Semi-Converters

Semi-converters, also called half controlled converters or asymmetrical converters, represent an important intermediate category between fully uncontrolled rectifiers and fully controlled converters. By combining SCRs with freewheeling diodes, semi-converters deliver controllable DC output while maintaining a better power factor than fully controlled converters. They are widely used in applications where energy reversal from load to supply is not required, such as resistive heating and unidirectional DC motor drives.

Core Concept Explanation

A semi-converter is a single phase bridge rectifier in which two SCRs are replaced by two diodes, and a freewheeling diode (FWD) is connected across the load. In a fully controlled bridge, all four switching devices are SCRs, allowing both positive and negative output voltages. In a semi-converter, only two SCRs are used (one per half cycle), and the other two positions are diodes. This asymmetrical arrangement allows control during forward conduction but prevents negative output voltage.

The freewheeling diode is the defining element of a semi-converter. When the SCR would normally stop conducting due to the supply voltage going negative, the inductor current in the load continues flowing. Without the FWD, this current would force the SCR into reverse conduction, which is not possible for a thyristor. The FWD provides a path for this continuing current through the load, clamping the output voltage to zero instead of allowing it to go negative. This is called freewheeling action.

The most significant advantage of semi-converters over fully controlled converters is improved power factor. In a fully controlled converter, both SCRs fire at α, causing the input current fundamental to lag the voltage by nearly α. In a semi-converter, one half cycle always has a diode conducting (which starts at the natural zero crossing), reducing the average delay angle. The input current waveform is asymmetrical, with one half cycle closer to the natural conduction point, resulting in a higher displacement power factor.

Semi-converters are classified as one quadrant converters because the output voltage is always positive (or zero) and current is always positive. Energy cannot be returned from the load to the supply. This makes them suitable for unidirectional DC motor drives and battery charging applications, but unsuitable for regenerative braking where energy must flow back to the supply.

Mathematical Expression

The average output voltage of a single phase semi-converter is given by integrating the supply voltage from α to π (SCR conduction) plus zero voltage from π to 2π (freewheeling), divided by the period 2π. The resulting formula is Vdc = (Vm / π) × (1 + cos α). Comparing this to the fully controlled full wave formula Vdc = (2Vm/π) × cos α, the semi-converter formula is always non-negative for all values of α from 0 to π. This confirms that output voltage can never go negative.

At α = 0, Vdc = 2Vm/π, identical to an uncontrolled full wave rectifier. At α = π, Vdc = 0. The ripple frequency of the semi-converter output is the same as the supply frequency (50 Hz), unlike the fully controlled bridge whose ripple frequency is 100 Hz. This higher ripple content means a larger filter inductance is required for smooth DC output. The displacement power factor of a semi-converter is approximately (1 + cos α)/2, which is always greater than cos α for the same α, confirming the power factor improvement.

Practical Understanding

In practical drives, the freewheeling diode has an important effect on motor performance. During the freewheeling interval, the motor armature current continues flowing through the FWD, maintaining a smoother current waveform. Without the FWD, the armature current would become discontinuous at light loads, causing jerky motor operation and torque ripple. The FWD thus improves the current continuity of the load, especially for inductive loads.

The trade-off of using a semi-converter is the inability to operate in the inverting mode. A fully controlled converter can have α > 90°, making Vdc negative and allowing power flow from the DC side back to the AC supply. This is essential for regenerative DC motor drives (braking). Semi-converters cannot do this because the FWD prevents negative output voltage. Applications that need regenerative braking must use fully controlled four quadrant converters or dual converters.

Given:
Single phase semi-converter
Supply Vs = 230 V (rms), 50 Hz
Firing angle α = 90°
Load: Resistive

Why this formula applies:
Semi-converter output uses (Vm/π)(1 + cosα)
because FWD clamps negative excursions to zero.

Formula:
Vdc = (Vm / π) × (1 + cosα)
Vm = 230 × √2 = 325.27 V

Substitution:
Vdc = (325.27 / π) × (1 + cos 90°)
Vdc = (325.27 / 3.1416) × (1 + 0)

Calculation:
Vdc = 103.56 × 1 = 103.56 V

Final Answer: Vdc = 103.56 V

Note: For fully controlled full wave at α=90°: Vdc = (2×325.27/π)×cos90° = 0 V
Semi-converter gives higher output than fully controlled at same α.

Exam Tip: The semi-converter formula Vdc = (Vm/π)(1+cosα) is derived for one quadrant only. A common GATE trap is using the fully controlled formula (2Vm/π)cosα for semi-converters. Also note: ripple frequency of semi-converter = supply frequency (f), not 2f as in fully controlled bridge.

Mechanism of Freewheeling

• During α to π: SCR T1 conducts, supply voltage drives current through load. Output voltage equals supply voltage.
• At ωt = π: Supply voltage reaches zero. For inductive load, inductor prevents current from dropping instantly.
• Beyond π: Freewheeling diode Df becomes forward biased. Load current diverts through Df, bypassing the SCR.
• During freewheeling: Output voltage is clamped to zero. SCR T1 sees reverse voltage and turns off naturally.
• At next α: SCR T2 fires for the next half cycle. Process repeats symmetrically.
• Net effect: Output is always non-negative. Power factor is better than fully controlled converter at same α.

Quick Revision

• Semi-converter uses 2 SCRs + 2 diodes + 1 freewheeling diode across load.
• Output formula: Vdc = (Vm/π)(1 + cosα). Valid for α from 0 to π, always non-negative.
• Ripple frequency = supply frequency f (not 2f). Higher filter inductance needed than fully controlled bridge.
• Power factor of semi-converter is higher than fully controlled converter at the same firing angle α.
• One quadrant operation only: Vdc ≥ 0 and Id ≥ 0. Cannot operate as inverter (no regeneration).
• Freewheeling diode prevents negative output voltage and improves current continuity in inductive loads.
• Exam trap: Do not use (2Vm/π)cosα for semi-converter. That formula is for fully controlled full wave only.