Description of operation
The thyristor is triggered by deriving the gate current from the anode of the
device. Current flow is initiated by an opto-coupler which provides the electrical
isolation. By maintaining the gate current long enough to reach the latching
current, the system can be safely used in applications with large inductive loads.
The anode voltage of the thyristor must be at least 14V before the gate current
reaches 200mA. The full gate current is maintained until the thyristor triggers
and is terminated when the anode voltage goes below 14V. The rise time of the gate
current is approximately proportional to the supply voltage. Because the di/dt
rating of the thyristor does not decrease linearly with a reduction in gate current
rise time, the device is always triggered safely. The diode in the gate allows
triggering of amplifying-gate devices. The only requirement is a 12mA signal at
the control terminals. There is a small turn-on delay of approximately 30µs and the
full dv/dt can only be re-applied 100µs after removing the trigger signal from the
control terminals .
The trigger units use the anode voltage of the thyristor as a supply for the gate
trigger current. Therefore no power supply is needed for generating the trigger
www.DataSheet4U.cpoumlses. As long as there is a control current flowing into the control terminals
of the trigger unit, the thyristor remains triggered. This is a useful feature
when the thyristor is required to control highly inductive loads. It is therefore
recommended that the unit be driven by a continuous pulse instead of a train of
pulses as is the case with pulse transformers. It must be noted that there is a
small delay of about 30µs from the time of applying the control signal until the
thyristor is actually triggered. The unit can also trigger amplifying gate
Because the anode voltage of the thyristor is used as the power supply for the
trigger pulses, the anode to cathode voltage applied to the thyristor must be
greater than the onset voltage of about 12V before a trigger current can be
produced. The trigger current increases with the anode voltage until it reaches
its maximum of 1. 2A at about 25V. Since the onset voltage is small it can be
ignored for sinusoidal mains operation.
The unit produces a trigger pulse with a rise time of 1A/µs at 300V which is more
than adequate for all applications. Note that this figure is reached at 150V with
a 1200V unit and increases about linearly with the anode voltage.
Upon application of the control current the thyristor receives a fast rising gate
current which is fully maintained until the anode voltage of the thyristor
collapses. This kind of operation accounts for the low power dissipation of the
trigger unit. On the other hand if the gate - cathode terminals are short
circuited, while a current is fed into the control terminals, the trigger unit will
be destroyed due to excessive power dissipation.
The gate of the thyristor is current driven from a voltage source of 30V. This
eliminates the need for very short gate leads in order to achieve a high rise of
gate current and reduces the possibility of undesired triggering. Twisted gate
leads in the order of 1m can be used, even in very hostile environments. It is good
engineering practice to twist gate and cathode leads anyway, and the anode lead as
close to the twisted pair as much as possible. If the snubber network is also to be
accommodated by the same leads, the loop inductance and wire resistance needs to
be kept small to ensure reliable triggering.
The voltage rating of the trigger unit follows the same guidelines as those
applicable to the thyristor. For example in the case of the 1200V unit, the
recommended maximum AC voltage should not exceed 400V while the peak transient
voltage can not go above 1200V. Voltages in excess of 1200V will destroy the unit.
Similarly the 2000V and 2200V units can handle a mains voltage not exceeding 660V .
Triggering is generally initiated by a control current less than 6mA. In order to
accommodate the aging effect of the opto-coupler and its temperature dependence at
elevated temperatures, it is good engineering practice to drive the unit with 12mA.
The input consists of the light emitting diode of the opto-coupler in parallel with
a reverse polarity protection diode. It is good practice to connect the (0)
terminals for the control signal to the zero-volt line of the logic control.
In some applications it may be desirable to trigger the thyristor when the anode
voltage exceeds a predetermined value. This voltage can for example be tailored to
suite the ratings of the MOV connected across the thyristor. In such an event,
slow rising large energy transients can be diverted by triggering the thyristor,
thus avoiding damage to the MOV. This feature may also be useful in some
controlled rectifier applications. This version is supplied on request only and
exact details about the application will be required.