Page 168 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH005.3D ± 156 ± [153±176/24] 17.11.2001 10:15AM

156 Power semiconductor devices and converter hardware issues

circuits must be used in general to suppress transients which may result in high
voltages and ensure that the breakdown voltage (V BR ) is not exceeded as shown in
Figure 5.1(b). For current, usually the average value, the rms value and the peak
value for a given duration are considered. Due to thermal effect on these values,
typically the diodes operate below their rated value.
If the circuit requirements for voltage and current are higher than the ratings of a
given individual diode, a number of diodes may be connected in series and/or parallel
to share the voltage and/or current respectively. Exact voltage and current sharing is
not always feasible, since it is impossible that two similar devices exhibit the exact
same characteristics. To avoid problems of unequal sharing of voltage and/or cur-
rent, which in some cases may result in thermal differences and other problems,
external resistors or other devices may be used to ensure that the voltage and/or
current are shared as required.

5.2.2 Thyristor

The SCR, simply referred to as thyristor is a semi-controlled four-layer power
semiconductor with three electrodes, namely, the anode (A), the cathode (K) and
the gate (G). The circuit symbol of the thyristor and its structure are shown in
Figures 5.3(a) and (b) respectively.
Since it is a four-layer device, it possesses three junctions. A two-transistor based
analogy can be used to explain the operating characteristics of the device as shown in
Figures 5.3(c) and (d).

Fig. 5.3 Thyristor: (a) circuit symbol; (b) structure; (c) schematic structure of the two-transistor model; and (d)
two transistor equivalent circuit.