Working principle diagram and application diagram of silicon controlled thyristor

Working principle diagram and application diagram of silicon controlled thyristor

SCR is the abbreviation of silicon controlled rectifier element. It is a high-power semiconductor device with a four-layer structure with three PN junctions. It is generally composed of two thyristors connected in reverse. Its function is not only rectification, but also can be used as a non-contact switch to quickly turn on or off; realize inversion of direct current into alternating current; convert one frequency of alternating current into another frequency of alternating current, etc. SCRs, like other semiconductor devices, have the advantages of small size, high efficiency, good stability, and reliable operation. Its emergence has brought semiconductor technology from the weak current field to the strong current field, and has become a component that is eagerly adopted in industry, agriculture, transportation, military scientific research, as well as commerce and civil appliances. Currently, thyristors are widely used in automatic control, electromechanical applications, industrial electrical and home appliances.

In terms of appearance, there are three main types of thyristors: spiral type, flat type and flat bottom type. Spiral applications are common.

The thyristor has three poles - anode (A), cathode (C) and control electrode (G). The tube core is a four-layer structure composed of overlapping P-type conductors and N-type conductors. There are three PN junctions. It is very different in structure from a silicon rectifier diode with only one PN junction. The four-layer structure of the thyristor and the introduction of the control electrode lay the foundation for its excellent control characteristics of "controlling the big with the small". When thyristor is used, as long as a small current or voltage is applied to the control electrode, a large anode current or voltage can be controlled. At present, thyristor components with a current capacity of several hundred amps or even thousands of amps can be manufactured. Generally speaking, the thyristors with less than 5 amps are called low-power thyristors, and the thyristors with more than 50 amps are called high-power thyristors.

We can think of the first, second, and third layers counting from the cathode upward as an NPN transistor, while the second, third, and fourth layers form another PNP transistor. The second and third floors are shared by two overlapping pipes. The equivalent circuit diagram of Figure 1 can be drawn. When a forward voltage E is applied between the anode and the cathode, and a positive trigger signal is input between the control electrode G and the cathode C (equivalent to the base-radiator of BG2), BG2 will generate a base current Ib2. Amplified, BG2 will have a collector current IC2 amplified by β2 times. Because the collector of BG2 is connected to the base of BG1, IC2 is the base current Ib1 of BG1. BG1 then sends Ib1 (Ib2) amplified by β1’s collector current IC1 back to the base of BG2 for amplification. This cycle of amplification continues until BG1 and BG2 are completely turned on. In fact, this process is "on the verge of triggering". For the thyristor, when the trigger signal is added to the control electrode, the thyristor turns on immediately. The conduction time mainly depends on the performance of the thyristor.

Once the thyristor is triggered and turned on, due to cyclic feedback, the current flowing into the base of BG2 is not just the initial Ib2, but the current amplified by BG1 and BG2 (β1*β2*Ib2). This current is much larger At Ib2, it is enough to maintain the continuous conduction of BG2. Even if the trigger signal disappears at this time, the thyristor will still remain in the conductive state. Only when the power supply E is disconnected or the output voltage of E is reduced so that the collector current of BG1 and BG2 is less than the minimum value to maintain conduction, the thyristor can Shut down. Of course, if the polarity of E is reversed, BG1 and BG2 will be in a cut-off state due to the reverse voltage. At this time, even if the trigger signal is input, the thyristor cannot work. On the contrary, E is connected in positive direction, but the trigger signal is negative, and the thyristor cannot conduct. In addition, if no trigger signal is added and the forward anode voltage exceeds a certain value, the thyristor will still conduct, but it will be an abnormal working condition.

The controllable characteristic of thyristor, which controls conduction (large current through thyristor) through trigger signal (small trigger current), is an important feature that distinguishes it from ordinary silicon rectifier diodes.

Since the thyristor has only two working states: on and off, it has switching characteristics. This characteristic requires certain conditions to be transformed. These conditions are shown in Table 1.

Table 1 SCR turn-on and turn-off conditions

Status condition description

From off to on 1, the anode potential is higher than the cathode potential

2. The control pole has sufficient forward voltage and current, both of which are indispensable.

Maintain conduction 1. The anode potential is higher than the cathode potential

2. The anode current is greater than the holding current, and both are indispensable.

From turn-on to turn-off 1. The anode potential is lower than the cathode potential

2. The anode current is less than the holding current under any condition.

Application examples:

In practical applications of silicon controlled thyristors, the most diverse circuits are their gate trigger circuits, which can be summarized as DC trigger circuits, AC trigger circuits, phase trigger circuits, etc.

1. DC trigger circuit:

Figure 2 is an overvoltage protection circuit commonly used in televisions. When the E+ voltage is too high, the voltage at point A also becomes high. When it is higher than the voltage stabilization value of the voltage regulator tube DZ, the DZ channel passes, and the thyristor D is triggered. Daotong will short-circuit E+, causing the fuse RJ to blow, thus playing the role of overvoltage protection.

2. Phase trigger circuit:

The phase trigger circuit is actually a type of AC trigger circuit, as shown in Figure 3. The method of this circuit is to use an RC loop to control the phase of the trigger signal. When the R value is small, the RC time constant is small, and the phase shift A1 of the trigger signal is small, so the load obtains greater electrical power; when the R value is large, the RC time constant is large, and the phase shift A2 of the trigger signal is small. Large, so the load gets less electrical power. This typical stepless adjustment circuit of electric power is used in many electrical products in daily life.

The main parameters of silicon controlled silicon are:

1. Rated average on-state current

Under certain conditions, the average value of the 50 Hz sinusoidal half-wave current that can pass continuously between the anode and the cathode.

2. Forward blocking peak voltage

When the control electrode is open without a trigger signal and the anode forward voltage has not exceeded the conduction voltage, the forward peak voltage can be repeatedly applied to both ends of the thyristor. The peak forward voltage that the thyristor can withstand cannot exceed the parameter value given in the manual.

3. Reverse negative interruption peak voltage

When a reverse voltage is applied to the thyristor and it is in the reverse off state, the reverse peak voltage can be repeatedly applied to both ends of the thyristor. When used, the parameter value given in the manual cannot be exceeded.

4. Control pole trigger current

Under the specified ambient temperature, a certain voltage is applied between the anode and the cathode, which is the minimum control electrode current and voltage required to turn the thyristor from the off state to the on state.

5. Maintain current

At the specified temperature, the control electrode is open and the minimum anode forward current necessary to maintain the conduction of the thyristor is maintained.

Using thyristor technology to control the lighting system has the following features: fast voltage adjustment, high precision, real-time adjustment in time intervals, voltage stabilization, use of electronic components, relatively small size, light weight, and low cost. However, this voltage regulation method has a fatal flaw. Due to chopping, the voltage cannot achieve sine wave output, and a large number of harmonics will appear, causing harmonic pollution to the power grid system and causing great harm. It cannot be used in capacitor compensation circuits. (Modern lighting design requirements stipulate that the power factor in the lighting system must be above 0.9, while the power factor of gas discharge lamps is generally below 0.5, so capacitors are designed to compensate for the power factor.) In developed countries abroad, there are explicit regulations on electrical Restrictions on the harmonic content of equipment. In China, large cities such as Beijing, Shanghai, and Guangzhou have restricted the integration of equipment with excessive harmonic content into the power grid for use.

When using thyristor technology to control the illumination of the lighting system, filtering equipment can be installed to effectively reduce harmonic pollution.

In recent years, many new thyristor components have come out, such as fast thyristors suitable for high-frequency applications, bidirectional thyristors that can be controlled in two directions by positive or negative trigger signals, and bidirectional thyristors that can be controlled by positive trigger signals. It is turned on, a thyristor that uses a negative trigger signal to turn it off, etc.

Application introduction------Application of thyristor in dimmer:

Triac dimmer is currently the mainstream equipment in the field of stage lighting and ambient lighting.

Various dimmers used in lighting systems are essentially AC voltage regulators. Old-fashioned transformers and rheostat dimming are achieved by adjusting the amplitude of voltage or current, as shown in the figure below. u1 is the waveform of 220V alternating current without voltage regulation. The voltage waveform after voltage regulation is u2. Since its amplitude is smaller than u1, the light becomes dim. In this dimming mode, although the amplitude of the sinusoidal alternating current is changed, the essence of its sinusoidal waveform is not changed.

Compared with transformers and resistors, thyristor dimmers have a completely different dimming mechanism. They use phase control methods to achieve voltage regulation or dimming. For ordinary reverse blocking thyristors, the thyristor characteristics are as follows: when the forward anode voltage is applied to the thyristor and the appropriate forward control voltage is applied at the same time, the thyristor is turned on; this conduction The pass will remain on even after the gate control voltage is removed, and it will not turn off until the reverse anode voltage is added or the anode current is less than the holding current of the thyristor itself. Ordinary silicon-controlled dimmers use this characteristic of silicon-controlled silicon to achieve leading-edge trigger phase-controlled voltage regulation. At a certain moment t1 (or a certain phase angle wt1) after the sine wave alternating current crosses zero, a trigger pulse is added to the control electrode of the thyristor to turn on the thyristor. According to the switching characteristics of the thyristor introduced earlier , this conduction will be maintained until the end of the positive half cycle of the sine wave. Therefore, in the positive half cycle of the sine wave (i.e., the 0~p interval), the thyristor does not conduct between 0 and wt1. This range is called the control angle, often represented by a; while between wt1 and p, the thyristor conducts , this range is called the conduction angle, often represented by j. In the same way, in the negative half cycle of the sine wave alternating current, another thyristor in reverse connection (for two one-way thyristors in anti-parallel connection or a two-way thyristor) is applied at time t2 (that is, the phase angle wt2) Trigger the pulse to turn it on. This goes on and on, controlling the conduction of the sine wave for every half cycle to obtain the same conduction angle. If the application time (or phase) of the trigger pulse is changed, the size of the conduction angle j (or control angle a) is changed. The larger the conduction angle, the higher the voltage the dimmer outputs and the brighter the light. From the above-mentioned thyristor dimming principle, we can know that the voltage waveform output by the dimmer is no longer a sine wave, unless the dimmer is in a fully conductive state, that is, the conduction angle is 180° (or p). It is precisely because the sine wave is cut and the waveform is damaged that it will cause problems such as interference to the power grid...

Good dimming equipment should take necessary measures to try to reduce the interference caused by using thyristor technology.