thyristor

2. A thyristor is a four-layer
semiconductor device, consisting of
alternating P type and N type materials
(PNPN). A thyristor usually has three
electrodes: an anode, a cathode, and a
gate (control electrode).
Thyristors are used in motor speed
controls, light dimmers, pressure-control
systems, and liquid-level regulators

3.  Thyristor is designed to handle large current and
voltage, it is used mainly in electrical power circuit
with system voltage more than 1 kV or currents
more than 100 A. The main advantage of using
thyristors as power control device is that as the
power is controlled by periodic on – off switching
operation hence (ideally) there is no internal power
loss in the device for controlling power in output
circuit. Thyristors are commonly used in some
alternating power circuits to control alternating
output power of the circuit to optimize internal
power loss at the expense of switching speed.

4.  In this case thyristors are turned from
forward-blocking into forward-conducting
state at some predetermined phase angle of
the input sinusoidal anode–cathode voltage
waveform. Thyristors are also very popularly
used in inverter for converting direct power to
alternating power of specified frequency.
These are also used in converter to convert
an alternating power into alternating power of
different amplitude and frequency.This is the
most common application of thyristor.

5. The operationof the thyristor can be
understood from Fig. 2.

6.  When the thyristor cathode is more positive
than the anode then junctions J1 and J3 are
reverse biased and the device blocks. When
the anode is more positive than the cathode,
junctions J1 and J3 are forward biased. As J2
is reverse biased, then the device still blocks
forward voltage. If the reverse voltage across
J2 is made to reach its avalanche breakdown
level then the device conducts like a single
forward-biased junction


7. The ’two transistor’ model of Fig. 4

8.  The ’two transistor’ model of Fig. 4 can be used to
consider the p-n-p-n structure of a thyristor as the
interconnection of an npn transistor T1 and a pnp
transistor T2. The collector of T1 provides the base
current for T2. Base current for T1 is provided by
the external gate current in addition to the collector
current from T2. If the gain in the base-collector
loop of T1 and T2 exceeds unity then the loop
current can be maintained regeneratively. When
this condition occurs then both T1 and T2 are
driven into saturation and the thyristoris said tobe
’latched’. Theanode tocathode current is then only
limited by the external circuit