Page 64 - Power Electronics Handbook

Series and parallel operation 57
1.14.2 PIvllkl operation

Several power semiconductor devices, connected in parallel, all have the
same voltage across them when they are conducting, but due to unequal
characteristics they carry different currents. This is illustrated for a
thyristor pair in Figure 1.34. Once again it is possible to swamp the
inequality of the device voltage drops by seriesconnected impedances,
such as inductors or resistors, or by current-sharing reactors. Inductances
are effective only during transient conditions, when the current is
changing. Current-sharing reactors, where the imbalance in different parts
of the reactor forces equal current flow in the various parallel arms, are
expensive and bulky. Resistors are cheaper, but dissipate power.

V vdtr
(a) (b)
1.34 Unequal current distribution in paralleled thyristors: (a) paralleled thyristors;
(b) spread in forward characteristics

Figure 1.35 shows two thyristors connected in parallel with sharing
resistors. If VI and V, are the maximum and minimum voltage drops across
the thyristors and Zl and Zz the permitted variations in the shared current,
then equation (1.28), which gives the desired value of the sharing resistor R,
can be derived from equations (1.26) and (1.27).

(1.26)
(1.27)

R= v2 - v1 (1.28)
11 - 12
The series resistors force d.c. more equally through paralleled power
devices, but fail to compensate for unequal turn-on times, latching currents
or holding currents. For instance, in a paralkl-conncctcd set of thyristors,
if one device turns on before the rest, it will be forad to momentarily carry
the full load current. This need not be serious so long as the di/& or surge

IVFi

12
LV2A Flgwe 1.35 Series resistance for forced current sharing