Abstract: This paper details the influence of gate triggering of thyristors on the turn-on characteristics of thyristors in high-voltage soft starters.
Keywords : thyristor, gate, trigger, voltage, current, pulse;
Abstract: This paper describes a high-voltage soft starter in the thyristor gate trigger characteristics of thyristor opening.
Key words: thyristor, gate, trigger, voltage, current, pulse;
1 Introduction
A thyristor is a current-controlled bipolar semiconductor device. Therefore, the gate of a thyristor requires its driving unit to provide an extremely steep spike current pulse signal to the gate. This ensures reliable triggering of the thyristor under various possible operating conditions.
The gate trigger pulse of a thyristor has a significant impact on the switching on and off of the thyristor. Therefore, the trigger voltage and current delivered by the gate drive unit of the thyristor must be greater than the maximum value of the trigger voltage UGT and trigger current IGT specified by the device gate, and sufficient margin must be allowed.
In practical applications, many manufacturers of thyristor-based high-voltage soft starters provide trigger pulse signal values that are at a critical level. Therefore, in some extremely harsh working environments, the thyristor may fail to trigger reliably.
2. Introduction to Thyristors
A thyristor is short for silicon controlled rectifier, also known as a silicon controlled rectifier. It was previously simply called a thyristor. The world's first thyristor product was developed by General Electric in 1957 and commercialized in 1958. A thyristor has a PNPN four-layer semiconductor structure with three terminals: anode, cathode, and gate. The operating conditions for a thyristor are: a forward voltage applied and a trigger current at the gate. Derivative devices include fast thyristors, bidirectional thyristors, reverse-conducting thyristors, and light-controlled thyristors. It is a high-power switching semiconductor device, represented in circuits by the symbols "V" or "VT" (formerly "SCR" in older standards), as illustrated in Figure 1.
Figure 1. Symbol of a thyristor
Thyristors possess the characteristics of silicon rectifier devices, enabling them to operate under high voltage and high current conditions. Furthermore, their operation is controllable, and they are widely used in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency converters.
3. Operating conditions of thyristors
Since thyristors only have two working states, on and off, they have switching characteristics. These characteristics require certain conditions to be converted, as shown in Table 1.
4. Main parameters of thyristors
4.1 Main Current Parameters of Thyristors
(1) Maintaining current IH
At room temperature, the minimum current required to keep a thyristor conducting when the control electrode is open is called the holding current. When the forward current is less than the IH value, the thyristor will turn off automatically. The IH value is typically tens to hundreds of milliamperes. It is related to the junction temperature; the higher the junction temperature, the smaller the IH, and the more difficult it is for the thyristor to turn off.
(2) Stop the current IL
IL is the minimum anode current required to maintain the thyristor's conduction state immediately after it transitions from the blocking state to the conducting state and the gate trigger signal is removed. The value of the latching current depends on the operating conditions, and is generally 2 to 4 times the holding current IH.
4.2 Key Gate Parameters of Thyristors
(1) Gate trigger current IGT
Gate trigger current IGT refers to the minimum gate DC current required to switch a thyristor from a blocking state to a conducting state under specified ambient temperature and a minimum voltage of 6V between the anode and cathode of the thyristor.
(2) Gate trigger voltage VGT
The gate trigger voltage VGT is the minimum gate DC voltage required to switch a thyristor from a blocking state to a conducting state under specified ambient temperature and a certain positive voltage between the anode and cathode of the thyristor. It is generally around 1.5V.
Because the gate current-voltage characteristics of thyristors exhibit significant variation, the standard only specifies the lower limits of IGT and VGT. When selecting devices, attention should be paid to the measured parameter values indicated on the product certificate. The current and voltage supplied to the gate by the trigger unit should be appropriately greater than the values listed on the thyristor's certificate of conformity, but should not exceed its peak values IFGM and UFGM. The average gate power PG and maximum gate power PGM should also not exceed the specified values.
5 Gate I-V Characteristics
A PN junction J3 exists between the gate and cathode of a thyristor, as shown in Figure 2. The gate voltage-current characteristic refers to the relationship between the forward gate voltage Ug and the gate current Ig at this PN junction, as shown in Figure 3.
Figure 2. Two-transistor model of a thyristor.
Because the gate current-voltage characteristics of thyristors are highly variable, it is impossible to find a typical representative curve. Therefore, a region between a limiting high-resistance gate characteristic and a limiting low-resistance gate characteristic can be used to represent the gate current-voltage characteristics of all thyristors, as shown in the shaded area of Figure 3. This region is called the gate current-voltage characteristic region. This region is the reliable triggering region. For a thyristor in normal use, its gate trigger current and voltage should both fall within this region. The reliable triggering region is defined by the gate forward peak current IFGM, the allowable instantaneous maximum power PGM, and the forward peak voltage UFGM.
Figure 3. Gate current-voltage characteristics of thyristors
Therefore, when designing a trigger circuit, the voltage and current of the trigger pulse generated by the trigger circuit must be greater than the gate trigger voltage and current of the corresponding thyristor in order to ensure that a qualified thyristor can work normally.
6. The effect of trigger pulse on thyristor turn-on
Thyristor triggering is a process; that is, the thyristor needs a certain amount of time to turn on. It is not an instantaneous process. The thyristor can only turn on when the anode current, i.e. the main circuit current, rises above the thyristor's latching current IL.
Because the positive feedback process inside the thyristor requires time, the increase in anode current cannot be instantaneous. The time from the moment the gate current steps up until the anode current rises to 10% of its steady-state value is called the delay time td. Simultaneously, the voltage drop between the anode and cathode decreases. The time required for the anode current to rise from 10% to 90% of its steady-state value is called the rise time.
The turn-on time tgt is defined as the sum of the two, i.e., tgt = td + tr. The turn-on time tgt of a common thyristor is about 6μs.
6.1 The effect of trigger pulse amplitude on thyristor turn-on
The delay time of a conventional thyristor decreases as the gate current increases. To shorten the turn-on time, the actual trigger current is often much larger than the specified trigger current.
For example, the turn-on time corresponding to a trigger pulse amplitude of Ig = 1A is 2.1μs, the turn-on time corresponding to Ig = 200mA is 3.2μs, and the turn-on time corresponding to Ig = 80mA is 20.2μs.
It is evident that the gate trigger current amplitude of a thyristor has a significant impact on the turn-on speed of the device; a high gate trigger current can significantly reduce the turn-on time of the device.
6.2 The effect of trigger pulse rise time steepness on thyristor turn-on
The turn-on time depends not only on the amplitude of the trigger pulse, but also on the steepness of the trigger pulse rise time. When the trigger pulse amplitude Ig = 500mA, a pulse rise time of 0.5μs corresponds to a turn-on time of 6μs, while a pulse rise time of 1.5μs corresponds to a turn-on time of 6.3μs.
Therefore, the steepness of the trigger pulse rise time has a significant impact on the thyristor's turn-on speed. A longer trigger pulse rise time effectively reduces the gate trigger current. Conversely, a steeper trigger pulse with a shorter rise time results in a shorter thyristor turn-on time.
Furthermore, if the trigger pulse is not wide enough, the thyristor will not be able to be triggered to conduct. Therefore, the width of the trigger pulse should be at least 6μs, and is generally taken as 20 to 50μs. For large inductive loads, since the current rises more slowly, the trigger pulse width should be increased. Otherwise, when the pulse ends, the main circuit current has not yet risen above the thyristor's latching current IL, and the thyristor will be turned off again. Therefore, the pulse width should not be less than 300μs, and is usually taken as 1ms, which is equivalent to 18° electrical angle of a 50Hz sine wave.
6.3 Gate Trigger Criteria for Thyristors in High-Voltage Soft Starters
Given the impact of the gate trigger pulse characteristics on the thyristor turn-on process, a good trigger pulse can shorten the device's turn-on time and reduce turn-on losses. To ensure the safe and reliable operation of thyristor-based high-voltage soft starters in any environment, manufacturers of thyristor-based high-voltage soft starters should ensure that the trigger pulse current amplitude IG is much greater than IGT, the pulse rise time tr is less than 1μs, and the gate pulse width is greater than 300μs in their products.
7. Conclusion
The amplitude and steepness of the gate trigger pulse affect the turn-on time of the thyristor. Increasing the amplitude and steepness shortens the turn-on time. Shortening the turn-on time reduces the turn-on losses in the circuit, which is beneficial for the safe operation of the thyristor in high-voltage soft-start applications.
References
[1] Huang Jun, Wang Zhaoan. Power Electronic Converter Technology [M]. Machinery Industry Press, 2004: 6-20.
[2] Wang Yi, Zhao Kaiqi, Xu Dianguo. Research on power factor angle in soft starter control system of motor [J]. Proceedings of the Chinese Society for Electrical Engineering, 2002 (8): 82-87.
[3] Xu Shizhang. Electrical Machines [M]. 3rd ed. Beijing: China Machine Press, 1996.
[4] Xu Honggang. Principle and application of soft starter [J]. Energy Technology, 2002(6):132-135.
[5] Wang Yufeng, Ma Guangcheng, Wang Changhong, et al. Study on oscillation phenomenon during the starting process of thyristor-controlled induction motor [J]. Journal of Electrical Machines and Control, 2002, 6(3): 186-190.
About the Author
He Xiaoping, male, is an electronics engineer specializing in power electronics and motor drives. He currently works at the R&D Center of Harbin Jiuzhou Electric Co., Ltd.
