SWITCH-MODE CONCEPT
The types of regulator so far described have the advantage of simplicity but are wasteful of energy, which is dissipated as heat in the regulating device itself and in any associated ballast resistor. A more efficient method of regulating power uses a control element which itself absorbs virtually no power – a switch. The principle of switching energy into an integrator was described in Chapter 10, where a field-scan sawtooth waveform at 50 Hz was built up by feeding 15 kHz pulses into an LC store. Control was effected by varying the ‘on’ period of the switch, i.e. the duty-cycle or mark-space ratio. Ina switch-mode power supply unit (SMPSU) the mark-space ratio of a chopper switch (which may be one or more thyristors, but is more commonly an npn transistor) is varied to suit the requirements of the load. When little energy is required the switch dwells only a short time in the on position, briefly dumping a charge into an inductive or capacitive reservoir to be drawn on throughout the ‘off’ period of the chopper switch. When the energy requirement in the load is large a monitor circuit automatically opens up the mark-space ratio to increase the charge-dumping period. In this way the energy drawn from the primary power source exactly matches the energy required by the load. There are many ways in which chopper power supplies can be arranged.