How the two-way thyristor works

Photocoupler

How the two-way thyristor works

The working principle of the two-way thyristor 1. 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. When anode A is applied with a forward voltage, both BG1 and BG2 tubes are in an amplified state. At this time, if a forward trigger signal is input from the gate G, the BG2 has a base stream ib2 flowing through it, and the collector current ic2=β2ib2 is amplified by the BG2. Since the collector of BG2 is directly connected to the base of BG1, ib1 = ic2. At this time, the current ic2 is amplified by BG1, so that the collector current ic1 of BG1 = β1ib1 = β1β2ib2. This current flows back to the base of BG2, which is positive feedback, which increases the ib2. As a result of the forward feed loop, the current of the two tubes increases sharply, and the thyristor turns the saturation on.

Due to the positive feedback effect of BG1 and BG2, once the thyristor is turned on, even if the current of the gate G disappears, the thyristor can maintain the conduction state, since the trigger signal only acts as a trigger, and does not turn off. Function, so this thyristor is not switchable. Since the thyristor has only two operating states: on and off, it has a switching characteristic that requires certain conditions to be converted. 2, triggering conduction when a forward voltage is applied to the gate G (see Figure 5) Because J3 is positively biased, the holes in the P2 region enter the N2 region, and the electrons in the N2 region enter the P2 region to form the trigger current IGT. On the basis of the internal positive feedback of the thyristor (see Figure 2), plus the effect of the IGT, the thyristor is turned on in advance, resulting in the OA section of the volt-ampere characteristic of Figure 3 shifting to the left, the larger the IGT, the characteristic left Move faster.

TRIAC characteristics

What is a two-way thyristor: IAC (TRI-ELECTRODE AC SWITCH) is a three-pole AC switch, also known as a triac or a two-way thyristor. TRIAC is a three-terminal component, the three ends of which are T1 (second terminal or second anode), T 2 (first terminal or first anode) and G (control electrode) are also a gate control switch, and SCR maximum The difference is that the TRIAC can be turned on regardless of the forward or reverse voltage, its symbol structure and appearance, as shown in Figure 1. Because it is a bidirectional component, regardless of the polarity of the voltage of T1 and T2, if the gate has a signal, T1 and T2 are in a conducting state; otherwise, if the gate triggering signal is applied, there is a pole between T1 and T2. High impedance.

(a) symbol (b) construction

Figure 1 TRIAC

2. Trigger characteristics of TRIAC:

Since the TRIAC is a controllable bidirectional thyristor, the four combinations of the polarity of the gate voltage VG and the voltage VT1T2 between the anodes are as follows:

(1). VT1T2 is positive and VG is positive.

(2). VT1T2 is positive and VG is negative.

(3). VT1T2 is negative and VG is positive.

(4). VT1T2 is negative and VG is negative.

It is generally best to use symmetrical conditions (1 and 4 or 2 and 3) to achieve symmetrical results for positive and negative half cycles. The most convenient control method is the control state of 1 and 4, because the gate signal is the same as VT1T2. polarity.

Figure 2 TRIAC VI characteristic curve

Figure 2 shows the VI characteristic curve of TRIAC. Comparing this figure with the VI characteristic curve of SCR, it can be seen that the characteristic curve of TRIAC is similar to SCR, except that the positive and negative voltages of TRIAC can be turned on, so the curve of the third quadrant Similar to the curve of the first quadrant, the TRIAC can be regarded as the breakdown voltage of the T1-T2 of the two SCR anti-parallel TRIACs. It can also be seen that the voltage of the positive and negative half cycles can make the TRIAC turn on, generally making the TRIAC cut off. The method is the same as the SCR, that is, trying to reduce the current between the two anodes to the holding current below the TRIAC is cut off.

Trigger of TRIAC:

The phase control of TRIAC is similar to that of SCR. It can be triggered by DC signal, AC phase signal and pulse signal. The difference is that TRIAC can still be triggered when V T1-T2 is negative voltage.

Phase control of TRIAC:

The phase control of TRIAC is similar to that of SCR, but because TRIAC can be dual-conducting, it can be triggered in both positive and negative half cycles, and can be used as full-wave power control. Therefore, TRIAC has the advantages of SCR and is more convenient for AC power control. Fig. 3(a) shows the TRIAC phase control circuit. The excitation angle can be changed only by adjusting the RC time constant appropriately. Figures 3(b) and (c) show the VT1-T2 and the load at the excitation angle of 30 degrees. The voltage waveform, generally TRIAC can control the load is much smaller than the SCR, in general, about 600V, 40A or less.

(A)

(B) Voltage waveform at both ends of the AC voltage waveform (C)

5. Trigger device:

The trigger circuit of TRIAC is similar to SCR. It can be triggered by RC circuit with trigger circuit composed of UJT, PUT, DIAC and other components. The trigger delay angle of these components. It can be adjusted by changing the resistance value used by the circuit. The variation range is between 0° and 180°, and it can be turned on in both positive and negative half cycles. In industrial power control, the voltage feedback is often used to adjust the trigger delay angle. The voltage feedback is used to represent the actual situation of the load, and the system is started to perform good closed loop control. This feedback control delay angle is usually done by UJT or TCA785.

experiment:

Application circuit description

As shown in the figure, the TRIAC phase control circuit composed of TCA785 has the same operation principle as the TCR785 phase control circuit of SCR. Since the TRIAC can be turned on during the positive and negative half cycles of the power supply, the 14th pin (controls the excitation angle of the positive half cycle). ) and the 15th foot (control the negative half-cycle firing angle) must be used. By changing the VR1 to change the control voltage value of the 11th pin, the excitation angle can be adjusted to control the brightness of the bulb.

Using TCA785 for phase control of TRIAC

Principle and application of two-way thyristor structure

The bidirectional thyristor is developed on the basis of ordinary thyristor. It can replace not only two thyristors connected in parallel with reverse polarity, but also requires only one trigger circuit. It is an ideal AC switch device. Its English name TRIAC means the three-terminal two-way AC switch.

Construction principle

Although the bidirectional thyristor can be viewed as a combination of two common thyristors, it is actually a power integrated device composed of seven transistors and a plurality of resistors. Low-power two-way thyristors are generally packaged in plastic, and some have heat sinks, as shown in Figure 1. Typical products are BCMlAM (1A/600V), BCM3AM (3A/600V), 2N6075 (4A/600V), MAC218-10 (8A/800V), and the like. High-power triacs are mostly packaged in the RD91 package. The main parameters of the triac are shown in the attached table.

The structure and symbol of the triac are shown in Figure 2. It belongs to the NPNPN five-layer device, and the three electrodes are T1, T2, and G, respectively. Since the device can be bi-directionally connected, the two electrodes except the gate G are collectively referred to as the main terminal, and T1 and T2 are used. Indicates that it is no longer divided into an anode or a cathode. It is characterized in that when the voltages of the G pole and the T2 pole are relative to T1, the voltage is positive, T2 is the anode, and T1 is the cathode. Conversely, when the voltages of the G and T2 poles are negative with respect to T1, T1 becomes the anode and T2 becomes the cathode. The volt-ampere characteristics of the triac are shown in Figure 3. Since the forward and reverse characteristic curves are symmetrical, they can be turned on in either direction.

Detection method

The following describes the method of determining the bidirectional thyristor electrode using the multimeter RXl file, and also checks the triggering capability.

1. Determine the T2 pole

As can be seen from Fig. 2, the G pole is very close to T1 and is far from T2. Therefore, the positive and negative resistances between G and T1 are small. When measuring the resistance between any two legs with RXl file, only the low resistance exists between G-T1, the positive and negative resistances are only tens of ohms, and the positive and negative between T2-G and T2-T1. The resistance is infinite. This shows that if one foot and the other two feet are not connected, it is definitely the T2 pole. In addition, the T2-pole is usually connected to the small heat sink by using a bidirectional thyristor in the TO-220 package, and the T2 pole can also be determined accordingly.

2. Distinguish between G pole and T1 pole

(1) After finding the T2 pole, first assume that one of the remaining two legs is the T1 pole and the other leg is the G pole.

(2) Connect the black pen to the T1 pole, the red pen to the T2 pole, and the resistance to infinity. Then use the red tip to short-circuit T2 and G, and add a negative trigger signal to the G-pole. The resistance should be about ten ohms (see Figure 4(a)), which proves that the tube is turned on and the conduction direction is T1-T2. Then disconnect the red tip from the G pole (but still connect to T2). If the resistance remains the same, it is proved that the tube can maintain the conduction state after the trigger (see Figure 4(b)).

(3) Connect the red test lead to the T1 pole, the black test lead to the T2 pole, then short-circuit the T2 and G, and add a positive trigger signal to the G pole. The resistance value is still about ten ohms. If the resistance is not after the G pole is disconnected, Change, it means that after the tube is triggered, it can maintain the conduction state in the T2-T1 direction, so it has the bidirectional triggering property. This proves that the above assumption is correct. Otherwise, it is assumed that it does not match the actual situation. It is necessary to make another assumption and repeat the above measurement. Obviously, in the process of identifying G and T1, the triggering ability of the triac is also checked. If the measurement is made according to which assumption, the triac will not be triggered to conduct, which proves that the tube is damaged. For the tube of lA, it can also be detected by RXl0 file. For tubes of 3A and above, the RXl file should be selected, otherwise it is difficult to maintain the conduction state.

typical application

The two-way thyristor can be widely used in industries, transportation, household appliances, etc., and realizes various functions such as AC voltage regulation, motor speed regulation, AC switch, automatic opening and closing of street lamps, temperature control, table lamp dimming, stage dimming, etc. Used in solid state relay (SSR) and solid state contactor circuits. Figure 5 is a proximity switch circuit composed of a triac. R is the gate limit current resistance and JAG is the dry reed pipe. Normally JAG is disconnected and the triac TRIAC is also turned off. The JAG pulls in only when the small magnet moves closer, turning the triac on and turning the load power on. Due to

The current of the reed switch is very small, only a few microseconds, so the life of the switch is very long.

Figure 6 shows the internal circuit of a zero-crossing triggered AC solid state relay (AC-SSR). It mainly includes input circuit, optocoupler, zero-crossing trigger circuit, switching circuit (including bidirectional thyristor), and protection circuit (RC absorption network). When the input signal VI (generally high) is applied and the AC load supply voltage passes through zero, the triac is triggered to turn the load power on. Solid-state relays have low drive power, no contact, low noise, strong anti-interference ability, short pull-in, short release time, long life, compatible with TTL\CMOS circuits, and can replace traditional electromagnetic relays.