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

Power supplies 225Chapter 7Power suppliesThe power supply is a vital but often neglected part of any electronic product. It is theinterface between the noisy, variable and ill-defined power source from the outsideworld and the hopefully clear-cut requirements of the internal circuitry. For thepurposes of this discussion it is assumed that power is taken from the conventional acmains supply. Other supply options are possible, for instance a low-voltage dc bus, orthe standard aircraft supply of 400Hz 48V. Batteries we shall discuss separately at theend of this chapter.7.1 GeneralA conceptual block diagram for the two common types of power supply − linear andswitch-mode − is given in Figure 7.1.7.1.1 The linear supplyThe component blocks of a linear supply are common to all variants, and can bedescribed as follows:• input circuit: conditions the input power and protects the unit, typicallyvoltage selector, fuse, on-off switching, filter and transient suppressor• transformer: isolates the output circuitry from the ac input, and steps down(or up) the voltage to the required operating level• rectifier and reservoir: converts the ac transformer voltage to dc, reduces theac ripple component of the dc and determines the output hold-up time whenthe input is interrupted• regulation: stabilises the output voltage against input and load fluctuations• supervision: protects against over-voltage and over-current on the outputand signals the state of the power supply to other circuitry; often omitted onsimpler circuits.7.1.2 The switch-mode supplyThe advantage of the direct-off-line switch-mode supply is that it eliminates the 50Hzmains transformer and replaces it with one operating at a much higher frequency,typically 30−300kHz. This greatly reduces its weight and volume. The componentblocks are somewhat different from a linear supply. The input circuit performs a similarfunction but requires more stringent filtering. This is followed immediately by arectifier and reservoir which must work at the full line voltage, and feeds the switchelement which chops the high-voltage dc at the chosen switching frequency.The transformer performs the same function as in a linear supply but now operates