Conversion Power electronics is the technology associated with efficient conversion and control of electric power by using power semiconductor devices. Power electronics encompasses the use of electronic components, the application of circuit theory and design techniques, and the development of analytical tools toward efficient electronic conversion, control, and conditioning of electric power. Definition given by IEEE Power Electronics Society.B.Goals of Power Electronics Efficient conversion, conditioning, or processing and control of electric power using solid-state semiconductor devices in order to supply high quality power to the load causing minimum pollution of environment and the utility supply circuit. To develop different types of efficient Power electronics controllers ranging from few watts to several Megawatts keeping in view the recent trend in Energy conservation and Energy exploration. Develop Integrated Power Electronics Modules (IPEM) based power electronics processes using an integrated systems approach,improve the quality, reliability, and cost-effectiveness of power electronics systems; and reduce both the time and effort associated with design cycles for systems application.

Fig 2. Block Diagram of a Power Electronics ProcessorConverter has power switching semiconductor devices and energy storing elements like inductors and capacitors. Resistive elements are avoided in converters because they cause power loss and reduce efficiency. Controller switches on/off the switching devices present in the converter.

Fig 3. Example of a Power Processor A power processor may have more than one stage of power conversion.Fig 2 shows an example of a power processor containing 2 Converters (AC-DC and a DC-AC) which converts a fixed voltage and fixed frequency AC Source to Variable voltage and variable frequency AC employed in a AC Motor

Power ElectronicsSimulation and ComputingSignal ProcessingAnalog ElectronicsElectro-MagneticsSystems and Control TheoryElectrical MachinesPower SystemsCircuit TheorySolid State PhysicsMicro-controllersFig 4. Multi Disciplinary Nature of Power ElectronicsII.POWER CONVERTERS

A Power Electronic Converter processes the available form to another having a same/different frequency and/or voltage magnitude. There can be five basic types of converters depending upon the function performed: 1. AC-DC Converters 2. DC-AC Converters 3. Fixed DC- Variable DC Converters 4. Fixed AC- Variable AC (Fixed Frequency) Converters 5. Fixed AC- Variable AC (Variable Frequency) Converters

Fig 5. Different types of Power Converter Topologies and Their Symbolsiii.ac-dc converters/rectifiers Converter rectifier circuits are used to convert AC power into DC Power. The Converter circuits employing only diodes are called Uncontrolled Rectifiers . The output voltage does not vary for a given load and given AC supply. Therefore, it is called an Uncontrolled Converter.The converter circuit which employs both Diodes and thyristors (SCRs) is called a semi or half controlled converters. The controlled output voltage and current are unidirectional and power flows in one direction i.e from source to load. The converter circuit employing only SCRs is called Fully controlled converters. The direction of the current remains the same but the output DC voltage can be reversed, to allow the power from load to the supply.Fig 6. Classification of AC-DC Converters/Rectifiers In the study of AC-DC Converters, SCRs and diodes are assumed to be ideal switches. During Turn-ON there is no voltage drop across SCR or Diode. No reverse current exists during reverse blocking modeTransition time is negligible i.e Zero .Holding current is zeroConsider a Single phase half controlled rectifier with RL Load as shown in fig.7(a) and the corresponding current and voltage waveforms are shown in fig.7(b). The thyristor T is fired at wt = and the extinction angle is .During postive half cycle of supply voltage Thyristor is forward biased at wt = , the thyristor is turned on by gating signal. The load voltage becomes equal to source Voltage VS.

A.Single-phase Half wave phase controlled rectifier with RL Load Load Inductance forces load current io to rise gradually.After sometime io reaches the maximum value and then decreases as shown in fig. 7(b). At wt=, VO is zero but io is not zero due to load Inductance and SCR will not turn off untill io reduces below holding value. At wt=, io reduces to zero and SCR is turned Off. Fig 7(a).Single phase half wave rectifier with RL Load

After wt= and till wt=2 VO=0 and io=0.= (-) is the conduction angle. Average DC output voltage,

RMS Output Voltage,

Fig 7(b).Volatge and current waveforms for Single phase half wave rectifier with RL Load