Single-Phase Semi-Converter

Introduction

A controlled rectifier converts fixed AC into adjustable DC by delaying when its thyristors (SCRs) are switched on. In the fully-controlled bridge, all four devices are SCRs, so the output can swing negative and the converter can even return power to the source (inversion). The single-phase semi-converter takes a simpler, cheaper route: it replaces two of those SCRs with ordinary diodes, forming a half-controlled bridge.

This one change means the output voltage can be smoothly controlled from maximum down to zero — but it can never go negative. The semi-converter therefore works in a single quadrant (positive voltage, positive current) as a pure controlled rectifier, and as a bonus it draws a better input power factor than the full converter at the same firing angle. In this tutorial we cover its circuit, the step-by-step modes of operation with a resistive load, the full set of voltage and current waveforms, and all the key formulas.

What is a Single-Phase Semi-Converter?

A single-phase semi-converter (also called a half-controlled bridge rectifier) is a single-phase AC-to-DC converter built from a mix of controlled and uncontrolled devices: two thyristors (T1, T2) provide the phase control, while two diodes (D1, D2) provide an uncontrolled return path. A freewheeling diode D_m is usually connected across the load to carry the inductive load current when the supply is not delivering power.

By delaying the SCR turn-on by a firing (delay) angle α, the average DC output is varied. The average output voltage of a single-phase semi-converter is:

Because the diodes and the freewheeling path prevent the output from ever reversing polarity, the semi-converter is a one-quadrant converter: it can only rectify (deliver power from AC to DC), and cannot feed energy back from the DC load to the AC source.

Types of Single-Phase Semi-Converter

Depending on how the two thyristors and two diodes are arranged in the bridge, a single-phase semi-converter is built in one of two configurations. Both use 2 SCRs + 2 diodes, both are half-controlled and one-quadrant, and both give the same average output voltage V_o = (V_m/π)(1 + cosα) — they differ mainly in device arrangement and gate-drive requirements.

1. Symmetrical Semi-Converter

In the symmetrical configuration the two thyristors (T1, T2) occupy the two upper arms and share a common cathode connected to the positive DC terminal, while the two diodes (D1, D2) occupy the two lower arms with a common anode at the negative terminal. Because both SCR cathodes sit at the same point, their gate-drive (triggering) circuits can share a common reference, which makes control simpler. In the positive half-cycle T1 and D1 conduct; in the negative half-cycle T2 and D2 conduct. This is the configuration analysed on this page.

2. Asymmetrical Semi-Converter

In the asymmetrical configuration each leg of the bridge contains one thyristor and one diode. The two thyristors therefore do not share a common cathode, so each SCR generally needs its own isolated gate-drive circuit. The freewheeling action during the zero-output interval is provided mainly by the two diodes. Electrically it delivers the same controlled DC output as the symmetrical converter; the choice between the two is driven by triggering, isolation and layout considerations.

Circuit Diagram & Construction

A single-phase semi-converter (symmetrical configuration) is built from four bridge devices plus a freewheeling diode:

• Two thyristors (SCRs) — T1 and T2 — form the controlled (upper) half of the bridge.
• Two diodes — D1 and D2 — form the uncontrolled (lower) half of the bridge.
• One freewheeling diode — D_m — connected directly across the load.
• A load that is usually inductive (R–L), such as a DC motor armature or a filtered DC supply.

The AC supply feeds the bridge; T1 pairs with D1 to handle the positive half-cycle, and T2 pairs with D2 to handle the negative half-cycle. Whichever pair conducts, the load current is steered in the same direction, giving a DC output. For a purely resistive load the diodes alone provide the return path; for an inductive (R–L) load the freewheeling diode D_m clamps the output to zero (and keeps the current flowing) whenever the supply voltage tries to go negative.

Modes of Operation (R Load)

Consider the symmetrical semi-converter feeding a resistive (R) load, with the two thyristors fired at a delay angle α. Over one full input cycle (0 to 2π) the circuit passes through three distinct modes. In each conducting mode a thyristor and a diode form a series path for the load current; between them there is a dead interval in which the output is zero.

Mode 1 — T1 & D1 conduct (α ≤ ωt < π)

During the positive half-cycle thyristor T1 is forward-biased. When T1 is fired at ωt = α, it turns on and conducts together with diode D1, connecting the load directly across the supply. Current flows source → T1 → load → D1 → source, so the output equals the supply voltage, v_o = v_s = V_m sinωt, and the load current is i_o = v_o/R. Before firing (0 to α) no device conducts, so the output stays at zero — this is how the delay angle controls the average voltage.

Mode 2 — No device conducts, v_o = 0 (π ≤ ωt < π+α)

At ωt = π the supply voltage falls to zero, so T1 (with D1) turns off by natural commutation. In the interval from π to π+α the next thyristor T2 has not yet been fired, and with a resistive load there is no stored energy to keep current flowing. Hence no device conducts, the load current is zero, and the output voltage is held at v_o = 0 V.

Mode 3 — T2 & D2 conduct (π+α ≤ ωt < 2π)

In the negative half-cycle thyristor T2 is forward-biased. When T2 is fired at ωt = π+α, it conducts with diode D2, again connecting the load to the supply — now current flows source → T2 → load → D2 → source. The bridge steers this current through the load in the same direction as before, so the output is the rectified magnitude v_o = |V_m sinωt|. At ωt = 2π the supply returns to zero, T2 and D2 turn off, and the whole cycle repeats.

Input & Output Waveforms (Voltage & Current)

Every trace below is computed directly from the modes above (drawn for a firing angle α = 45°, symmetrical semi-converter with an R load). The six panels share one time axis (ωt): the AC supply voltage, the conducting devices, the output voltage, the output current, and the individual thyristor and diode currents.

Reading the Waveform

• Supply voltage v_s: the ordinary AC sine, V_m sinωt, over one full cycle (0 to 2π). The pink dashed lines mark the firing instants α and π+α.
• Devices conducting: T1·D1 power the load from α to π (Mode 1); both pairs are off from π to π+α (Mode 2, v_o = 0); then T2·D2 power the load from π+α to 2π (Mode 3).
• Output voltage v_o: a chopped sine hump from α to π, zero from π to π+α, and another hump from π+α to 2π. It never goes negative — one-quadrant operation. The orange dashed line is the average, V_o = (V_m/π)(1 + cosα).
• Output current i_o: for a resistive load i_o = v_o/R, so it has exactly the same chopped-sine shape as v_o, scaled by 1/R. Its average is I_o = V_o/R.
• Thyristor currents i_T1, i_T2: each SCR carries the load current only while it conducts — i_T1 is a hump from α to π, and i_T2 is a hump from π+α to 2π. Each SCR conducts once per cycle.
• Diode currents i_D1, i_D2: because a diode is in series with its thyristor, i_D1 = i_T1 (α to π) and i_D2 = i_T2 (π+α to 2π). The diode carries the same current, at the same time, as its partner SCR.
• R vs R–L load: with an inductive load plus freewheeling diode D_m, the voltage waveform is unchanged, but the currents (i_o, and the device currents) become flat-topped blocks instead of humps, because the inductor smooths the current to a nearly constant value.

Key Parameters & Formulas

For an ideal single-phase semi-converter feeding its load, with V_m the peak supply voltage and α the firing angle:

The average voltage varies smoothly from 2V_m/π (full output, α = 0°) down to 0 (α = 180°), which is exactly how the semi-converter controls DC power to the load.

Effect of the Firing Angle α

The firing angle α is the single control knob of the converter. Delaying it reduces the conducting portion of each half-cycle, so both the average and RMS output fall. A few representative points from V_o = (V_m/π)(1 + cosα):

Unlike the full converter, the semi-converter's output cannot become negative no matter how large α is — so it cannot operate in the inversion mode and cannot feed energy back to the AC supply.

Semi-Converter vs Full-Converter

Both are single-phase phase-controlled bridges, but they make very different trade-offs:

Advantages & Disadvantages

Advantages

• Lower cost: two diodes replace two SCRs, so fewer gate-drive and control circuits are needed.
• Better input power factor than a full converter at the same firing angle, thanks to the freewheeling action.
• Simpler control: only two thyristors need firing pulses.
• Smooth DC control from maximum down to zero by adjusting α.
• The freewheeling diode reduces load-current ripple and protects the SCRs from large reverse transients.

Disadvantages

• One-quadrant only: output cannot go negative, so it cannot invert or return energy to the source.
• Not suitable for applications needing regenerative braking or reversible DC drives.
• Output contains ripple and harmonics that may need filtering for sensitive loads.
• Input current is non-sinusoidal, introducing harmonics into the AC supply.

Applications

• DC motor speed control where one-directional (motoring-only) operation is enough — fans, pumps, conveyors.
• Battery charging and regulated DC power supplies with adjustable output.
• Electroplating, anodizing and electrolysis that need controllable DC current.
• Traction and industrial drives supplying large DC motors from a single-phase AC line where reverse power flow is not required.
• Heater and lighting power control for resistive DC loads.

Frequently Asked Questions – FAQs

Related Topics

• Single-Phase Full-Wave Controlled Rectifier (Full Converter)
• Single-Phase Half-Wave Controlled Rectifier
• Single-Phase Full-Wave Centre-Tapped Rectifier
• SCR Firing-Angle Calculator
• What is a Rectifier?