Page 51 - Power Electronics Handbook

44 Power semiconductor devices
A thyristor can be forced into conduction if a rapidly rising voltage is
applied across it, even though the magnitude of the voltage is less than its
peak value. This is called the dv/dt rating of the thyristor, and it causes the
thyristor to turn on due to current flow through the capacitance associated
with the depletion layer (C,) of the reverse-biased junction, given by
equation (1.16).
dv
i= Cdx (1.16)

Techniques which improve this rating are also illustrated in the next
section.

Hold

i

(a)
Ib) Mean current ow (A)
7000 - I I I I

6000 - -

From: No load followed by -
rated load or rated load

01 I I I I I
1 10 io* 103 104 105
(C) Number of cycles at 50 Hz
Figure 1s Thyristor Characteristics: (a) static characteristics; (b) current curves; (c) surge
rating;