Page 230 - Power Electronics Handbook

220 Phase-controlled rectification and inversion
The diodes and capacitor C, are again rated at twice the peak of the a.c.
supply, whilst capacitor C1 is rated at the same voltage as the supply, but it
must be capable of carrying the r.m.s. load current.
The bridge voltage doubler, shown in Figure 9.42(c), consists of a full
wave bridge and a symmetrical voltage-doubler arrangement, so it is
capable of good stability and regulation. In this circuit diodes D2 and D3
supply the load current in alternate half cycles at the same time that the
capacitors are being discharged, so that the output voltage is closer to twice
the peak of the input a.c., even on heavy load currents. The ripple
frequency is again twice that of the supply, and the diodes each have to be
rated at twice that of the peak of the ax. supply, even though four are
used, since the capacitors effectively bypass the a.c. signals.
Voltage-multiplication factors greater than two can again be obtained by
a variety of circuits, a few being shown in Figure 9.43. The voltage-tripler
circuit of Figure 9.43(a) can be considered to be a combination of the diode
pump voltage doubler and a half-wave rectifier circuit. Twice the supply
voltage appears across capacitor C, due to the action of C1, D2 and D3,
whilst the half-wave rectifier D1 charges capacitor C3 to the peak of the a.c.
supply, so that the voltage across CD is three times that of the supply. The
voltage-quadrupler circuit of Figure 9.43(b) is made up of a combination of
two diode pump circuits, each giving twice the supply voltage across
capacitors C3 and C,, so the voltage across CD on no load is equal to four
times that of the peak of the a.c. supply.
A more universal voltage multiplier is shown in Figure 9.43(c), this being
referred to as the Cockcroft-Walton circuit. Each capacitor is charged to
the voltage of the peak of the a.c supply plus that of the capacitor below it,
the peak of the output voltage accumulating as one proceeds up the ladder.