Page 353 - Power Electronics Handbook

Design of inverter circuits 343

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0.1 - I

Flgure 13.57 Variation of peak thyristor turn-off current and watt loss factors with peak load
current

From Figure 13.57 it is seen that watts loss is a minimum at a
commutation factor of 0.8, the variation between 0.6 and 1.0 being slight.
Below 0.6 the peak current and commutation loss increase steeply,
although available turn-off time also increases. Working on the minimum
loss point the values of Lo and C can be found as in equations (13.32) and
(13.33), respectively.

(13.32)

(13.33)
The contribution to device ratings by the commutation interval is
directly dependent on time r2 in Figure 13.56. This is the period required
for the current in TH1 to decay from Zpk to zero due to losses across TH,
and D1 (assumed to be a constant and equal to Av) and due to the
'effective loss' resistance (Re) of the THI-L-D1 loop. To reduce this time
to a minimum means introducing an external resistance in series with D1,
which gives greater loss during normal operation, with a subsequent
reduction in efficiency. There is no one acceptable solution for all cases.
Some inverters may be using devices which are overrated, so that higher
r.m.s. currents may be acceptable, whilst in others efficiency may not be
important so losses across a larger Re are tolerated. In any case it is always
important to ensure that rz is less than half a cycle of inverter operation, to
prevent commutation failures.
If ZD1 and ZT1 represent the commutation current, in r.m.s. terms,
through the diode and thyristor respectively, and Tis the inverter periodic
time, Figures 13.58 and 13.59 show plots of ZDl~(Z")/Zupk~) and
against
&d(7')/lqpk~) supply voltage. These are calculated €or values of
and C given by equations (13.32) and (13.33) and for A V = 2.5 V and
Re = O.~VB/Z~(~~). figures show the advantage of using fast turn-off
These