About Triac dimming
author: Sincerelux
2024-09-19
TRIAC, also known as a silicon-controlled rectifier (SCR), is a high-power semiconductor device with a four-layer structure composed of three P-N junctions. It is generally constructed by connecting two thyristors in reverse. Its functionality is not only rectification but also rapid on/off switching without contact, conversion of DC to AC, and frequency conversion. Like other semiconductor devices, thyristors have the advantages of small volume, high efficiency, good stability, and reliable operation. Their existence has allowed semiconductor technology to enter the field of strong electricity and become an element widely adopted in industrial, agricultural, transportation, and military research, as well as commercial and domestic appliances. Currently, thyristors have a wide range of applications in automatic control, mechatronics, industrial electric systems, and home appliances.
There are mainly three types of thyristors: spiral, flat, and flat-bottomed. The spiral type is more widely used. The thyristor has three electrodes, the anode (A), cathode (), and gate (G). The core of the thyristor is a four-layer structure composed of P-type and N-type conductors, with three P-N junctions. This structure is different from the silicon rectifier diode, which only has a single P-N junction. The four-layer structure of the thyristor and the introduction of the gate laid the foundation for its excellent "control big with small" control characteristics. When a small current or voltage is applied to the gate, a large current or voltage can be controlled between the anode and cathode. Thyristors with a current capacity of several hundred amperes to more than one thousand amperes have been manufactured. Thyristors below 5 amperes are generally called small-power thyristors, while those above 50 amperes are called large-power thyristors. One can see from Figure 1 that the first, second, and third layers counted from the cathode constitute NPN transistors, and the second, third, and fourth layers constitute another PNP transistor. The second and third layers are overlappingly shared by both transistors. When a positive voltage E is applied between the anode and cathode, and a positive trigger signal is input between the gate (G) and cathode (C) (similar to the base-emitter connection of BG2), BG2 will generate a base current Ib2, which is amplified and generates a collector current IC2 that is amplified by β2 times. Because the collector of BG2 is connected to the base of BG1, IC2 becomes the base current of BG1. BG1 further amplifies Ib1(b2) by β1 and sends it back to the base of BG2. This process continues until BG1 and BG2 are completely turned on. In fact, this process is a "one-trigger discharge." For a thyristor, once a trigger is applied to the gate, it will immediately turn on. The conductivity time of the thyristor mainly depends on its performance.
Once the thyristor is triggered, due to the feedback loop, the current flowing into the base of BG2 is no longer just the initial lb2 but is amplified by the BG1 and BG2 transistors (81 * B2 * 1b2), which is far greater than b2 and enough to maintain continuous conduction of BG2. Even if the trigger signal disappears at this time, the thyristor will still remain in a conducting state. Only when the power supply E is turned off or the output voltage of E is lowered such that the collector current of BG1 and BG2 is less than the minimum value to maintain conduction, can the thyristor be turned off. Of course, if the polarity of E is reversed, BG1 and BG2 will be cut off by the reverse voltage. At this time, even if an input trigger signal is provided, the thyristor will not work. Conversely, if E is connected as positive and the trigger signal is negative, the thyristor will not conduct. In addition, if no trigger signal is applied, the thyristor will conduct when the positive anode voltage is greater than a certain value, but this is an abnormal operating condition.
The controllable characteristic of the thyristor, which controls conductivity (with a large current) by the trigger signal (a small trigger current), is its important feature distinguishes it from ordinary silicon rectifier diodes.
There are mainly three types of thyristors: spiral, flat, and flat-bottomed. The spiral type is more widely used. The thyristor has three electrodes, the anode (A), cathode (), and gate (G). The core of the thyristor is a four-layer structure composed of P-type and N-type conductors, with three P-N junctions. This structure is different from the silicon rectifier diode, which only has a single P-N junction. The four-layer structure of the thyristor and the introduction of the gate laid the foundation for its excellent "control big with small" control characteristics. When a small current or voltage is applied to the gate, a large current or voltage can be controlled between the anode and cathode. Thyristors with a current capacity of several hundred amperes to more than one thousand amperes have been manufactured. Thyristors below 5 amperes are generally called small-power thyristors, while those above 50 amperes are called large-power thyristors. One can see from Figure 1 that the first, second, and third layers counted from the cathode constitute NPN transistors, and the second, third, and fourth layers constitute another PNP transistor. The second and third layers are overlappingly shared by both transistors. When a positive voltage E is applied between the anode and cathode, and a positive trigger signal is input between the gate (G) and cathode (C) (similar to the base-emitter connection of BG2), BG2 will generate a base current Ib2, which is amplified and generates a collector current IC2 that is amplified by β2 times. Because the collector of BG2 is connected to the base of BG1, IC2 becomes the base current of BG1. BG1 further amplifies Ib1(b2) by β1 and sends it back to the base of BG2. This process continues until BG1 and BG2 are completely turned on. In fact, this process is a "one-trigger discharge." For a thyristor, once a trigger is applied to the gate, it will immediately turn on. The conductivity time of the thyristor mainly depends on its performance.
Once the thyristor is triggered, due to the feedback loop, the current flowing into the base of BG2 is no longer just the initial lb2 but is amplified by the BG1 and BG2 transistors (81 * B2 * 1b2), which is far greater than b2 and enough to maintain continuous conduction of BG2. Even if the trigger signal disappears at this time, the thyristor will still remain in a conducting state. Only when the power supply E is turned off or the output voltage of E is lowered such that the collector current of BG1 and BG2 is less than the minimum value to maintain conduction, can the thyristor be turned off. Of course, if the polarity of E is reversed, BG1 and BG2 will be cut off by the reverse voltage. At this time, even if an input trigger signal is provided, the thyristor will not work. Conversely, if E is connected as positive and the trigger signal is negative, the thyristor will not conduct. In addition, if no trigger signal is applied, the thyristor will conduct when the positive anode voltage is greater than a certain value, but this is an abnormal operating condition.
The controllable characteristic of the thyristor, which controls conductivity (with a large current) by the trigger signal (a small trigger current), is its important feature distinguishes it from ordinary silicon rectifier diodes.
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