Abstract: This paper discusses the application of thyristors in thyristor excitation equipment and locomotives, and analyzes the selection of important parameters of thyristors under different situations, circuits, and load conditions to improve equipment operation and extend service life. Keywords: Thyristor; Parameters; Selection Power electronic thyristors, formerly known domestically as silicon controlled rectifiers and internationally as SCR elements, are the most important components in silicon rectifier devices. The rationality of their parameter selection directly affects the equipment's performance. Reasonable selection of thyristors can improve operational reliability and service life, ensure production, and reduce equipment maintenance costs. This paper discusses the relevant electrical parameters of thyristors used in magnetic separation and locomotive equipment, which are frequently used in Leshan Metallurgical Machinery Rolling Mill. Under normal circumstances, the two most important parameters of the thyristors provided by the equipment manufacturer's drawings are rated current (A) and rated voltage (V). The device parameter requirements from the user department are also limited to these two parameters. For fast or medium-frequency thyristors in frequency converters, an additional commutation turn-off time (tg) parameter is generally acceptable. However, from the perspective of improving equipment performance and service life, we can also select certain parameters of the thyristor based on the characteristics of the equipment when choosing thyristor devices. Based on the static characteristics of the thyristor, the following points are discussed regarding the selection of thyristor device parameters: 1. Selection of Forward and Reverse Voltages When there is no signal at the gate and the control current Ig is 0, the thyristor is reverse biased when a voltage (J2) is applied between the anode (A) and cathode (K). Therefore, the device is in a high-impedance state, called the forward blocking state. If UAK is increased to a certain value VBO, the thyristor suddenly switches from blocking to conducting. This VBO value is called the forward breakover voltage. This conduction is abnormal and will shorten the device's lifespan. Therefore, a sufficiently high forward repetitive blocking peak voltage (VDRM) must be selected. When a reverse voltage is applied between the anode and cathode, the first and third PN junctions (J1 and J3) of the device are reverse biased and in a blocking state. When the reverse voltage is increased to a certain value VRB, the reverse state of the thyristor suddenly changes from blocking to conducting, which is a reverse breakdown and will damage the device. Furthermore, VBO and VRB values ​​decrease with repeated voltage applications. In the case of inductive loads, such as the rectifier of a magnetic separator, a very high voltage (∈=-Ldi/dt) is generated during turn-off. If there is no good absorption circuit in the circuit, this voltage will damage the thyristor. Therefore, the device must also have a sufficient reverse withstand voltage VRRM. When the thyristor operates in a converter (such as an electric locomotive), it must be able to repeatedly withstand a certain overvoltage at the power supply frequency without affecting its operation. Therefore, the forward and reverse peak voltage parameters VDRM and VRRM should be at least 2-3 times the peak voltage of normal operation. Considering potential surge voltage factors, when selecting alternative parameters, only the next higher value should be chosen. 2. Selecting Rated Operating Current Parameters The rated current of a thyristor is the maximum average on-state current IT under certain conditions. Specifically, it is the maximum allowable average on-state current at a stable rated junction temperature, under ambient temperature of +40℃ and specified cooling conditions, in a circuit with a resistive load, a single-phase sinusoidal half-wave power frequency, and a conduction angle of not less than 170℃. However, in general converter operation, there are current imbalances among the thyristors in each arm. In most cases, a thyristor cannot operate at a 170℃ conduction angle; it is usually less than this angle. Therefore, it is necessary to select a thyristor with a slightly higher rated current, generally 1.5-2.0 times its average normal current. 3. Selecting Gate (Control Stage) Parameters When a control signal is applied to the gate of a thyristor, it takes a certain amount of time to change it from blocking to conducting. This time is called the turn-on time, tgt. It consists of the delay time td and the rise time tx. tgt is the time interval from a predetermined point at the leading edge of the gate current pulse (e.g., when the gate current rises to 90% of its final value) to the instant when the on-state anode current IA reaches 10% of its final value. tx is the time it takes for the anode current to rise from 10% to 90%. It is evident that the turn-on time tgt is related to the trigger voltage and current of the thyristor gate, the junction temperature of the thyristor, the anode voltage before turn-on, and the anode current after turn-on. For ordinary thyristors, tgt is below 10μs. When the inductance of the external circuit is large, it can reach tens or even hundreds of μs or more (due to the slow rise of the anode current). When selecting thyristors, especially when using them in series or parallel connections, thyristors with similar gate triggering characteristics should be selected for use in the same device, particularly in series or parallel connections within the same arm. This improves the reliability and lifespan of the equipment. If thyristors with significantly different triggering characteristics are connected in series, the forward voltage will not be evenly distributed, damaging the thyristor with the longer trigger gate voltage (tgt). Conversely, in parallel connection, the thyristor with the shorter trigger gate voltage (tgt) will receive a larger current and be damaged, which is detrimental to the thyristor device. Therefore, the triggering voltage and triggering current of thyristors connected in series or parallel on the same arm should be as consistent as possible; that is, they should be used in pairs. In equipment where thyristors cannot be allowed to misfire due to interference, such as motor speed control, thyristors with slightly higher gate triggering voltage and current (e.g., triggering voltage VGT > 2V, triggering current IGT > 150mA) can be selected to prevent misfire. In circuits with strong trigger pulse power, thyristors with slightly higher triggering voltage and current can also be selected. In magnetic separation equipment, especially in older narrow-pulse triggering circuits, thyristors with lower VG and IG can be selected, such as VGT < 1.5V and IGT ≤ 100mA. This can reduce the risk of phase loss due to trigger failure. The above explanations illustrate that VGT and IGT parameters should be selected in certain situations. (The above examples are for reference parameters of a 500A thyristor.) 4. Selecting the Turn-Off Time (tg): After the anode current decreases to 0, if a forward anode voltage is immediately applied, the thyristor will turn on again even without a gate signal. If the device is subjected to a reverse bias voltage for a certain period of time before the forward anode voltage is applied again, it will not turn on erroneously. This indicates that the thyristor needs a certain amount of time to recover its blocking capability after being turned off. The minimum interval from when the current passes 0 to when the device can block the reapplied forward voltage is the turn-off time tg of the thyristor, which consists of the reverse recovery time t and the gate recovery time t. The tg of a typical thyristor is about 150-200μs, which is usually sufficient for the use of converters at general power frequency, but some selection is possible in the case of large inductive loads. In medium-frequency inversion applications, such as medium-frequency devices, locomotive choppers, and variable frequency speed control, the turn-off time parameter must be carefully selected. Generally, the turn-off time of a fast thyristor (i.e., a KK-type thyristor) is 10-50μs, and its operating frequency can reach 1K-4KHZ; the turn-off time of a medium-speed thyristor (i.e., a KPK-type thyristor) is 60-100μs, and its operating frequency can reach several hundred to 1KHZ, which is the frequency conversion frequency of locomotives. 5. Selecting the voltage rise rate (dμ/dt) and current rise rate (di/dt): When a thyristor is in the blocking state, if a positive voltage is applied across its terminals, even if the applied voltage value does not reach its positive maximum off-peak voltage VDRM, as long as the rate of rise of the applied voltage exceeds a certain value, the device will turn on. This is because the capacitance of the PN junction causes charging, which acts as a trigger, causing the thyristor to turn on. Different specifications of thyristors have different specified dμ/dt values, which should be considered when selecting a thyristor to ensure sufficient dμ/dt. Generally, a 500A thyristor has a dμ/dt of 100-200V/μs. For locomotives operating at frequencies below several hundred Hz, KK or KPK thyristors with a dμ/dt between 200-1000V/μs should be selected. When a trigger pulse is applied to the gate, the thyristor first conducts in a small region near the gate, then gradually expands until the entire junction is conducting. Therefore, if the anode current rises too quickly at the initial conduction point, it may cause partial burnout of the PN junction. Thus, the current rise rate of the thyristor should be carefully selected; the on-state current rise rate (di/dt) should meet the circuit requirements. A typical thyristor (500A) has a di/dt of 50-300A/μs. Under power frequency conditions, such as magnetic separators, a di/dt below 50A/μs is sufficient. Under frequency conversion conditions, such as electric locomotives, a di/dt above 100A/μs is required. When the anode voltage is high and triggering occurs at its peak, the requirements for dμ/dt and di/dt are relatively high. In addition to avoiding operation of the equipment under these conditions, the dμ/dt and di/dt of the thyristor must be selected with higher parameters. Furthermore, when a large capacitor circuit is directly connected during turn-on, a thyristor with a larger di/dt must be selected. 6. Selecting the latching current IL and holding current IH: When the thyristor gate is triggered and conducts, if the anode current IA has not yet reached the latching current IL, the thyristor returns to the blocking state once the trigger pulse disappears. If IA > IL, the thyristor remains on even after the gate pulse signal is removed. For inductive loads such as magnetic separators, attention should be paid to the rate of increase of the load current (i.e., the anode current). The rate of increase of the load current is important for whether the thyristor can continue to conduct after the gate pulse disappears, as shown in Figure (1): When the load current increases rapidly, IA>IL before the pulse disappears, and the flow of IA is not affected after the pulse disappears. If IA increases slowly, IA will not be affected after the pulse disappears. In terms of ensuring reliable triggering and maintaining conduction of the thyristor, it is understood that some semiconductor material manufacturers have introduced Japanese circuit technology: "wide pulse train trigger circuit". The pulse train has a steep leading edge and a wide width (the pulse train is 180° wide, while a narrow pulse is only 30° to 50°, and a strong trigger pulse is only about 90°). The triggering is fast and reliable, and because it is a pulse train, the power consumption is particularly small (the power consumption of a strong trigger wide pulse is particularly large, which is an important drawback). As shown in Figure (2): Figure 1 The pulse train width of this circuit effectively ensures the thyristor's continued conduction, and the maintenance current parameter of the thyristor does not need to be specified. It is understood that this circuit also offers options for voltage regulation, current regulation, or current density regulation operation. It features limited current operation performance and overcurrent blocking protection, an integral "soft start" characteristic to reduce the inrush current to the thyristor, and retains blocking protection interfaces for over-temperature and undervoltage switching signals, significantly improving the reliability and service life of the equipment. Magnetic separators at Guangdong Dabaoshan Iron Mine and other mines have achieved excellent results using this circuit. In conclusion, when selecting thyristor device parameters, specific parameters should be carefully considered based on different applications, circuit conditions, and load characteristics to ensure better equipment operation, higher reliability, and longer lifespan. References [1] Huang Jun. Power Electronic Converter Technology [M]. Machinery Industry Press, 1992. [2] Ren Wanqiang. Thyristor High Power Factor Adjustable Fluorescent Lamp Electronic Ballast [J]. China Lighting Electric, 2007, (4). [3] Wang Wu. Characteristics and Test of Thyristor Series Voltage Regulating Capacitor Reactive Power Compensation Device [J]. Power Electronics Technology, 2006, (5). [4] Li Xiang. Analysis and Simulation of Three-Phase Three-Wire AC Voltage Regulating Circuit Based on Thyristor [J]. Journal of Wanxi University, 2006, (2).