The single phase fully controlled rectifier allows conversion of single phase AC into DC. Normally this is used in various applications such as battery charging, speed control of DC motors and front end of UPS (Uninterruptible Power Supply) and SMPS (Switched Mode Power Supply).
All 4 devices used are thyristors. The turn-on instants of these devices are dependent on the firing signals that are given. Turn-off happens when the current through the device reaches zero and it is reverse biased at least for a duration equal to the turn-off time of the device specified in the data sheet.
When an uncontrolled (diode) converter is to be simulated, all 4 devices should be fired at a delay angle of 0⁰. When a semi-converter is to be simulated, the lower two devices can be fired at 0⁰ and 180⁰ respectively and the upper two devices are fired at α and 180⁰+α.
Normally in a fully controlled converter, the current transfer takes place from T1 to T2 instantaneously without any time delay. But when a source inductance is present , the stored energy in Ls has to be expended before the current transfer or commutation takes place from T1 to T2. Because of this, T1 and to T2 will conduct simultaneously from α to α+u (where u is the overlap angle) causing short circuiting of the DC load during the overlap period u. This is known as commutation overlap.
Fig.1.1. Single-phase fully controlled converter
When the thyristor converter has a resistive load, both the DC current and voltage will be in phase. As soon as the positive half-cycle ends, the current through thyristors T1 and T11 will be reaching zero after which they would be reverse biased due to the supply voltage embarking into negative half-cycle. This causes the voltage pulses to be spreading from firing angle α to π as shown in the Fig.1.2. Both the current in the DC link (Idc) and the current in the source (Is) will be discontinuous.
Fig.1.2 Waveforms for single-phase converter with resistive load for a firing angle of 30°
Fig 1.3 Waveforms for a firing angle of 60° with highly inductive load and Ls being present
Zero voltage period (in Vdc- DC output voltage) from about 43.5 to 45 msec indicates the commutation overlap interval