Single-Phase Semi Converter: Single Phase Semi Converter is also known as a half-controlled converter. A single-phase half controlled or semi converter utilizes SCRs(thyristor) and diodes to convert AC power to DC power. Due to the utilization of diodes and thyristors, it provides limited control over the level of DC output voltages.
This section will discuss the single-phase semi-converter, its circuit diagram, operation, waveform, average, and RMS output voltage expression.
Circuit Diagram of Single Phase Semi Converter
The figure below shows the circuit diagram of single phase semi-converter bridge circuit. The bridge circuit consists of two thyristors and three diodes connected across an RLE load. Among these, two thyristors (T1, T2) and two diodes(D1, D2) are utilized in the bridge connection while another diode (FD) is connected across the bridge circuit or load. The free-wheeling diode(FD) finds its application in partially recovering the energy stored in the inductor back to the load such that no energy is fed back to the source.
The Waveform of Single-Phase Semi Converter
The figure below shows the waveform of a single-phase half-controlled converter.
The operation of a single-phase semi-converter is discussed based on the above waveform for definite time intervals.
1. At ωt=0 to ωt=π
During the interval of ωt =0 to ωt=α, thyristor T1 and diode D1 come into operation. After ωt=0, thyristor T1 becomes forward biased only if Vmsinωt is greater than the load circuit emf E. The thyristor T1 is then triggered at ωt=α during which Vmsinα > E. When T1 is triggered, it gets ON and the load current io flows through the RLE load through thyristor T1 and diode D1. The thyristor T1 will be conducting from ωt =α to ωt=π.
During ωt=α to ωt=π, the voltage waveform across the load terminal(Vo) will be a replica of the source voltage(VS).
2. At ωt =π to ωt=2π
Just after the ωt=π i.e. ωt=πt, the supply voltage gets negative, this causes thyristor T1 to be reverse biased and it gets turned off. But on the other side, this reversal of supply voltage at ωt=πt causes the freewheeling diode (FD) to be forward biased. So now the load current io gets transferred from T1D1 to FD. As the freewheeling diode (FD) gets forward biased, load terminals get short-circuited and the voltage across the load(Vo) becomes zero.
During this period, thyristor T2 will be in reverse biased condition only if the source voltage is greater than E. For ωt=π+α, source voltage (Vs) is greater than E, and T2 is triggered from FD to T2 D2. After the T2 gets triggered, FD gets reverse biased.
3. After ωt=2π
Just after ωt=2π, the thyristor T2 gets reverse biased and is turned off and on the other hand, FD gets forward biased, and load current io gets transferred to FD.
This cycle continues until the AC supply is supplied to the circuit.
Role of Freewheeling Diode
In the single-phase semi converter, we can observe from its waveform that for time intervals α to π, T1 D1 conducts and for the interval (π+α) to 2π, T2D2 conducts during which power is supplied from the source o the RLE load.
During this period, a certain amount of energy is stored by the inductor(L), a definite amount in the load circuit emf, and in the resistor (R), a certain amount of energy is dissipated in the form of heat.
During the period π to π+α and 2π to 2π+α, the energy stored in the inductor is partially fed back to the load circuit through the freewheeling diode(FD). This energy that is being fed back to the load circuit is partially dissipated in the resistor as heat and partially added to the load circuit emf E.
Hence, the freewheeling diode(FD) provides a freewheeling path for the energy that is stored in the inductor during a certain interval to be fed back to the load again.
Average and RMS Output Voltage
The expression for average output voltage (Vo) of single-phase semi-converter is given by
The expression for RMS output voltage (Vorms) is
Circuit Turn Off Time of Single-Phase Semi Converter
The circuit turn off time for a single-phase half controlled converter is