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Power supplies 233of harmonic distortion and its seriousness depends on the susceptibility of other loadsand components in the network.Large systems installations, in which there are many electronic power supplies offairly high rating fed from the same supply, are the main threat. In domestic premises,the switch-mode supplies of TV sets are the main offenders; in commercial buildings,the problem is worst with switch-mode supplies of PCs and their monitors, andfluorescent lamps with electronic ballasts. The current peaks always occur together andso reinforce each other. A network which is dominated by resistive loads such asheating and filament-lamp lighting can tolerate a proportion of high crest factor loadsmore easily.Power factor correctionThe “peakiness” of the input current waveform is best described in terms of itsharmonic content and legislation now exists in Europe, under the EMC Directive, tocontrol this. The European standard EN 61000-3-2:2000 places limits on the amplitudeof each of the harmonic components of the mains input current up to the 40th (2kHz at50Hz mains frequency), and it applies to virtually all electrical and electronic apparatusup to an input current of 16A, although products other than lighting equipment with arated power of less than 75W are exempt. The limits, although not particularly stiff, arepretty much impossible for a switched-mode power supply to meet without sometreatment of the input current. This treatment is generically known as “Power FactorCorrection” (PFC).In this context, Power Factor (PF) is the ratio between the real power, as transferredthrough the power supply to its load with associated losses, and the apparent powerdrawn from the mains: RMS line voltage times RMS line current. A purely resistiveload will have a PF of unity, but since peaks increase the RMS current, one drawing apeaky waveform will have a PF of 0.5–0.75. “Correcting” the PF towards unity requiresthat the input current waveform is made nearly sinusoidal, so that its harmonic contentis much reduced. This is done by a second switching “pre-regulator” operating directlyat the mains input. The usual topology is a boost regulator, as shown in Figure 7.7.The input rectifier supplies a full-wave-rectified half-sine voltage across C IN . Thiscapacitor is too low a value to affect the 50Hz input current significantly, but highenough to act as an effective reservoir at the switching frequency (typically 50–100kHz). One sense input of the switching controller comes from this input voltage,and the controller is designed to maintain an average input current through the inductorin phase with this voltage. It does this by varying the switching pulse width orfrequency as the input voltage changes. The rectified output is a dc voltage slightlyhigher than the highest peak supply voltage, which forms a reasonably well-regulatedinput to the main SMPS converter – which can of course be for any application, not justfor an electronic power supply.Naturally the addition of a second switching converter increases the cost of the totalpower supply, and contributes to more interference which must be filtered out at themains input. Neither of these disadvantages are excessive, and commercial PFC powersupply modules are now widely available. If you are designing your own, several ICmanufacturers offer controllers specifically for the purpose, such as the L4981A/B,L6561, UC3853-5, and MC33626/33368. An extra advantage of the PFC pre-regulatoris that almost by definition it will work over a wide input voltage range; so that a byproductof including it is that a single power supply will cover all worldwide markets(section 7.2.1), and will also have a uniform and predictable response to dips andinterruptions (section 7.3.2).
234 The Circuit Designer’s CompanioninputI avge boost inductorC IN I LcontrollerV inC OUTloadcurrent sensevoltage envelopeboost inductor current I Laverage current I avgeFigure 7.7 Power factor correction7.2.6 FrequencyThe UK and European mains frequency is held to 50Hz ±1%. The American supplystandard is 60Hz. The difference in frequencies does not generally cause any problemfor equipment that has to operate off either supply (provided that it’s designed inEurope!), since mains transformers and reservoir circuits that perform correctly at 50Hzwill have no difficulty at 60Hz. The sensitivity of the power supply circuits to supplyvoltage droops at 60Hz should be less than at 50Hz since the ripple amplitude is only83% of the 50Hz figure, and the minimum voltage will thus be higher (Figure 7.8).50Hz60Hzsame dV/dt, same maximum voltagelower rippleamplitude,hence higherminimum voltageFigure 7.8 Ripple voltage versus frequencyThe ±1% tolerance on the mains frequency is slightly misleading because thesupply authorities maintain a long-term tolerance very much better than this. Diurnalvariations are arranged to cancel out, and this allows the mains to be used as a timingsource for clocks and other purposes. If you are planning to use the mains frequency forinternal timing then you will need to incorporate some kind of switching arrangementif the equipment will be used on both US and European systems.
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The Circuit Designer’s Companion
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NewnesAn imprint of ElsevierLinacre
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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xii Contents
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2 The Circuit Designer’s Companio
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10 The Circuit Designer’s Compani
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Grounding and wiring 25Cable type R
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Grounding and wiring 27Shielding an
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Grounding and wiring 29successive h
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Grounding and wiring 31(a) Two audi
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Grounding and wiring 331. Side-by-s
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Grounding and wiring 35ABV pZ out =
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Grounding and wiring 37times will d
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Grounding and wiring 39of Z o . Thu
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Printed circuits 41material. The co
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Printed circuits 43board cost you s
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Printed circuits 45Board ABoard ABo
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Printed circuits 47which they can w
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Table 3.3 Survey of capacitor types
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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