SCR Triggering

SCR triggering is the process of applying a gate signal to turn the thyristor from forward blocking to forward conduction mode. It is essential for phase controlled rectifiers and static AC switches.

Core Concept

The most reliable method is gate triggering. A positive pulse is applied to the gate terminal with respect to the cathode. This injects charge carriers and initiates regenerative action.

Pulse triggering uses high frequency pulse trains via a pulse transformer. This isolates the control circuit from the high voltage power circuit. A typical firing angle alpha of 45 degrees yields moderate output voltage.

Other methods include dv/dt triggering and thermal triggering. However these are considered unwanted parasitic turn on mechanisms rather than intentional control methods.

Key Formulas

Gate power dissipation Pg equals gate voltage Vg multiplied by gate current Ig. The firing angle alpha determines the conduction angle which is 180 minus alpha for a half wave load.

Given: Vg = 5 V, Ig = 100 mA, Duty Cycle = 0.2
Formula: Average Power = Vg * Ig * Duty Cycle
Steps: Pavg = 5 * 0.1 * 0.2 = 0.5 * 0.2
Final Answer: 0.1 W

Exam Tip: Pulse triggering is preferred over continuous DC triggering because it reduces gate power dissipation and allows for pulse transformer isolation.

Performance Parameters

• Minimum gate trigger current Igt: 10 mA to 200 mA
• Minimum gate trigger voltage Vgt: 1 V to 3 V
• Maximum gate power Pgm: typically 5 W
• Pulse width: 10 to 50 microseconds
• Turn on time tgt: 1 to 3 microseconds

Quick Revision

• Gate current initiates conduction
• Pulse trains reduce thermal stress
• Isolation provided by pulse transformers
• UJT oscillators commonly generate pulses
• R and RC triggering offer basic phase control
• Exam trap: Thinking an SCR can be turned off by negative gate current