19-05-2014, 03:34 PM
Trigger Circuits for Thyristors
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Normally, thyristors are switched on by the application of a
voltage signal at the gate terminal of the device as shown in
Fig. The gate voltage (vG) is generated with the help of a
gate drive circuit, which is called a firing or triggering circuit.
Thus, thyristors can be switched on by a slow-rising rectified
ac signal, a sharp single pulse, a constant-magnitude dc signal,
or a train of high-frequency pulses as shown in Fig.. The
thyristor switches on as soon as vG exceeds the critical gate
trigger voltage (VGT ) level. This depends on the gate-to-
cathode junction temperature (Tj), anode current (iA), and
the supply voltage. The ideal switching signal for a thyristor
should have an adequate amplitude of current for sufficient
duration with a short rise time as shown in Fig. The
initial high magnitude and rapid rise of the gate current
quickly turn on the device completely. Thus, carriers spread
rapidly throughout the surface of blocking junction (J2). This
decreases switching losses and increases the initial di dt
capability of the device. After a few microseconds, a small
gate current (slightly higher than the minimum value required
for triggering IGT ) can be maintained. Ordinarily, a contin-
uous gate signal is not required, but inductive circuits neces-
sitate a sustained gate signal initially, until successful triggering
takes place. For reliable operation of controllers, gate trigger
signals are normally supplied during the entire on period (e.g.,
converters of dc drive). However, for a resistive load, a single
sharp rising pulse is sufficient for triggering. For high-power
applications, it is a common practice to isolate the control
and triggering circuits from the power circuit (consisting of
thyristors). Otherwise, the effects of high voltage and high
current transients may cause misoperation or damage to the
low-power control and trigger circuits. For this purpose, pulse
transformers or optocouplers are used for low- and medium-
power semiconductor devices. For higher power applications,
for example, HVDC transmission systems, fiber-optic cables
are used to isolate the control circuit from the power circuit. A
5-mW light trigger power source was found sufficient for
switching a light-activated silicon-controlled rectifier (LASCR)
with a rating of 4 kV, 3 kA.
Simple Gate Trigger Circuits forThyristors
Simple trigger circuits can be realized by R or RC network.
They are cheap and consume little power. However, the
control and hence the load output voltage (vo), are susceptible
to the device temperature variations. Moreover, feedback
control cannot easily be incorporated.
Resistance Trigger Circuits
A step dc voltage, a slow rising dc signal, or a rectified positive
half-wave signal can be used to trigger thyristors. When the
voltage applied to the gate terminal exceeds the VGT level,
triggering takes place. Figure shows a trigger circuit and
waveforms for an ac circuit. Before conduction of the SCR, the
input supply voltage (v) appears across the SCR.
Neglecting R2 and the voltage drop across D1