1.2 Filtered Full Wave RectifierThe filtered full wave rectifier is created from the FWR by adding a capacitor across theoutput. Figure 2. Filtered full wave rectifierThe result of the addition of a capacitor is a smoothing of the FWR output. The output isnowa pulsating dc,withapeak topeak variationcalled ripple.The magnitudeoftherippledepends ontheinput voltagemagnitudeandfrequency, thefilter capacitance, andtheloadresistance.To describe the source of the voltage ripple, consider the performance of the filtered fullwave rectifier above. The input to the rectifier is a sinewave of frequency f. et !ibe the fullwave rectified signal input to the filter stage of the rectifier and !obe the output. !ican beappro"imated as the absolute value of the rectifier input, with frequency 2f. Figure #. $utput %!i& and input %!o& of a filtered full wave rectifier'n the time period from T( to T), the diode *) %or *#, depending on the phase of thesignal&isforwardbiasedsince!i+!,)%appro"imatetheforwardbiaseddiodeasashortcircuit&. The capacitor ,) charges and the voltage across the load R increases. From T) to T2,the diodes *) and *2 are reverse biased %open circuit& because !cap+ !i, and the capacitordischarges through the load R with a time constant of R, seconds.The voltages between times T) and T2 lie along a capacitor discharge curve. -long thisline, Thepeaktopeak%pp&rippleisdefinedasthevoltagedifferencebetween !ma"and!min. 'f , is large, such that R, ++ T2 . T), we can appro"imate the e"ponential as. Then /ince T2 . T) 0 T12, where T is the period of the sine wave, then

Peak Current Levels*iodes in the bridge conduct only in the time period from T( to T). The diode currentmust replace the charge lost by the capacitor during its discharge.' 2 d31dT 2 ,4d!1dT-s the magnitude of the filter capacitor increases, the peak current through the diodesmust increase to replace the charge in less time. Therefore it is not always best to choose thelargest value of ,) available.'n a dc power supply, you can rely on the stages following theFFWR to significantly improve the voltage regulation. 1.3 The Shunt Regulator- shuntregulatormaybeplacedbetweenthefilteredfullwaverectifierandtheloadresistance %impedance&. 'ts purpose is to minimi5e the variation in the voltage across the load, aseither the input voltage or the output resistance changes.Figure 6. Filtered FWR and shunt regulatorThis regulator is called a shunt because it provides an additional path for current to flow,so that some current can bypass the load. The shunt regulator consists of a 5ener diode and aresistor.The5enerdiodehasanearlyconstant voltagedropwhenusedinreversebias. Theresistor ischosentomaintainthe5ener initsproper workingregion, whereit canprovideregulation and not e"ceed a ma"imum power limit.- simple model for the 5ener diode is a dc supply %battery& with a value of !5o, where!5o is the effective 5ener voltage,, !5 is the rated breakdown voltage, and R5 isthe effective resistance of the 5ener, given by the inverse of the slope of the '! curve in theworking region.Figure 7. Filtered FWR and shunt regulator with the 5ener diode replaced with its circuit model 'n the 5eners working region, R5 is small %(.) to 7( ohm &. For voltages less than the kneevoltage, R5 is very high, and for purposes of hand calculations can be considered to be an opencircuit.Figure 8. ,urrent.voltage characteristic of a 5ener diode9ou can show for the circuit above that where ' is the current through the load. The )st term in this equation is constant since it dependsonly on the diode 5ener voltage and two resistances. The 2nd and #rd terms depend on the inputvoltage and load current, both of which may change with time. These terms must be minimi5edfor quality regulation. Inut Sensitivit! and Load Sensitivit!-ssume the input to the shunt regulator is !dc :1. !ripple. For !in 2 !in%ma"& 2 !dc :!ripple, additional current is available from the source. To keep !o 2 ' R constant, some ofthat current must be shunted through the 5ener diode. -s long as '5 ; '5%ma"&, as defined by thema"imum power dissipation for the 5ener, the circuit will safely regulate. ,hoose R to preventthe 5ener from e"ceeding its ma"imum current limit. For !in2 !in%min& 2 !dc. !ripple, current drops. To keep !o2 '4Rconstant, thecurrent through the 5ener diode must be reduced. To maintain regulation, '5 must not be reducedbelow the knee current. ,hoose R to maintain sufficient current through the 5ener< The shunt regulator has several ma=or problems which prevent its common use as the solepre.regulation stage in dc power supplies