SCR, short for thyristor rectifier, is a four-layer high-
Power Semiconductor Device with three PN junctions, also known as a thyristor. It has the characteristics of small size, relatively simple structure and strong function, and is one of the more commonly used semiconductor devices. The device is widely used in various electronic equipment and electronic products, and is used for controllable rectification, inverter, frequency conversion, voltage regulation, and non-contact switching.
concept
The thyristor is also called a Silicon Coin Rolled Rectifier (SCR). It has grown into a large family since its inception in the 1950s. Its main members are unidirectional thyristors, bidirectional thyristors, light-controlled thyristors, reverse-thyristing thyristors, turn-off thyristors, fast thyristors, and so on.
A thyristor device is a very important power device that can be used for high voltage and high current control. The thyristor device is mainly used in the switching aspect to switch the device from a closed or blocked state to an on or off state, and vice versa. The thyristor device has a close relationship with the bipolar transistor. The conduction process of both is related to electrons and holes, but the switching mechanism of the thyristor is different from that of the bipolar transistor, and the device structure is different and controllable. Silicon devices have a wide range of current and voltage control capabilities. Today's thyristor devices are rated for currents from a few milliamps to more than 5,000 amps and rated voltages in excess of 10,000 volts. The working principle of the basic thyristor device will be discussed below, followed by some high power and high frequency thyristor devices.
structure
Today, we use a unidirectional thyristor, which is commonly referred to as a common thyristor. It consists of four layers of semiconductor material with three PN junctions and three external electrodes (Fig. 2(a)): first layer P The electrode drawn by the semiconductor is called anode A, the electrode of the third layer of P-type semiconductor is called gate G, and the electrode of the fourth layer of N-type semiconductor is called cathode K. As can be seen from the circuit symbol of the thyristor [Fig. 2(b)], it is a unidirectional conductive device like the diode. The key is to add a control electrode G, which makes it have completely different operating characteristics from the diode. .
P1N1P2N2 four-layer three-terminal device based on silicon single crystal, started in 1957, because its characteristics are similar to vacuum thyristors, so it is known internationally as silicon thyristor, referred to as thyristor T, and because of thyristor Initially in terms of static rectification, it is also known as a
Silicon Controlled Rectifier component, referred to as a thyristor SCR.
In terms of performance, thyristor not only has unidirectional conductivity, but also has more controllability than silicon rectifying components (commonly known as "dead silicon"). It has only two states: on and off.
The thyristor can control high-power electromechanical devices with milliampere current. If this frequency is exceeded, the average switching current is allowed to decrease due to the significant increase in component switching losses. At this time, the nominal current should be degraded.
The advantages of thyristor are many, for example: controlling high power with low power, power amplification up to several hundred thousand times; fast response, turning on and off in microseconds; no contact operation, no spark, no noise; High efficiency, low cost, etc.
The weakness of thyristor: static and dynamic overload capability is poor; it is easily interfered and misdirected.
The thyristors are mainly classified into a bolt shape, a flat plate shape and a flat bottom shape.
Structure of thyristor element
Regardless of the shape of the thyristors, their dies are four-layer P1N1P2N2 structures consisting of P-type silicon and N-type silicon. See Figure 1. It has three PN junctions (J1, J2, J3), from the J1 structure. The P1 layer leads to the anode A, the cathode K is taken out from the N2 layer, and the gate G is taken out from the P2 layer, so it is a four-layer three-terminal semiconductor device.
How thyristor components work
The thyristor is a four-layer three-terminal structural component of P1N1P2N2. There are three PN junctions. When analyzing the principle, it can be regarded as consisting of a PNP tube and an NPN tube. The equivalent diagram is shown as the main thyristor in the right figure. Working characteristics
In order to intuitively understand the working characteristics of the thyristor, let's look at this teaching board (Figure 3). The thyristor VS is connected in series with the small bulb EL and is connected to the DC power source through the switch S. Note that the anode A is connected to the positive pole of the power supply, the cathode K is connected to the negative pole of the power supply, and the control pole G is connected to the positive pole of the 1.5V DC power supply through the push button switch SB (the KP1 type thyristor is used here. If the KP5 type is used, it should be connected to the 3V DC. The positive pole of the power supply). This connection of the thyristor to the power supply is called a forward connection, that is, a positive voltage is applied to the thyristor anode and the control electrode. Now we close the power switch S, the small bulb does not light, indicating that the thyristor is not conducting; press the button switch SB again, input a trigger voltage to the control pole, the small bulb lights up, indicating that the thyristor is turned on. What inspired this demonstration experiment?
This experiment tells us that to turn on the thyristor, one is to apply a forward voltage between its anode A and cathode K, and the other is to input a forward trigger voltage between its gate G and cathode K. After the thyristor is turned on, release the push button switch to remove the trigger voltage and still maintain the conduction state.
Thyristor characteristics
"At the touch of a hair." However, if the anode or the control electrode is applied with a reverse voltage, the thyristor cannot be turned on. The role of the gate is to turn the thyristor on by applying a positive trigger pulse, but it does not turn it off. So, what method can be used to turn off the turned-on thyristor? Turn off the turned-on thyristor, disconnect the anode power supply (switch S in Figure 3) or make the anode current less than the minimum value that maintains conduction. Maintain current). If an alternating voltage or a pulsating DC voltage is applied between the thyristor anode and the cathode, the thyristor will turn itself off when the voltage crosses zero.
How to test the quality of thyristors
Can you use a multimeter to distinguish the three electrodes of a thyristor? How do you test the quality of a thyristor?
The three electrodes of a common thyristor can be measured with a multimeter ohmic block R x 100 gear. As we all know, there is a PN junction between the thyristors G and K (Fig. 2(a)), which is equivalent to a diode. G is the positive electrode and K is the negative electrode. Therefore, according to the method of testing the diode, two of the three poles are found. One pole, measuring its positive and negative resistance, the resistance is small, the multimeter red pen is connected to the control pole G, the black meter is connected to the cathode K, and the remaining one is the anode A. To test the quality of the thyristor, you can use the teaching board circuit just shown (Figure 3). Turn on the power switch S, press the button switch SB, the light bulb is good, no light is bad.
