PRINCIPLES OF OPERATION
(Block Diagram Overview)
Throughout this discussion, refer to both the block diagram of Figure A-1 and the schematic diagrams at the rear of the manual. The input ac line voltage is first applied to the prereg- ulator which operates in conjunction with the SCR control cir- cuit (preregulator control circuit) to rectify the tap switched AC voltage. This preregulator minimizes the power dissipated in the series regulating elements by controlling the dc level across the input filter capacitor, depending on the output volt- age.
To achieve this, tap switching is accomplished by four SCRs and one bridge diode (CR10, CR12, CR15, CR18 and CR13) and the SCR control circuit. By selecting different SCR firing combinations from SCR control circuit, these circuits allow the input capacitors (C7 and C8) to charge to one of four discrete voltage levels, depending on the output voltage required.
The main secondary winding of the power transformer has three sections (N1, N2, and N3), each of which has a different turns ratio with respect to the primary winding. At the begin- ning of each half-cycle of the input ac, the control circuit determines whether one pair, both or none of the SCR will be fired. If neither SCR is fired, the bridge diode (CR13) receives an ac input voltage that is determined by N1 turns and the input capacitors charge to a corresponding level. If SCR CR15 and CR18 are fired, input capacitors charge to the volt- age determined by N1+N2 turns. Similarly, if CR10 and CR12 are fired the capacitors are charged by N1 + N3. Finally, if all SCRs are fired simultaneously, input capacitors charge to its highest voltage level determined by N1 + N2 + N3 turns.
The SCR control circuit determines which SCRs are to be fired by monitoring the output voltage and comparing these values against a set of three internally derived reference lev- els. These three reference levels are translated into boundary lines to allow the output characteristic to be mapped into four operating regions (Figure A-2). The boundary lines, which are invisible to the user, are divided into four operating regions (V1, V2, V3, and V4) to minimize the power dissipation in the
