12-05-2011, 03:41 PM
Generally speaking, thyristor power supplies, conventional switch-mode PSUs and the line O/P stage in a T.V. have a lot in common. They all work on the principle of energy being stored in one reactive component being switched into another reactive component with the thyristor or transistor acting as the switch. It's the on/off ratio of that switch that determines the ratio of Vin to Vout. The diode that always features in such circuits simply completes the current path when the switch goes off. Consequently, with all that switching going on, to avoid the use of large reactive components and to reduce switching losses, the switch frequency is made high - way above 50 Hz (in the vast majority of cases). Since the switch - by definition - must be either on or off, the losses in it (compared to a linear regulating device, which, in effect, acts as a variable power resistor) are a lot less. The price to be paid for this dramatically improved efficiency is, of course, greater circuit complexity compared to a linear regulator PSU. The major problem when a thyristor is used - as opposed to a transistor - is that unlike a transistor, the only way to switch a thyristor off is either to totally break the anode - cathode current or by making the anode negative with respect to the cathode: often not easily arranged. Once the thyristor is on, the gate has no further control over the anode-cathode current. This unique feature of a thyrsistor in a PSU configuration is, I suspect, one of the main reasons as to why thyristors are infrequently used in switch-mode PSUs: it's far easier to do the 'switching off' with a conventional bi-polar transistor; other comparable advantages are minimal.
Al.
Al.
