Average load current in a phase-controlled rectifier: On which parameter(s) does the average load current delivered by a thyristor predominantly depend (assume mains frequency fixed and gate drive above threshold)?

Introduction / Context:
In AC phase-controlled rectifiers, the average DC output is regulated by delaying the thyristor firing instant within each half-cycle. Understanding which control variable truly sets the average load current helps avoid common misconceptions about gate drive magnitude and repetition rate.

Given Data / Assumptions:

• Supply frequency is fixed (e.g., 50/60 Hz).
• Gate current amplitude exceeds the necessary trigger threshold.
• Load is such that conduction follows the phase control behavior.

Concept / Approach:

The average load current is determined by the average DC output voltage, which depends on firing angle α (e.g., Vdc ∝ cos α in a full converter, or Vdc ∝ (1 + cos α) in a semi-converter). Firing frequency is tied to the mains and not a control variable. Once the gate pulse exceeds threshold, its magnitude does not set the steady-state average load current; it only guarantees reliable triggering.

Step-by-Step Solution:

Verification / Alternative check:

Converter equations in textbooks express Vdc as functions of α; gate drive amplitude appears only in trigger reliability, not in Vdc formulas.

Why Other Options Are Wrong:

• Firing frequency: locked to mains; not an independent knob.
• Gate current magnitude: above threshold it does not change Vdc.
• “All of the above” is incorrect for the reasons above.

Common Pitfalls:

Assuming stronger gate pulses “force” more current; the converter is voltage-controlled by phase delay, the load sets the current from that voltage.

Final Answer:

firing angle