Page 277 - Power Electronics Handbook

D.C. to d.c. converter circuits 267
current path D3, the supply, D2 and R1. The value of this resistor R1 is
chosen such that the resonant circuit is overdamped.
Thyristor TH2 turns off once C1 has charged to its peak voltage and
commences its resonant discharge back into the supply. Thyristor TH1 is
fired to turn off the main thyristor TH3 and this places the commutation
Ll-C1 circuit across the main thyristor. The capacitor provides the load
current and after the peak value of current is reached it resonates through
TH1 and D3 and recharges with plate a positive, after which TH1 goes off,
the load current free-wheeling through D4. If the capacitor resonance
caused it to be charged to greater than the supply voltage with plate a
positive, then this voltage will discharge back to the value of the supply
voltage through the path R1, D1, the supply and D4. After this, TH3 can be
refired to commence the load cycle, followed by TH2 to prime C1 for the
next commutation period.
During regeneration the load voltage exceeds that of the supply and
current flows from the load to the supply via diode D3. When TI€, is fired
regeneration stops and the load current free-wheels through this thyristor.
Thyristor TH1 is fired at the same time as TH4, or soon afterwards, and it
causes capacitor C1 to charge to the supply voltage with plate a positive,
any excess voltage being dissipated in the path R1, D1, the supply, and D4,
as before. To commutate TH, thyristor TH2 is turned on and regeneration
is again commenced.

12.2.3 Circuit enhancements
Thyristor commutation circuits were discussed and classified in Chapter 11.
Although these classification systems represent rather rigid boundaries
between different groups of circuits, it is sometimes required to modify the
circuit within a group to obtain an enhancement of one of its parameters,
and such modifications are discussed in this section. It is important to
appreciate that none of these alterations is sufficient in itself to change the
classification group of a circuit.
There are three enhancements which will be described in this section:
(i) Higher-frequency operation. The maximum and minimum output
voltage from a chopper, operating in a mark-space control mode, is
limited by the set and reset times of the commutation capacitor. This
limits the maximum operating frequency which can be used and still
give a useful output voltage range.
(ii) Reduction of commutation losses, which again limits the operating
frequency. A chopper or inverter may be designed to give small
minimum on and off times and adequate output voltage control range
at high frequencies, but excessive commutation losses reduce the
efficiency obtained and may make it unsuitable for use at these
frequencies.
(iii) Commutation voltage boosting. The higher the commutation
capacitor voltage, the smaller its size, for the same commutation
energy output, which may lead to cheaper components. If the voltage
boost is proportional to load current, optimisation of components at
all outputs can be obtained.