The Circuit Designer's Companion - diagramas.diagram...

238 The Circuit Designer’s Companionfigure is used for the transformer in the above example then the peak V tx off-load atmaximum input voltage will rise to 20.2V. At the same time the diode forward voltagedrops at low current will be much less, say 0.6V each, so the possible maximum voltageat the reservoir capacitor could be around 19V. Thus even the common 16V ratedelectrolytic will not be adequate for this circuit. For higher voltage outputs, themaximum input voltage can even exceed the voltage rating of the regulator itself, andyou have to invest in a pre-regulator to hold the maximum to a manageable level. Notethat this condition is not the worst-case for regulator power dissipation, because theregulator is not passing significant load current.Maximum regulator dissipationIn fact maximum series-pass dissipation does not necessarily occur at full load current,because as the current rises the voltage across the series-pass element falls. Themaximum dissipation will occur at less than full output if the voltage dropped acrossthe dc supply’s equivalent series resistance is greater than half the difference betweenthe no-load input voltage and the output voltage. Figure 7.10 shows this graphically.I L·R sV rI LV r·I LI LRegR sV oc Vin,dc V outR s is equivalent series resistance of supplyFigure 7.10 Peak power dissipationPeak power dissipationoccurs when I L·R S = 0.5(V oc − V out )Minimum load requirementA further problem, particularly with switch-mode supplies, is that the stability of theregulator cannot always be assured down to zero load. For this reason some rails haveto be run with a minimum load, such as a bleed resistor, to remain within specification,and this represents an unnecessary additional power drain. Many circuits, of course,always take a minimum current and so the minimum load is not then a problem.7.2.10 Rectifier and capacitor selectionThe specification of the rectifiers and capacitors is dominated by surge and ripplecurrent concerns.Reservoir capacitorThe minimum capacitor value is easily decided from the required ripple voltage:C = I L /V ripple · twhereI L is the dc load currentV ripple is the acceptable ripple voltageFor mains inputs, t is about 2ms less than the ac input period, 8ms for 50Hz or6ms for 60Hz full-waveA more accurate value can be derived from Schade’s curves (see footnote, page 236)which have been reprinted in numerous textbooks, but remember that the tolerance on