The use of bidirectional thyristors in AC voltage regulation circuits

At present, bidirectional thyristors are commonly used for communication voltage regulation, which have the advantages of small size, light weight, high efficiency, and easy use. They have significant effects on improving production efficiency and reducing costs. However, they also have poor overload and anti-interference capabilities, and can interfere with the power grid and self interfere when controlling large inductive loads. Let's talk about how thyristors can avoid the above problems in their use.

1 Sensitivity

A bidirectional thyristor is a three terminal component, but we no longer refer to its two poles as anode and cathode, but as T1 and T2 poles. G is the control pole, and the voltage applied to the control pole, whether positive or negative, can cause the control pole to conduct. Under the four conditions shown in Figure 1, bidirectional thyristors can be triggered to conduct, but the triggering sensitivity is different from each other. The minimum gate current IGT that ensures the bidirectional thyristor can enter a conducting state is different, with (a) having the highest triggering sensitivity and (b) having the lowest triggering sensitivity. In order to ensure triggering while minimizing gate current, (c) or (d) should be selected as the triggering method.

2. Protection against thyristor overload

There are many advantages to thyristor components, but their overload capacity is poor. Short term overcurrent and overvoltage can cause damage to the components. Therefore, in order to ensure the normal operation of the components, it is necessary to have the following conditions: (1) the applied voltage can exceed the forward turning voltage, otherwise the control electrode will not work; (2) The average on state current of a thyristor is generally taken as 1.5 to 2 times the maximum current from a safety perspective; (3) To ensure reliable triggering of the control pole, the triggering current applied to the control pole is generally greater than its rated value. In addition, protective measures must also be taken. The general protection measure for overcurrent is to connect a fast fuse in series in the circuit, with a rated current of about 1.5 times the average current of the thyristor. Its connection position can be on the AC side or DC side. When it is on the AC side, the rated current is higher, and the former is generally used. Overvoltage protection often occurs in circuits with inductance, or in surge voltage caused by interference on the AC side or overvoltage caused by transient processes on the AC side. Due to the high peak and short duration of overvoltage, resistance and capacitance absorption circuits are often used to suppress it.

Avoiding interference with the power grid and self interference when controlling large inductive loads

When a thyristor component controls a large inductive load, there may be interference with the power grid and self interference. The reason is that when the thyristor component controls a circuit connected to an inductive load to open or close, the current path in its coil is cut off, and its rate of change is extremely large. Therefore, a high voltage is generated on the inductor, which is applied to both ends of the switch contact through the internal resistance of the power supply. Then, the induced voltage is discharged repeatedly until the induced voltage is lower than the voltage necessary for discharge. During this process, a large pulse beam will be generated. These pulse beams are superimposed on the supply voltage and transmit interference to the supply line or to the surrounding space in the form of radiation. These pulses have high amplitude and wide frequency, making switch points with inductive loads a strong source of noise.

3.1 To prevent or reduce noise, the general methods for handling phase-shift controlled AC voltage regulation include inductor capacitor filter circuits, resistor capacitor damping circuits, bidirectional diode damping circuits, and other circuits.

3.1.1 Inductive capacitor filtering circuit, as shown in Figure 2 (a), consists of a resonant circuit composed of inductance and capacitance, with a low-pass cut-off frequency of f=1/2 π Ic, generally taking the low frequency of tens of kilohertz.

3.1.2 Bidirectional diode damping circuit, as shown in Figure 2 (b). Due to the reverse series connection of diodes, they are insensitive to the polarity of input signals. When the load is excited by the power supply, the suppression circuit has no effect on the load. When the current in the inductive load coil is cut off, transient current flows through the suppression circuit, thus avoiding the discharge of induced voltage through the switch contact and reducing noise. However, it is required that the reverse voltage of the diode should be higher than any transient voltage that may occur. Another requirement is that the rated current value must meet the circuit requirements.

3.1.3 Resistance capacitance damping circuit, as shown in Figure 2 (C), utilizes the characteristic of the capacitor voltage not being able to change abruptly to absorb the spike like overvoltage generated during thyristor commutation, and limits it within the allowable range. A series resistor is used to prevent capacitor and inductor oscillation during thyristor blocking, providing damping effect. In addition, a resistor capacitor circuit also has the function of accelerating thyristor conduction.

Another method to prevent or reduce noise is to use the on-off ratio to control the AC voltage regulation method. The principle is to use a zero crossing trigger circuit, which controls the bidirectional thyristor to conduct and cut off when the power supply voltage crosses zero, that is, the control angle is zero, so as to obtain a complete sine wave on the load. However, its disadvantage is that it is suitable for systems with a time constant larger than the on-off cycle, such as thermostats.