Basic requirements for thyristor trigger circuits

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Thyristors are semiconductor devices that can be turned on by applying specific external conditions. To switch a thyristor from the off state to the on state, a forward voltage must be applied to its anode, and a positive control signal must be sent to its gate. Once the thyristor is triggered, the gate no longer has any influence. The device will automatically turn off when the anode current drops below the holding current, typically during the zero crossing of the power supply cycle.

Because the gate loses control once the thyristor is turned on, the main purpose of the gate control circuit is to provide a threshold-level signal with a certain width to initiate the switching process. This type of circuit is commonly referred to as a trigger circuit. A well-designed trigger circuit ensures reliable and consistent operation of the thyristor in various applications.

The basic requirements for a thyristor trigger circuit include:

1. Trigger Signal Type: The trigger signal can be AC, DC, or pulse-based. It should only be active when the gate is positively biased relative to the cathode. To minimize gate losses, pulse signals are often used. Common waveforms are shown in the figure below.
2. Sufficient Power: The trigger pulse must have enough voltage and current to exceed the gate trigger voltage and current of the thyristor. Due to variations in thyristor characteristics and temperature sensitivity, the trigger circuit must ensure reliable triggering without exceeding the gate's maximum limits (typically Vcm < 10V and Igm < 10A).
3. Phase Shift Range: The phase shift range of the trigger pulse must match the requirements of the converter system. For example, a three-phase half-wave rectifier may require a 150° phase shift under inductive loads, while a fully controlled rectifier with resistive load needs a 120° range. In more complex systems like a three-phase full-bridge converter, the phase shift range might extend up to 180°, though practical applications often limit it slightly.
4. Pulse Width and Steepness: The pulse width must be sufficient to allow the anode current to reach the holding level before the pulse ends. For resistive loads, this could be as short as 10 μs, but inductive loads may require up to 100 μs. The leading edge of the pulse should be steep—ideally steeper than 10V/μs or 800mA/μs—to ensure simultaneous triggering of multiple thyristors in parallel or series.
5. Synchronization: The trigger pulse must be synchronized with the main supply voltage to ensure consistent triggering on each cycle. This synchronization is essential for maintaining the stability and performance of the thyristor-based system.

In real-world applications, designing a thyristor trigger circuit requires careful consideration of all these factors to ensure safe, efficient, and reliable operation. Whether it's for power conversion, motor control, or industrial automation, a properly designed trigger circuit plays a critical role in the overall performance of the system.