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

228 The Circuit Designer’s Companion300Power supply cost250200Cost £150100500LinearSMPS, open framSMPS, DIN rail0 50 100 150 Power W200 250 300Figure 7.2 Price versus power rating for standard power suppliesauthorities are obliged to maintain their voltage at the point of connection to thecustomer’s premises within ±6%, to which is added an allowance for local loadingeffects. If the voltage tolerance is applied to the UK/Europe nominal then the inputvoltage range becomes 207−253V or 195−253V. This range must be handledtransparently by the power supply circuitry.To cope simultaneously with both the American supply voltage, which may dropbelow 100V, and the European voltages is difficult for a linear supply although it ispossible to design “universal” switch-mode circuits which can accept such a wide range(see the comment at the end of section 7.2.5). Historically, this problem was handledby using a mains transformer with a split primary (Figure 7.3) which can be connectedin series or parallel by means of a discreetly mounted voltage selector switch. This hasL115V 230V115VFigure 7.3 Split-primary transformer wiringN115Vthe disadvantage that the switch may be so discreet that the user doesn’t know about it,or else it may not be discreet enough and the user may be tempted to fiddle with it. Thisis not a real problem in the US, but applying 230V to a unit which is set for 115V willat least annoy the user by blowing a fuse, and at worst cause real damage. Universalswitch-mode supplies are therefore popular.7.2.2 CurrentThe maximum continuous input current should be determined by the output load and thepower conversion efficiency of the circuit. The main interest in this parameter is that itdetermines the rating of the input circuit components, especially the protective fuse. Youhave to decide whether an overload on the output will open the input circuit fuse or