From The Circuit Designer's Companion, Second Edition
The power supply is a vital but often neglected part of any electronic product. It is the interface between the noisy, variable and ill-defined power source from the outside world and the hopefully clear-cut requirements of the internal circuitry. For the purposes of this discussion it is assumed that power is taken from the conventional ac mains supply. Other supply options are possible, for instance a low-voltage dc bus, or the standard aircraft supply of 400Hz 48V. Batteries we shall discuss separately at the end of this chapter.
A conceptual block diagram for the two common types of power supply linear and switch-mode is given in Figure 7.1.
Figure 7.1: Power supply block diagrams
7.1.1 The linear supply
The component blocks of a linear supply are common to all variants, and can be described as follows:
input circuit: conditions the input power and protects the unit, typically voltage 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 the ac ripple component of the dc and determines the output hold-up time when the input is interrupted
regulation: stabilises the output voltage against input and load fluctuations
supervision: protects against over-voltage and over-current on the output and signals the state of the power supply to other circuitry; often...
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