Fast-acting thyristors Ordinary thyristors cannot operate at high frequencies. This is because the device requires a certain amount of time to turn on or off, and a rapid rise in anode voltage can cause mis-conduction; a rapid rise in anode current can burn out the device. Improvements in manufacturing processes and structure have led to the development of thyristors suitable for high-frequency applications, which we call fast-acting thyristors. They have the following characteristics: I. Short turn-off time (toff) When a conducting thyristor is de-currentized, it does not immediately "turn off." If a forward voltage is immediately applied, it will continue to conduct. The time required from the de-current being cut off until the control electrode regains control capability is called the turn-off time, denoted by t0^1. The turn-off process of a thyristor is actually the process of the disappearance of stored charge carriers. To accelerate this process, gold doping is used in the manufacture of fast-acting thyristors, adding gold to the silicon to reduce the lifetime of minority carriers in the base region. The more gold doping in the silicon, the smaller t0^1 is, but excessive gold doping can affect other performance characteristics of the device. II. Fast Turn-On Speed ​​and High Current Rise Rate (dI/dt) of Thyristors Triggered by the Control Electrode. Thyristors always conduct first in the cathode region closest to the control electrode, then gradually expand outwards until the entire area is conducting. Large-area thyristors require 50-100 microseconds or more to achieve full-area conduction. When the initial conduction area is small, the initial current rise rate must be limited; otherwise, localized overheating will occur, affecting component performance and even causing burnout. This phenomenon is more severe at high frequencies. To address this, a method similar to integrated circuits is used to fabricate a small thyristor for amplifying the trigger signal on the same silicon chip as the main thyristor. After the control electrode triggers the small thyristor, its initial conduction current flows laterally across the silicon chip to the cathode of the main thyristor, triggering the main thyristor. This effectively achieves strong triggering, accelerating component conduction and improving the current rise rate tolerance. III. High Voltage Rise Rate (dV/dt) of Thyristors. Thyristors consist of three P-N junctions. Each junction acts as a capacitor. When the junction voltage changes rapidly, a large displacement current flows through the component, which is equivalent to the trigger current of the gate electrode. This can cause the thyristor to mis-conduct. This is why ordinary thyristors cannot withstand high voltage rise rates. To effectively prevent this mis-conduct, fast thyristors employ a short-circuit emitter junction structure. The cathode and gate electrode are short-circuited in a specific geometry. In this way, even with a high voltage rise rate, the current amplification factor of the thyristor remains almost zero, preventing mis-conduct. Only when the voltage rise rate further increases, the junction capacitance displacement current further increases, and the voltage drop at the short-circuit point is sufficiently large, can the thyristor conduct. When a thyristor with a short-circuit emitter junction structure is triggered by the gate electrode current, the gate electrode current also initially flows from the short-circuit point to the cathode. Only when the gate electrode current is sufficiently large, the voltage drop across the short-circuit point resistance is sufficiently large, and the PN junction is forward biased, can it be triggered to conduct like a component without a short-circuit emitter junction. Therefore, fast thyristors have better anti-interference capabilities. The production and application of fast thyristors are progressing rapidly. Currently, high-power fast thyristors with currents of several hundred amperes, withstand voltages of over 1000 volts, and turn-off times of only 20 microseconds are available. Components for high-frequency inverters with operating frequencies up to tens of kilohertz have also been developed. These products are widely used in high-power DC switches, high-power medium-frequency induction heating power supplies, ultrasonic power supplies, laser power supplies, radar modulators, and DC electric vehicle speed control. [b]Reverse-conducting thyristors[/b] Previously, urban trams and subway locomotives used DC power for easy speed control, employing DC switches to increase or decrease circuit resistance, changing the circuit current to control vehicle speed. However, this method had drawbacks such as inability to smoothly start and accelerate, large switch size, short lifespan, and high power consumption at low speeds (consumed in the starting resistor during deceleration). Since the advent of reverse-conducting thyristors, using them to control and regulate vehicle speed has not only overcome these shortcomings but also reduced power consumption and improved locomotive reliability. A reverse-conducting thyristor is formed by connecting a diode in reverse parallel to a regular thyristor (both are fabricated on the same silicon chip). Its equivalent circuit and symbol are shown in Figure 1. Its characteristic is that it can conduct large currents in reverse. Because its anode and cathode are connected in reverse parallel to the diode, it can quickly release the large current and high voltage generated when an inductive load is turned off. Currently, reverse-conducting thyristors with a withstand voltage of 1500-2500V, a forward current of 400A, a current absorption of 150A, and a turn-off time of less than 30 microseconds can be produced. Turn-off thyristors: Once a regular thyristor is turned on, the control electrode loses its function and cannot control the turn-off of the thyristor. A turn-off thyristor is a device that uses a positive control electrode pulse to trigger conduction and a negative control electrode pulse to turn off the anode current, restoring the blocking state. Therefore, a single turn-off thyristor can be used to make a DC contactless switch or a chopper. Its structure and symbol are shown in Figure 2. The process of turning off a turn-off thyristor (SCR) using a positive control electrode pulse is exactly the same as that of a regular thyristor. After the device is turned on, if a sufficiently large negative pulse is applied to the control electrode, the anode current will be completely pulled to the control electrode and flow out. There is no conducting current in the P2-N2 junction. At this time, the entire device acts like a cutoff transistor P1N1P2 after the current has disappeared. This is the basic principle of turning off a turn-off thyristor using a negative control electrode pulse. Turn-off thyristors have a faster turn-off speed than regular thyristors, allowing them to operate at higher frequencies. However, they also have disadvantages such as being difficult to manufacture into large components. They are mainly used in high-voltage DC switches, engine devices, high-voltage pulse generators, and overcurrent protection circuits. Bidirectional Thyristor The most important characteristic of a bidirectional thyristor is that regardless of whether terminal 1 is connected positive and terminal 2 is connected negative, or terminal 1 is connected negative and terminal 2 is connected positive, it can be triggered to conduct using a control electrode pulse that is positive or negative for terminal 2. In other words, it can control conduction in two directions using either a positive or negative control electrode pulse. One point that needs special clarification is that in an AC circuit, the charge carriers stored on the conducting side of a triac can diffuse to the other side, effectively adding a control electrode current to the blocking side. This can cause the device to automatically turn on and become uncontrolled. The faster the current on the conducting side decreases, or the faster the voltage on the blocking side rises, the more severe this mis-conduction phenomenon becomes. Its performance in this respect is worse than that of two triacs connected in anti-parallel. Caution should be exercised when applying this technology. Triacs are mainly used in AC control circuits, such as temperature control, lighting adjustment, explosion-proof AC switches, and DC motor speed control and commutation circuits. (Edited by: Chen Dong)