Page 60 - Rashid, Power Electronics Handbook

3 Thyristors 45

A

i
n 3
J 4 p 1

J 1
n 1
J 2 v
p 2
J 3
n 2

K
FIGURE 3.23 Cross section and iv plot of a Triac.

A low-power device similar to the BCT, but in existence for blocking. In addition, L protects the thyristor from reverse
1
many years, is the Triac. A simpli®ed cross section of a Triac is over-currents.
shown in Fig. 3.23. A positive voltage applied to the anode
with respect to the cathode forward-biases J , while reverse-
1
biasing J ; J , and J are shorted by the metal contacts. When J
2 4 3 2 3.7.2 Gate Circuits
is biased to breakdown, a lateral current ¯ows in the p -region.
2
This lateral ¯ow forward-biases the edge of J , causing carrier It is possible to turn on a thyristor by injecting a current pulse
3
injection. The result is that the device switches into its into its gate. This process is known as gating the thyristor. The
thyristor mode and latches. Applying a reverse voltage causes most important restrictions are on the maximum peak and
the opposite behavior at each junction, but with the same duration of the gate pulse current. In order to allow a safe
result. Figure 3.23 also shows the iv plot for a Triac. The turn-on commutation, the current pulse should be high
addition of a gate connection allows the breakover to be
controlled at a lower forward voltage.

R 1
L 1
3.7 Gate Drive Requirements
D 1
3.7.1 Snubber Circuits
To protect a thyristor, from a large di=dt during turn-on and a
large dv=dt during turn-off, a snubber circuit is needed. A
general snubber topology is shown in Figure 3.24. The turn-on
snubber is made by inductance L (often L is stray inductance C 2
1
1
only). This protects the thyristor from a large di=dt during the
turn-on process. The auxiliary circuit made by R and D 1 T
1
allows the discharging of L when the thyristor is turned off.
1
R 2
The turn-off snubber is made by resistor R and capacitance D 2
2
C . This circuit protects a GTO from large dv=dt during the
2
turn-off process. The auxiliary circuit made by D and R 2
2
allows the discharging of C when the thyristor is turned
2
on. The circuit of capacitance C and inductance L also FIGURE 3.24 Turn-on (top elements) and turn-off (bottom elements)
2
1
limits the value of dv=dt across the thyristor during forward snubber circuits for thyristors.