POWER SUPPLY SYSTEMS
In strict terminology, the power supply for operating such things as TVs, videorecorders and cameras is a primary energy source like domestic mains electricity, a battery or a generating set. In our context, the power supply unit (PSU) is not a generator but a controller of energy; the less energy it dissipates within itself (manifest as heating of components) the better. All the above-mentioned energy sources are subject to fluctuation: mains supplies can vary ±6% of nominal voltage due to load and distribution factors; batteries have a falling output voltage throughout their discharge cycle; and the volt- age supplied by a generator depends on engine speed and the load imposed, both mechanical and electrical. Natural sources of energy such as windpower and sunlight (both are used for powering electronic and transmitting equipment, sunlight being the sole source of energy in geostationary TV satellites) are even more erratic, calling not only for stabilisation, but even for a back-up supply if they fail altogether.
Nor are supply fluctuations the only factor. Unless the source impedance of the electrical energy supply is zero – impossible to achieve in practice – variations in current demand (load) in the equipment have an effect on the line voltage. As electronic devices become more efficient their current consumption follows more closely the demand made upon them – a TV set’s requirement may vary over 2:1 with beam current changes; the current required by a portable videore- corder depends on the function (i.e. standby, rewind, pause) in use, and different sections of a TV camera (zoom, viewfinder, or built-in cassette recorder) will come in and out of use as occasion demands. For any equipment, then, the stabilised power supply section has several functions; some or all of the following may be required:
1. Maintenance of constant output voltage against supply fluctua- tions
2. Maintenance of constant output voltage against varying load
3. Transformation of supply voltage to required load-operating voltage; usually down, sometimes up
4. Rectification of a.c. supplies to render d.c. operating line
5. Facility for adjustment of stabilised output voltage to set cor- rect conditions and take up tolerances in load
6. Provision of safety cut-outs to prevent damage and overheating in the event of an internal fault or malfunction in the load
7. In the case of mains-powered PSUs an even, low and equal demand on both half-cycles of the domestic a.c. supply
8. A minimum absorption of power in the stabilising circuits themselves, crucial where primary energy is at a premium, and a significant contribution to reliability by keeping operating temperatures low
9. Low ripple voltage on stabilised lines
10. Low internal impedance for good decoupling between different sections of the equipment using the same supply line