Page 270 - Power Electronics Handbook
P. 270

260   Forced commutation techniques
                       the  supply  voltage  VB. The  clamping action  of  diode  D2 prevents  C1
                       charging to a higher voltage. To turn TH1 off thyristor THz is fired, causing
                       capacitor C1 to discharge through Lz and coupling a voltage pulse via L1 to
                       THI, so  turning  it  off. The voltage on C1 falls to zero,  after which D1
                       conducts,  carrying the current due to energy stored in   and the load.
                       Thyristor  THz turns off when the current in b has fallen to below the
                       device-holding value.  C, is  not  normally required,  but  by  including it
                       thyristor  THl  can  be  fired  before  THz has  turned  off, since  then  C,
                       discharges through L1 coupling a pulse to THz via Lz and so turning it off.
                       This allows a wider voltage control range.
                         Assuming a symmetrical system, with C1 = C, = C and L1 =   = L the
                       values of L and C can be found from equations (1 1.11) and (1 1.12). These
                       equations  illustrate  that  this  commutation  method  is  not  suitable  for
                       low-voltage high-current applications, since it would require a large value
                       of  commutation capacitor and an impractical small value of  inductance.

                                                                                 (11.11)


                                                                                 (1 1.12)


                       The following can be noted about the circuit in Figure 11.13.
                       (i)  The system can  be  operated  in  a variable-frequency or  a variable
                            mark-space mode.
                       (ii)  The minimum on and off times are approximately equal and are given
                            by d(LIC1) and d(I&)     respectively, for low load current.
                       (iii)  Due to the clamping action of D1 and D2, the commutation voltage is
                            fixed at VB, irrespective of  the load current.
                       (iv)  An  unsuccessful  commutation  would  not  allow  the  circuits  to
                            re-attempt commutation.
                       (v)  The charging pulse for C1 and the discharge pulse for C, flow through
                            TH1 increasing its current rating.
                       (vi)  A low-impedance failure path exists across the d.c. supply.
                         In  comparing  the  various  commutation  methods  described  in  this
                       chapter,  generally  parallel-capacitor commutation circuits are  the  most
                       flexible  and  are  suitable  for  operating  from  a  wide  range  of  supply
                       voltages. However, in applications where the supply voltage is high and the
                       load  current  small, parallel  capacitor-inductor  commutation  should be
                       considered,  since  these  are  often  simpler  than  the  corresponding
                       parallel-capacitor circuits. Coupled-pulse commutation is more expensive,
                       due to the use of  a transformer, and it finds more frequent use in inverter
                       circuits.  Similarly, series  capacitor  commutation  is  not  often  used  for
                       choppers, due to its limited output voltage range, but is often employed in
                       sine wave inverters.
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