Page 317 - Power Electronics Handbook
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Inverter circuits   307

                   13.2.4.2 Commutation loss reduction
                   Commutation losses can be reduced, as was done for choppers, by several
                   techniques.  For example, Figure 13.1 l(b) illustrated a popular push-pull
                   parallel  commutated  inverter  configuration,  which  is  not  suitable  for
                   high-frequency operation  due to its large commutation loss.  When TH1
                   fires, capacitor C discharges through inductor L1 and recharges to twice the
                   supply voltage. When this has been completed diode D1 conducts, feeding
                   back the inductive load current to the supply, and the energy stored in L1
                   during  the  commutation  interval  is  dissipated  in  the  loop  L1-D1-TH1.
                                   Load









                                                   D2
                                                      Figure 13.27 ‘Lossless’ push-pull  inverter

                   Figure  13.27 shows a modified push-pull  circuit which has a much lower
                   commutation loss, four thyristors being used to avoid the possibility of short-
                   circuit  of  the  commutation capacitor  during  the  turn-off  process  and  to
                   eliminate the  need  for  a  series  inductor.  Suppose that TH, and  TH3 are
                   conducting and TH,  has been fired to prime the commutation capacitor, after
                   which this thyristor turns off. To  reverse the load current thyristor TH,  is
                   fired, the capacitor discharging and recharging, with reverse polarity, through
                   TH3, this  thyristor  also  going  off  when  the  capacitor  has  completed its
                   charge. There is negligible commutation loss in the system since the stored
                   energy in a commutation inductor has been avoided.
                     Bridge inverter coupled-pulse circuits of  the McMurray-Bedford  typ,
                   as shown in  Figure  13.20, also suffer from relatively high  commutation
                   losses. For instance, if  TH1 is conducting then capacitor C1 is discharged
                   and C, is charged to VB. To turn TH1 off thyristor TH2 is fired, which
                   couples  a  pulse  to  the  top  device,  turning  it  off. Capacitor  C, then
                   discharges to zero voltage, after which time the energy stored in inductor
                   L1 is dissipated in the free-wheeling path TH2-D2-L1.
                     Several  modifications can  be  made  to  reduce  the  commutation  loss
                   caused by the circulating current in Figure 13.20, for instance by  tapping
                   the load. Figure 13.28 shows the addition of an auxiliary transformer with a
                   feedback  winding,  which  gives  a  better  performance.  During  the
                   commutation interval, for example when THz is fired as before, the energy
                   stored in L1 will induce a voltage in winding 1 of  the transformer, which
                   will feed back the energy from inductor L1 to the supply via the step-up
                   winding 4 and bridge rectifier D7. There is an overshoot voltage on   due
                   to this feedback action, as seen with chopper circuits, but it is small. For
                   example, for a 1:20 turns ratio between windings 1 and 4 it will be 1/20th of
                   the supply voltage.
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