Voltage across thyristors: half-wave vs full-wave (M-2) converters with resistive load A single-phase half-wave controlled converter feeding an R load and a single-phase full-wave (M-2) converter feeding an R load are both fired at angle α. How do the thyristor voltage waveforms compare?

Introduction / Context:
Converter topology determines device stress and voltage blocking patterns. Even with the same firing angle α and purely resistive loads, the thyristor voltages in half-wave (one device conduction per positive half-cycle) and full-wave M-2 (two devices conducting alternately in each half-cycle) differ significantly. Recognizing these waveform differences is essential for device selection and snubber design.

Given Data / Assumptions:

• Both loads are purely resistive.
• Firing angle α is the same in both circuits.
• Ideal devices; source inductance neglected.

Concept / Approach:

In the half-wave circuit, the controlled device sees full reverse half-cycle voltage stress and blocks for the entire negative half. In the M-2 full-wave circuit (center-tapped), two SCRs alternately conduct; each device’s blocking and conduction intervals are mirrored across half-cycles. Voltage across each thyristor thus has a distinct pattern compared with the single-device half-wave case, leading to non-similar waveforms.

Step-by-Step Solution:

Verification / Alternative check:

Plotting v_SCR(t) for both topologies demonstrates different blocking intervals and polarities even for the same α.

Why Other Options Are Wrong:

“Similar” or “identical only at α = 0” misrepresents fundamental topology differences; “may be similar” is vague and incorrect for ideal cases.

Common Pitfalls:

Comparing output load voltage waveforms instead of device voltage stress; ignoring the second device in M-2.

Final Answer:

are not similar