Page 59 - Power Electronics Handbook

52 Power semiconductor devices

Pl
"1 "1
\ \ \ \ \ \ \ \ i

la)
Yb
Current 4
II
gate current

I c
Voltage

G
IV Ll
(C) (d)
Figure 1.29 Triac: (a) five-layer representation; (b) construction; (c) characteristic;
(d) symbol

brought out so that a combination of light and gate current can be used to
control the device. The advantage of using light triggering is that the device
is now insensitive to electrical signals, which can cause faulty operation in
electrically noisy environments.
Figure 1.30(b) shows the construction of a photothyristor. In the absence
of light, junction Jz is reverse biased. When illuminated, hole-electron
pairs are created in the vicinity of 52 and are swept across to the anode and
cathode. This acts as a triggering current, and if it is large enough it will
turn on the thyristor. Usually a bias can be applied on the gate lead to vary
the threshold light level for turn-on.
The photothyristor is made from thin layers to enable greater light
penetration, but this also results in a lower blocking voltage. It is designed
with a larger junction to increase light sensitivity, but this has the effect of
making it more sensitive to variations in temperature and voltage, and in
increasing the turn-off time. Resistors can be added between the gate and
cathode to reduce its susceptibility to noise and dv/dt effects, but this
degrades its sensitivity to light triggering. Although a photothyristor has been
described here, other devices, wich are turned on by light, are also available,
such as photo triacs and photo Darlingtons (Rosen and Zutavern, 1994).