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.