Page 91 - Power Electronics Handbook

84 Power semiconductor control components
in order not to degrade other parameters, such as voltage rating.
Photothyristor and phototriac couplers require of the order of 10-3OmA to
trigger and can provide between 100 and 300mA of output current. The
thyristor turn-on time is typically 1-lop.
Figure 3.10 shows a few examples using optical couplers. A separate
low-voltage supply is used in Figure 3.10(a), so the voltage rating of the
optical coupler output stage is relatively low. The circuit of Figure 3.1qb)
does not require a separate gate power supply, since it derives this from the

Main
Loado supp'v

Y
I

(C)
Figure 3.10 Typical optical coupler applications: (a) thyristor control using a separate power
supply; (b) triac control using the load supply; (c) series thyristor control