Page 273 - Power Electronics Handbook
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D.C. to d.c. converter circuits 263
effects; (2) preventing rapid current changes, such as will occur due to the
reverse recovery of diode D1 unless a fast recovery diode is used; (3)
screening of circuits and leads; (4) use of input filters to prevent RFI being
conducted back into the supply.
The next section describes the various chopper circuits, primarily with
reference to a thyristor, which is the device most frequently used for
high-power applications. This is then followed by a descriptionof the
chopper voltage control methods and the design techniques for chopper
circuits. The chapter concludes with n description of the step-up d.c. to d.c.
converter and the control circuits which may be used with choppers.
12.2 D.C. to d.c. converter circuits
12.2.1 Chopper arrangements
The two major divisions of d.c. converter circuits can be considered to be
the output voltage level relative to the input, i.e. whether the chopper is
step-up or step-down, as introduced in Chapter 6. In the discussions which
follow, step-down choppers, which are the most common, are considered
and step-up choppers are covered in Section 12.5.
Three types of chopper circuits are available, depending on the type of
load being controlled, as shown in Figure 12.2. Gate turn-off switches are
shown as the power-control component and if thyristors are used then
commutation components will be required in a practical system. The basic
circuit of Figure 12.2(a) has the same operational mode as that of Figure
12.1 and the output is identical, load filtering being ignored. The current in
this system can only flow in one direction through the load, from a to b,
and the voltage is such that terminal a is always positive.
The regenerative chopper of Figure 12.2(b) is used with loads which
have regenerative capability, for example motor loads which require
electrical braking. These loads are capable of generating a voltage in excess
of the battery voltage, under certain conditions, and this phenomenon can
be used to feed energy back from the load to the supply, instead of
dissipating the excess energy in resistors. During normal chopper
operation gate turn-off switch TH1 regulates the load power and
free-wheeling diode D2 cames the inductive current of the load when no
power is being supplied from the d.c. source. The current in this mode
flows from a to b through the load and terminal a is positive, as for the
basic chopper of Figure 12.2(a). During the regenerative period the load
current reverses and flows out of terminal a, through diode D1 and into the
battery. The amount of regeneration can be controlled by operating gate
turn-off switch TH2, in either a variable-frequency or mark-space mode,
since when this device is on the load current will circulate through it rather
than being fed back to the supply. By operating TH2 it is also possible to
store energy in the load inductance, which is subsequently fed to the supply
when TH2 is off, so that this system works like a step-up chopper. It should
be noted that for regeneration in a chopper the polarity of the current has
changed, whereas for regeneration in a.c. to d.c. converters, described in
Chapter 9, the polarity of the voltage changes.
The reversing and regenerative chopper of Figure 12.2(c) can be
considered to be composed of two regenerative choppers, one set