Page 227 - Power Electronics Handbook

Voltage multiplication circuits 217

Table 9.2 Marmuice Mora

circuit PUlSeS -
V.V
reference Per V
cycle
Figure 9.1 0.45 0.636 3.142 1.414 1.0 1.571 3.142.
Figure 9.2 0.90 0.90 3.142 2.82 0.5 0.707 1.0
Figure 9.4 0.90 0.92 1.571 1.414 0.5 0.707 1.0
Figure 9.5 1.17 0.98 2.09 2.45 0.33 0.577 1.0
Figure 9.7 1.35 1.0 2.09 2.83 0.167 0.408 1 .o
Figure 9.8 1.17 1.0 2.42 2.83 0.167 0.289 0.5
Figure 9.11 2.34 1 .o 1.05 2.45 0.33 0.577 1.0

output voltage; vpk is the peak or crest working voltage of the devices; I,,
is the average current per device leg; Id is the d.c. load current; I,,,,, is the
r.m.s. current per leg; and zpk is the peak current per device leg.

9.7 Control circuits

The control or gate-firing circuits used for phase-controlled rectifiers are
essentially similar to those used for a.c. phase control, as shown in Figure
8.11, the functions being available within an integrated circuit. The input
detector determines the incidence of zero supply voltage, the delay section
provides a variable delay from this point, and this signal is then used to
energise the gate-drive circuits which fire the thyristors. There are two
further considerations. With source reactance the firing pulses must be of
sufficient width to overcome any overlap angle. This was illustrated in
Figure 9.27, where it was shown that a pulse width of at least pl is required
to ensure that the thyristor reaches its full conduction. This problem of
extended firng pulses is not as serious as for a.c. line control, where for
inductive loads a pulse duration of 90" is normally required. However,
controlled rectifier circuits have an added feature that the input waveform
could be distorted if line reactances are severe. Generally, the firing circuits
should be able to start the timing pulse when this voltage rises from the zero
voltage point.

9.8 Voltage multiplication circuits

The discussions so far have concentrated on power rectification, where the
output voltage is controlled to be at a value below that of the input. Io this
section a special application of power rectification is considered, in which
the level of current handled is relatively low, but the output voltage is
several orders of magnitude greater than that of the input. Voltage
multiplication can vary from a factor of two to several hundred, and there
are a variety of circuits which can be used, only a few of these being
introduced in this section.
Voltage doublers are first described, since they are the most well known,
and these can be categorised as symmetrical, diode pump or bridge. Figure