Lecture 9Controlled Rectifiers

The Controlled Half-wave RectifierNormal rectifiers are considered as uncontrolled rectifiers.

Once the source and load parameters are established, the dc level of the output and power transferred to the load are fixed quantities.

A way to control the output is to use SCR instead of diode. Two condition must be met before SCR can conduct:The SCR must be forward biased (VSCR>0)Current must be applied to the gate of SCR

Controlled, Half-wave R loadA gate signal is applied at t = , where is the delay/firing angle.

ExampleDesign a circuit to produce an average voltage of 40V across 100 load resistor from a 120Vrms 60 Hz ac source. Determine the power absorbed by the resistor and the power factor.

Briefly describe what happen if the circuit is replaced by diode to produce the same average output.

Example (Cont)SolutionIn such that to achieved 40V average voltage, the delay angle must beIf an uncontrolled diode is used, the average voltage would beThat means, some reducing average resistor to the design must be made. A series resistor or inductor could be added to an uncontrolled rectifier, while controlled rectifier has advantage of not altering the load or introducing the losses

Controlled, Half-wave R-L loadThe analysis of the circuit is very much similar to that of uncontrolled rectifier.

Controlled, Half-wave R-L load

Controlledfull-waverectifiers

Resistiveload

ThepowerdeliveredtotheloadThermscurrentinsourceisthesameasthermscurrentintheload.

Full wave Controlled Rectifier with RL load

Discontinuous and Continuous Operations

Discontinuous Mode

Analysisofthecontrolledfull-waverectifieroperatinginthediscontinuouscurrentmodeisidenticaltothatofthecontrolledhalf-waverectifier, exceptthattheperiodfortheoutputcurrentis .for Fordiscontinuouscurrentdiscontinuouscurrent :

Continuous Mode

continuouscurrent

R-LSourceloadFig.4-14TheSCRSmaybeturnedonatanytimethattheyareforwardbiased, whichisatanangle

Forcontinuouscurrentcase, theaveragebridgeoutputvoltageisaverageloadcurrentisTheacvoltagetermsareunchangedfromthecontrolledrectifierwithanR-Lload. Theaccurrenttermsaredeterminedfromcircuit.PowerabsorbedbythedcvoltageisPowerabsorbedbyresistorintheloadis

ControlledSingle-phaseconverteroperatingasaninverter

Copyright 2003 by John Wiley & Sons, Inc.Chapter 6 Thyristor ConvertersAverage DC output voltageAssuming AC side inductance is zeroNote that output voltage can go negative for alpha > 90 degrees. This means negative power flow or inversion