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
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