How to realize controlled rectification by SCR(tiristor)?

The most basic use of ordinary silicon controlled rectifiers is controlled rectification. The familiar diode rectifier circuit is an uncontrollable rectifier circuit. If the diode is replaced by a silicon controlled rectifier, a controlled rectifier circuit can be formed. I draw the simplest single-phase half-wave controllable rectifier circuit. During the positive half cycle of the sinusoidal AC voltage U2, if the control pole of VS does not input the trigger pulse Ug, VS still cannot be turned on. Only when U2 is in the positive half cycle and the trigger pulse Ug is applied to the control pole, the thyristor is triggered to turn on . Draw its waveform, you can see that only when the trigger pulse Ug arrives, the voltage UL output on the load RL. Ug arrives early, and the SCR is turned on early; Ug arrives late, and the SCR is turned on late. By changing the arrival time of the trigger pulse Ug on the control pole, the average value UL (the area of ​the shaded part) of the output voltage on the load can be adjusted. In electrical engineering, the half cycle of alternating current is often set as 180°, which is called electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the zero value to the moment of the trigger pulse is called the control angle α; the electrical angle at which the thyristor conducts in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to indicate the conduction or blocking range of the thyristor during the half cycle of the forward voltage. By changing the control angle α or the conduction angle θ, the average value UL of the pulse DC voltage on the load is changed, and the controllable rectification is realized.