Gate-drive choice for a single-phase full-wave AC regulator feeding an R–L load Which gate signal strategy is best suited for reliable firing?

Introduction / Context:
In AC voltage regulators using antiparallel SCRs feeding an R–L load, the instantaneous device voltage can vary rapidly and the anode current lags due to inductance. Gate-drive strategy must ensure the SCR actually turns on when anode-cathode voltage becomes forward biased.

Given Data / Assumptions:

• Single-phase full-wave AC regulator (two SCRs back-to-back).
• Load contains significant inductance L.
• Gate trigger threshold varies with temperature and device.

Concept / Approach:
Because inductive loads shift current zeroes and create uncertain forward-bias timing, a single narrow pulse may miss the exact forward-biased moment. A pulse train (a series of short pulses spanning a few milliseconds) greatly increases the probability of successful turn-on exactly when the device becomes forward-biased, while keeping average gate power low.

Step-by-Step Solution:
Inductance → current lags → forward bias may occur slightly after the planned α instant.Single short pulse may occur too early → misfire risk.Pulse train straddling α ensures at least one pulse coincides with proper forward bias → reliable triggering.

Verification / Alternative check:
Industrial gate drivers commonly use pulse trains for AC controllers, exactly to accommodate line and load variations and reduce gate heating.

Why Other Options Are Wrong:

• Short pulse or long single pulse is less robust to timing uncertainty.
• DC gate bias wastes power and can stress the gate without improving reliability.
• “Either (a) or (b)” is inferior to a well-timed pulse train.

Common Pitfalls:

• Assuming the voltage zero cross equals immediate forward bias on the intended device.
• Under-estimating gate power with continuous drive.

Final Answer:
A pulse train (repetitive pulses) around the firing instant