Phase-controlled converters, also known as line-commutated converters, are a class of power electronic circuits designed to transform a fixed AC voltage and frequency into a variable DC voltage output. This control is achieved by precisely managing the phase angle at which thyristors (typically SCRs) are triggered or “fired” during each AC cycle. This allows for a controllable, though pulsating, DC output.

3.1 2-Pulse Converter

A 2-pulse converter is the standard topology for single-phase, full-wave AC-DC conversion. It typically uses four SCRs in a full-bridge configuration (or two SCRs with a center-tapped transformer). By controlling the firing angle α—the point in the AC cycle at which the SCRs are triggered—the average DC output voltage can be varied. This topology produces two voltage pulses at the output for every one cycle of the single-phase AC input, hence the name “2-pulse.”

3.2 3-Pulse and 6-Pulse Converters

In a 3-pulse converter, used for three-phase systems, each of the three thyristors conducts for one-third (120°) of the supply cycle. By extending this principle, a 6-pulse converter is created, where the number of pulses is twice the number of phases in the AC supply. A key principle in converter design is that a higher number of pulses leads to greater utilization of the converter and a smoother DC output with less ripple.

3.3 The Effect of Source Inductance

In practical systems, the AC source always has some level of inductance. This source inductance prevents the instantaneous commutation (transfer of current) from one thyristor to the next. Instead, there is an “overlap interval” during which multiple thyristors conduct simultaneously. This effect has a tangible impact on performance, most notably by reducing the average DC output voltage of the converter.

3.4 Key Performance Parameters

The performance of AC-DC converters can be quantified using several standard parameters. Assuming ideal devices and resistive loads, these metrics are defined as follows:

• DC Voltage on Load (V_DC)
• RMS Voltage on Load (V_L)
• Form Factor (FF)
• Ripple Factor (RF)
• Efficiency (η) The general definition of efficiency is the ratio of DC output power to the total AC input power: η = P_DC / P_L. Assuming ideal switching devices with zero resistive losses (R_D = 0), this simplifies to a function of the form factor.
• Transformer Utilization Factor (TUF)

Following the conversion from AC to DC, the next logical step in many power systems is to precisely control that DC voltage using DC-DC conversion.