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

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.