Introduction to Electrical Power and Power Electronics

319Power Electronics Devices

Triac. It has ve semiconducting layers (PNPNP) and behaves like two PNPN

thyristors placed back to back. Its circuit symbol and the v–i characteris-

tic are shown in Figure 12.11. Because a single triac can conduct in both

directions, it can replace a pair of two thyristors in many ac voltage control

circuits and is widely used to step down the ac voltage by delaying the ring

Anode

I

d

Cathode

Gate

+

V

d

–

V

d

I

d

V

R

V

BO

at I

G

=

0

I

H

V

BO1

at I

G1

V

BO2

at I

G2

V

BO3

at I

G3

V

BO

at I

G

= 0

Gate currents

I

G3

> I

G2

> I

G1

> 0

FIGURE 12.10 Thyristor (SCR) voltage drop versus current characteristic.

I

d

Anode

Cathode

Gate

+

V

d

–

V

d

I

d

V

R

V

BO

at I

G

=

0

I

H

V

BO1 ...