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Power supplies 2517.4.2 HeatsinkingA necessary requirement for the continued health of any semiconductor device, be itmonolithic IC regulator, rectifier diode or power transistor, is that its junctiontemperature should stay within safe limits. Junction temperature is directly related topower dissipation, thermal resistance and ambient temperature, and the function of aheatsink is to provide the lowest possible thermal resistance between the junction andits environment – assuming the environment is always cooler.Chapter 9 includes a discussion of thermal management, including methods ofcalculating the size requirements of heatsinks. Here it is enough to say that the powersupply often represents the most concentrated source of heat in an item of equipment.As soon as its efficiency is roughly known, you should calculate the heat output andtake steps to ensure that the mechanical arrangement will allow an efficient heat flow.At the minimum this will involve ensuring that all components which will needheatsinking are positioned to allow this, and that the power unit’s positioning within theoverall equipment gives adequate thermal conductivity to the environment. Too manydesigns end up with a fan tacked onto the case as an afterthought!7.4.3 Safety approvalsMajor safety risks for power supplies are the threat of electric shock due to contact with“live parts”, and the threat of overheating and fire due to a fault. Safety is discussed ingreater depth in section 9.1. One of the important but forgotten functions of a powersupply is to ensure a safe segregation of the low-voltage circuitry, which may beaccessible to the user, from the high-voltage input, which must be inaccessible.Segregation is normally assured in a power supply by maintaining a minimum distancearound all parts that are connected to the mains, including spacing between the primaryand secondary of the transformer. This, of course, adds extra space to the designrequirements. Insulation of at least a minimum thickness may be substituted for emptyspace.There are many national and international authorities concerned with setting safetyrequirements. Foremost among these are UL in the United States, CSA in Canada, andthe CENELEC safety standards, implementing the Low Voltage Directive in Europe.As designer, you can either choose to apply a particular set of requirements for yourcompany’s market, or if you plan to export worldwide, you can discover the moststringent requirements and apply these across the board. A common specification isEN 60950-1 (IEC 60950-1), which is the safety standard for information technologyequipment and which is quoted by default by most off-the-shelf power supplies. If nosafety specification is quoted, beware.Most of the time it is legally necessary to have your product approved to safetyregulations, often it is also commercially desirable. Using a bought-in supply whichalready has the right safety approval goes a long way to helping your own equipmentachieve it. Note that there is a difference, on data sheets, between the words “designedto meet...” and “certified to...” The former means that, when you go for your own safetyapproval, the approvals agency will still want to satisfy themselves, at your expense,that the power supply does indeed meet their requirements. The latter means that thispart of the approvals procedure can be bypassed. It therefore puts the unit cost of thepower supply up, but saves you some part of your own approval expenses.
252 The Circuit Designer’s Companion7.5 BatteriesBattery power is mainly used for portability or stand-by (float) purposes. All batteriesoperate on one or another variant of the principle of electro-chemical reaction, in whichanode (negative) and cathode (positive) terminals are separated by an electrolyte, whichis the vehicle for the reaction. This basic arrangement forms a “cell”, and a batteryconsists of one or more cells. The chemistry of the materials involved is such that apotential is developed between the electrodes which is capable of sustaining a dischargecurrent. The voltage output of a particular cell type is a complex function of time,temperature, discharge history and state of charge.The basic distinction is between primary (non-rechargeable) and secondary(rechargeable) cells. This section will survey the various types of each shortly, but firstwe shall make a few general observations on designing with batteries.7.5.1 Initial considerationsWhen you know you are going to use a battery, select the cell type as early as possiblein the circuit and mechanical design. This allows you to take the battery’s propertiesinto account and increases the likelihood of a cost-effective result, as otherwise you willprobably need a larger, or more expensive, battery or will suffer a reduced equipmentspecification. Having made the selection, you can then design the circuit so that it worksover the widest possible part of the battery’s available voltage range. Some of thecheaper types deliver useful power over quite a wide range, with an endpoint voltageof 60–70% of nominal, and some of this energy will be lost if the design cannot copewith it. Also, check that the battery can deliver the circuit’s load current requirementsover the working temperature. This capability varies considerably for differentchemical systems. Rechargeable batteries can often be recharged only over a narrowertemperature range than they can be discharged.Always aim to use standard types if your specification calls for the user to be ableto replace the battery. Not only are they cheaper and better documented, but they arewidely distributed and are likely to remain so for many years. You should only need touse special batteries if your environmental conditions or energy density requirementsare extreme, in which case you have to make special provisions for replacement or elseconsider the equipment as a throwaway item.Voltage and capacity ratingsDifferent types of battery have different nominal open-circuit voltages, and the actualterminal voltage falls as the stored energy is used. Manufacturers provide a dischargecharacteristic curve for each type which indicates the behaviour of voltage against timefor given discharge conditions. Note that the open-circuit voltage can exceed thevoltage under load by up to 15%, and the operating voltage may be significantly lessthan the nominal battery voltage for some of the duration.The capacity of a battery is expressed in ampere-hours (Ah) or milliampere-hours(mAh). It may also be expressed in normalised form as the “C” figure, which is thenominal capacity at a given discharge rate. This is more frequently applied torechargeable types. Capacity will be less than the C rating if the battery is dischargedat a faster rate; for instance, a 15Ah lead-acid type discharged at 15 amps (1C) will onlylast for about 20 minutes (Figure 7.23).Three typical modes under which a battery can be discharged are constantresistance, constant current and constant power. For batteries with a sloping discharge
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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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2 The Circuit Designer’s Companio
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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 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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