POWER SUPPLY FILTERS AND REGULATORS A power supply filter ideally eliminates the fluctuations in the output voltage of a half wave or full-wave rectifier and produces a constant-level de voltage. Filtering is necessary because electronic circuits require a constant source of de voltage and current to provide power and biasing for proper operation. Filters are implemented with capacitors, as you will see in this section. Voltage regulation in power supplies is usually done with integrated circuit voltage regulators. A voltage regulator prevents changes in the filtered dc voltage due to variations in input voltage or load. After completing this section, you should be able 10 ‘© Explain and analyze the operation and characteristics of power supply filters and regulators © Explain the purpose of a filter = Describe the capacitor-input filter = Define ripple voltage and calculate the ripple factor = Discuss surge current in a capacitor-input filter = Discuss voltage regulation In most power supply applications, the standard 60 Hz ac power line voltage must be converted to an approximately constant de voltage. The 60 Hz pulsating de output of a half ‘wave rectifier or the 120 Hz pulsating output of a full-wave rectifier must be filtered to re- duce the large voltage variations, Figure 2-24 illustrates the filtering concept showing a nearly smooth dc output voltage from the filter. The small amount of fluctuation in the fil- ter output voltage is called ripple. \ ov ruttwave 4 [VV\ rectifier (2) Rectifier withouta filter VLA_LA | Fattwave A rectifier (8) Roster with Fike (output ipple is exaggerated) ia ——— Fihter 0| Capacitor-Input Filter A half-wave rectifier with « capacitor-input fiers shown in Figure 2-25. The filteris sim- ply a capacitor connected from the rectifier output to ground. R,, represents the equivalent resistance of & load. We will use the half-wave rectifier to illustrate the basic principle and then expand the concept to full-wave rectification, (0) The capacitor discharges through Ry ater peak of pesitive alteration when the diode i reverse-biased. ‘This disctarsing occurs during the portion of the input voltage indicated by the solid blue curveL -—h. pot (©) The capacitor charges buck to peak of input when the diode becomes forward-biased. This charting occurs ‘ring the portion ofthe input voltage indicated by the solid blue curve During the positive first quarter-cycle of the input, the diode is forward-biased, allowing the capacitor to charge to within 0.7 V of the input peak, as illustrated in Figure 2-25(a). ‘When the input begins to decrease below its peak, as shown in part (b), the capacitor retains its charge and the diode becomes reverse-biased because the cathode is more positive than the anode, During the remaining part of the cycle, the espacitor can discharge only through the load resistance at rate determined by the R;C time constant, which is normally long compared to the period of the input. The larger the ime constant, the less the eapacitor will discharge. During the first quarter ofthe next cycle, as illustrated in part (@), the diode will again become forward-biased when the input voltage exceeds the capacitor voltage by ap- proximately 0.7 V. Ripple Voltage As you have seen, the capacitor quickly charges at the beginning of a cy- cle and slowly discharges through R, after the positive peak of the input voltage (when the diode is reverse-biased). The variation in the capacitor Voltage due to the cbarging and dis- charging is called the ripple voltage. Generally, ripple is undesirable; thus, the smaller the tipple, the better the filtering action, as illustrated in Figure 2-26(a) Lage spp men les llesivePkerng (0) Sasi Hippie mas meee Rs Genel he ge te ‘apchor ate te smelt pe ese ap a ed FIGURE 2-26 altwave ripple voltage (green Foe) Fora given input frequency, the output Frequency ofa full-wave rectifier is twice that of a half-wave rectifier, as illusated in Figure 2-27. This makes atll-wave rectifier easier 10 filter heenuse ofthe shorter time between peaks, When filtered, the ful-wave rectified volt age has a smaller ripple than does a hall-wave votiage forthe same load resistance and ca pacitor values. The capacitor discharges less during the shorter interval bersteen full-wave pulses, 2s shown in Figuse 2-28, SANNA SL Sez. (a) Hatene haewave rected voltage. Sune supe vwapuchor Ripple fscharge rae (b) Fall wave Ripple Factor ‘The ripple factor (r) is an indication of the effecziveness of the filter and is defined as Yep) Vow Where Vip, is the peak-to-peak ripple voltage and Vpc is the de (average) value of the fil- ter’s output voltage, as illustrated in Figure 2-29. The lower the ripple factor, the better the filter. The ripple factor can be lowered by increasing the valve of the filter capacitor or in- creasing the load resistance.‘eFigune 2-29 Vand Voc detomine the pple factor Fora full-wave recifier with a capscior-inpat fer, approximations for the pe peak ripple voltage, Vin and the de value of the filter output voltage, Ves are given in following expressions: The variable Vag isthe unfiltered peak rectified voltage (Ly Equation2-11 Mn ™ (ts) Yam Equation 2-12 Vn (1 ~ ge) ‘These expressions ae derived in Appendix B. Determine the ripple factor for the filtered bridge rectifier with a load as indicated in Figure 2-30. vost ASViewe Oupai 9 Allies are 1N4OD1