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What is the functional difference between a MOS tube and a triode? These two components themselves can be regarded as a basic unit, an independent device. Even if the casing is disassembled, there is no difference with the naked eye. From the understanding of the working principle, it is difficult to understand. This time, from a simple touch Light circuit to feel the functional difference between the two.

The touch light circuit is as follows. The lower right corner of the figure also illustrates the circuit operation process. If you touch the G pole (gate) of the MOS tube and the positive pole of the power supply at the same time, the LED light will light up, and the light will continue to light even if you release your hand. , as to why this happens, it can be simply understood as: when the potential of the G pole rises, a conductive channel will be formed between the D pole (drain) and the S pole (source), and this channel is equivalent to a wire, so the line is on.

The voltage drop between the D pole and the S pole can also be measured with a multimeter when it is turned on. The voltage drop is close to 0 V, which was introduced in yesterday's article. If you touch the G pole and the negative pole of the power supply with your hand at the same time, then the LED light will go out. The reason for this is that the channel is pinched off, which is just the opposite of when it is turned on.

So if the MOS tube is replaced with a triode, is it still the same? The schematic diagram of the replacement is the circuit on the right in the figure below. In fact, it is replaced with a triode. At the same time, touch the base of the triode and the positive pole of the power supply (equivalent to the gate of the MOS tube and the positive pole of the power supply). The LED light will also be on, but when the hand is released After that, the LED light will go out immediately. Why is it different from using a MOS tube? Let's recall the conditions when the triode works in the amplified state: the emitter junction is forward-biased, and the collector junction is reverse-biased. If you touch the base and the positive pole of the power supply with your hand at the same time, these two conditions are satisfied, so the triode is turned on, and the LED light is also on; if you release your hand at this time, the reverse bias of the collector junction is still satisfied, but the forward bias of the emitter junction is still satisfied. It is not satisfied, so the triode will be cut off, and the external performance is that the LED light will be extinguished.

It can also be felt from this that MOS tubes are indeed easier to use than triodes. Indeed, MOS tubes have many advantages compared with triodes. For example, MOS tubes are voltage-controlled devices and triodes are current-controlled devices, so MOS tubes are more energy-efficient. The MOS tube only participates in the conduction of the majority carriers, and both the majority and minority carriers in the triode participate in the conduction, so the thermal stability of the MOS tube is better. The flexibility of MOS tube is better than that of triode. The MOS tube is easier to integrate in the process. When the MOS tube is turned on, the conduction voltage drop is small (close to 0 V, while the triode works in the amplified state and is greater than 0.3 V).

The advantages of MOS tubes are not limited to the points listed in the article. Interested friends can check the relevant information

Silicon Controlled Rectifier (SCR) for short, is a high-power electrical component, also known as thyristor. It has the advantages of small size, high efficiency and long life. In the automatic control system, it can be used as a high-power drive device to realize the control of high-power equipment with low-power controls. It has been widely used in AC/DC motor speed regulation system, power regulation system and servo system.

The thyristor is divided into two types: one-way thyristor and two-way thyristor. Triac is also called triac, or TRIAC for short. A triac is structurally equivalent to two unidirectional thyristors connected in reverse, and this thyristor has a bidirectional conduction function. Its on-off state is determined by the control electrode G. Adding a positive pulse (or negative pulse) to the gate G can make it conduct forward (or reverse) direction. The advantage of this device is that the control circuit is simple and there is no reverse withstand voltage problem, so it is especially suitable for use as an AC non-contact switch.

What everyone uses is a unidirectional thyristor, which is often referred to as an ordinary thyristor.

The thyristor is composed of four layers of semiconductor materials, with three PN junctions and three external electrodes [Figure 2(a)]: the electrode drawn from the first layer of P-type semiconductor is called anode A, and the third layer of P-type semiconductor is called anode A. The electrode drawn from the semiconductor is called the control electrode G, and the electrode drawn from the fourth layer of N-type semiconductor is called the cathode K. From the circuit symbol of the thyristor (Figure 2(b)), it can be seen that it is a unidirectional conductive device like a diode. The key is to add a gate G, which makes it have completely different working characteristics from the diode. .

The P1N1P2N2 four-layer three-terminal device with silicon single crystal as the basic material, started in 1957, because its characteristics are similar to vacuum thyristor, so it is commonly known as silicon thyristor in the world, referred to as thyristor T, and because thyristor Initially, in terms of static rectification, it is also called a silicon controlled rectifier element, or SCR for short.

In terms of performance, thyristor not only has unidirectional conductivity, but also has more valuable controllability than silicon rectifier components (commonly known as dead silicon). It has only two states of on and off.

The thyristor can control high-power electromechanical equipment with milliampere current. If the power exceeds this power, the switching loss of the components will increase significantly, and the average current allowed to pass will decrease. At this time, the nominal current should be degraded.

The thyristor has many advantages, such as: controlling high power with low power, the power amplification factor is as high as hundreds of thousands of times; the response is extremely fast, turning on and off in microseconds; contactless operation, no spark, no noise; High efficiency, low cost and so on.

Weaknesses of thyristor: poor static and dynamic overload capacity; easy to be misled by interference.

The thyristor is mainly classified in terms of shape: bolt-shaped, flat-shaped and flat-bottomed.

The structure of the thyristor element

Regardless of the shape of the thyristor, their die is a four-layer P1N1P2N2 structure composed of P-type silicon and N-type silicon. see picture 1. It has three PN junctions (J1, J2, J3), the anode A is drawn from the P1 layer of the J1 structure, the cathode K is drawn from the N2 layer, and the control electrode G is drawn from the P2 layer, so it is a four-layer three-terminal semiconductor device .

Schematic diagram and symbol diagram of thyristor structure

How it worksEdit

Structural original

The thyristor is a P1N1P2N2 four-layer three-terminal structural element with three PN junctions. When analyzing the principle, it can be regarded as composed of a PNP tube and an NPN tube, and its equivalent diagram is shown in the right figure. Triac: Triac is a silicon-controlled rectifier device, also known as a triac. This device can realize non-contact control of alternating current in the circuit, control large current with small current, and has the advantages of no spark, fast action, long life, high reliability and simplified circuit structure. From the outside, triacs are very similar to ordinary thyristors, and they also have three electrodes. However, except that one of the electrodes G is still called the control electrode, the other two electrodes are usually no longer called the anode and the cathode, but are collectively called the main electrodes T1 and T2. Its symbol is also different from that of ordinary thyristors. It is drawn by connecting two thyristors in reverse, as shown in Figure 2. Its model is generally represented by "3CTS" or "KS" in my country; foreign materials are also represented by "TRIAC". The specifications, models, shapes and electrode pin arrangements of triacs vary according to different manufacturers, but most of the electrode pins are arranged from left to right in the order of T1, T2, and G (when observing, the electrodes are pins down, facing the side marked with the characters). The shape and electrode pin arrangement of the most common plastic-encapsulated structure triacs on the market are shown in Figure 1 below.

Thyristor Characteristics

In order to intuitively understand the working characteristics of the thyristor, we first 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 supply 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 button switch SB (the KP1 type thyristor is used here, if the KP5 type is used, it should be connected to 3V DC positive side of the power supply). This connection between the thyristor and the power supply is called a forward connection, that is, the forward voltage is applied to the anode and the control electrode of the thyristor. Turn on the power switch S, the small light bulb does not light, indicating that the thyristor is not conducting; press the button switch SB again, and input a trigger voltage to the control electrode, the small light bulb is on, indicating that the thyristor is conducting. What did this demonstration experiment teach us?

This experiment tells us that to make the thyristor turn on, 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 control electrode G and cathode K. After the thyristor is turned on, release the button switch, remove the trigger voltage, and still maintain the on state.

Thyristor Features

"Triggered". However, if a reverse voltage is applied to the anode or gate, the thyristor cannot conduct. The function of the control pole is to turn on the thyristor by applying a positive trigger pulse, but it cannot turn it off. So, what method can be used to turn a conducting thyristor off? Turning a conducting thyristor off can either disconnect the anode power supply (switch S in Figure 3) or make the anode current less than the minimum value to maintain conduction (called maintain current). If an AC voltage or a pulsating DC voltage is applied between the anode and cathode of the thyristor, the thyristor will turn off by itself when the voltage crosses zero.

App types

Figure 4 shows the characteristic curve of the triac.

It can be seen from the figure that the characteristic curve of the triac is composed of the curves in the first and third quadrants. The curve in the first quadrant shows that when the voltage applied to the main electrode makes the polarity of Tc to T1 positive, we call it the forward voltage, and it is represented by the symbol U21. When this voltage gradually increases to be equal to the turning voltage UBO, the thyristor on the left of Figure 3(b) will be triggered to conduct, and the on-state current at this time is I21, and the direction is from T2 to Tl. It can be seen from the figure that the larger the trigger current is, the lower the turning voltage is. This situation is consistent with the trigger conduction law of ordinary thyristors. When the voltage applied to the main electrode makes the polarity of T1 to T2 When it is positive, it is called the reverse voltage and is represented by the symbol U12. When this voltage reaches the turning voltage value, the thyristor on the right side of Figure 3(b) will be triggered and turned on, the current at this time is I12, and its direction is from T1 to T2. At this time, the characteristic curve of the triac is shown in the third quadrant in Figure 4.

Four trigger modes

Because on the main electrode of the triac, no matter whether the forward voltage or reverse voltage is applied, and whether the trigger signal is forward or reverse, it can be triggered and turned on, so it has the following four trigger modes: ( 1) When the voltage applied by the main electrode T2 to T1 is a forward voltage, the voltage applied by the control electrode G to the first electrode T1 is also a forward trigger signal (FIG. 5a). After the triac is triggered and turned on, the direction of the current I2l flows from T2 to T1. It can be seen from the characteristic curve that at this time the triac trigger conduction law is carried out according to the characteristics of the second quadrant, and because the trigger signal is forward, so this trigger is called "first quadrant forward trigger" or called "first quadrant forward trigger". For I+ trigger mode. (2) If the forward voltage is still applied to the main electrode T2, and the trigger signal is changed to a reverse signal (Fig. 5b), after the triac is triggered and turned on, the direction of the on-state current is still from T2 to T1. We call this kind of trigger "the first quadrant negative trigger" or called I-trigger. (3) The reverse voltage U12 is applied to the two main electrodes (Fig. 5c), and the forward trigger signal is input. After the triac is turned on, the on-state current flows from T1 to T2. The triac works according to the characteristic curve of the third quadrant, so this kind of trigger is called Ⅲ+ trigger mode. (4) The reverse voltage U12 is still applied to the two main electrodes, and the reverse trigger signal is input (Fig. 5d). After the triac is turned on, the on-state current still flows from T1 to T2. This trigger is called the III-trigger method. Although the triac has the above four triggering methods, the triggering voltage and current required for triggering by a negative signal are relatively small. The work is more reliable, so in actual use, the negative trigger method is used more.

FeatureEdit

Commonly used are resistor-capacitor phase-shift bridge trigger circuits, unijunction transistor trigger circuits, transistor triode trigger circuits, trigger circuits that use small thyristors to trigger large thyristors, and so on.

The main parameters of the thyristor are:

1. Rated on-state average current IT Under certain conditions, the average value of 50 Hz sine half-wave current that can be continuously passed between the anode and the cathode.

2. Forward blocking peak voltage VPF When the control pole is open and no trigger signal is applied, 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 forward voltage peak value of the thyristor cannot exceed this parameter value given in the manual.

3. Reverse blocking peak voltage VPR When the thyristor is applied with reverse voltage and is in the reverse turn-off state, the reverse peak voltage applied to both ends of the thyristor can be repeated. When used, the parameter value given in the manual cannot be exceeded.

4. Trigger voltage VGT At the specified ambient temperature, when a certain voltage is applied between the anode and the cathode, the minimum gate current and voltage required for the thyristor to change from the off state to the on state.

5. The maintenance current IH is the minimum anode forward current necessary to maintain the conduction of the thyristor when the control pole is disconnected at the specified temperature. Many new thyristor components have come out one after another, such as fast thyristors suitable for high-frequency applications, bidirectional thyristors that can be controlled by positive or negative trigger signals to conduct in both directions, and can be turned on by positive trigger signals. , a thyristor that is turned off with a negative trigger signal, etc.

CategoryEdit

There are many ways to classify thyristors.

(1) Classification by turn-off, turn-on and control methods: thyristors can be divided into ordinary thyristors, bidirectional thyristors, reverse conduction thyristors, and gate off according to their turn-off, turn-on and control methods. Silicon controlled rectifier (GTO), BTG thyristor, temperature-controlled thyristor and light-controlled thyristor, etc.

(2) Classification by pin and polarity: SCR can be divided into two-pole thyristor, three-pole thyristor and four-pole thyristor according to its pin and polarity.

(3) Classification by package form: SCR can be divided into three types: metal package thyristor, plastic package thyristor and ceramic package thyristor according to its package form. Among them, metal-encapsulated thyristors are further divided into bolt-shaped, flat-plate, round shell, etc.; plastic-encapsulated thyristors are divided into two types with heat sink and without heat sink.

SCR switch

(4) Classification by current capacity: SCR can be divided into three types: high-power thyristor, medium-power thyristor and low-power thyristor according to current capacity. Usually, high-power thyristors are mostly packaged in metal shells, while medium and low-power thyristors are mostly packaged in plastic or ceramic packages.

(5) Classification by turn-off speed: thyristor can be divided into ordinary thyristor and high-frequency (fast) thyristor according to its turn-off speed.

(6) Zero-crossing triggering - generally it is power adjustment, that is, when the phase of the sinusoidal alternating current and the alternating current voltage is triggered at the zero-crossing point, it must be triggered at the zero-crossing point to turn on the thyristor.

(7) Non-zero-crossing triggering - no matter what phase the AC voltage is in, the thyristor can be triggered to turn on. The common one is phase-shift triggering, that is, by changing the conduction angle (angular phase) of the sinusoidal AC current, to change the output percentage .

Thyristor ModelEdit

According to the regulations of IBI144-75 of the Ministry of Machinery, ordinary thyristors are called KP-type thyristor rectifiers (also known as KP-type thyristors). The models of ordinary thyristors are marked in the following format:

Picture 1

There are 14 series for the rated speed state average swarming, as shown in Table 1-5. The forward and reverse repeating peak voltage levels stipulate that the tubes below 1000V are one level per 100V, and the tubes above 1000V are one level per 200V. Take the voltage divided by 100 as the level mark, as shown in Table 1-6.

The on-state average voltage groups are divided into 9 groups according to the voltage, which are represented by Yuwan, as shown in Table 1-1.

For example, KP500-12D represents an ordinary thyristor with an on-state average current of 500A, a rated (forward and reverse repetitive peak) voltage of 1200V, and a tube voltage drop (on-state average voltage) of 0.6---0.7V.

In summary, the summary is as follows:

(1) The thyristor is generally made into a bolt shape and a flat plate shape, with three electrodes. The die made of silicon semiconductor material is made of

Four layers of PNPN

(2) The thyristor must meet two conditions at the same time when it is turned off to turn on: (1) subject to the forward anode voltage; (2) subject to the forward gate voltage.

(3) After the thyristor is turned on, when the anode current is small and maintains the current In, the thyristor is turned off.

(4) The characteristics of thyristor are mainly: 1. Anode volt-ampere characteristic curve, 2. Gate volt-ampere characteristic area.

(5) The thyristor should be used within the range of rated parameters. The choice of thyristor mainly determines two factors:

Edit the main parameters

current

⒈ Rated on-state current (IT) is the maximum stable working current, commonly known as current. The IT of commonly used thyristors is generally one to dozens of amps.

⒉ Reverse repetitive peak voltage (VRRM) or off-state repetitive peak voltage (VDRM), commonly known as withstand voltage. The VRRM/VDRM of commonly used thyristors are generally several hundred volts to one thousand volts.

⒊ Control electrode trigger current (IGT), commonly known as trigger current. The IGT of commonly used thyristors is generally several microamps to tens of milliamps.

4. Under the specified ambient temperature and heat dissipation conditions, the average value of the current allowed to pass through the cathode and anode.

Package formEdit

Commonly used SCR packaging forms are TO-92, TO-126, TO-202AB, TO-220, TO-220ABC, TO-3P, SOT-89, TO-251, TO-252, SOT-23, SOT23- 3L, etc.

UseEdit

The most basic use of ordinary thyristors is controllable rectification. The well-known diode rectifier circuit belongs to the uncontrollable rectifier circuit. If the diode is replaced by a thyristor, a controllable rectifier circuit can be formed. Taking the simplest single-phase half-wave controllable rectifier circuit as an example, during the positive half cycle of the sinusoidal AC voltage U2, if there is no trigger pulse Ug input to the control pole of VS, VS still cannot be turned on, only when U2 is in the positive half cycle, in the When the trigger pulse Ug is applied to the control electrode, the thyristor is triggered and turned on. Draw its waveforms (c) and (d), only when the trigger pulse Ug arrives, there is a voltage UL output on the load RL. The earlier Ug arrives, the earlier the thyristor turns on; the later the Ug arrives, the later the thyristor turns on. By changing the arrival time of the trigger pulse Ug on the control electrode, the average value UL of the output voltage on the load can be adjusted. In electrical technology, the half cycle of alternating current is often set as 180°, which is called the electrical angle. In this way, in each positive half cycle of U2, the electrical angle experienced from the zero value to the moment when the trigger pulse arrives is called the control angle α; the electrical angle at which the thyristor is turned on in each positive half cycle is called the conduction angle θ. Obviously, both α and θ are used to represent the conduction or blocking range of the thyristor during the half cycle of the forward voltage. Controllable rectification is realized by changing the control angle α or conduction angle θ and changing the average value UL of the pulsed DC voltage on the load.

1: Low-power plastic-encapsulated triacs are usually used as sound and light control lighting systems. Rated current: IA is less than 2A.

2: Large; medium-power plastic-sealed and iron-sealed thyristors are usually used as power-type regulated voltage regulator circuits. Like adjustable voltage output DC power supply and so on.

3: High-power high-frequency thyristor is usually used in industry; high-frequency melting furnace, etc.

IdentificationEdit

There are three main types of thyristors in terms of shape: spiral type, flat type and flat bottom type, and the application of spiral type is more. A thyristor has three electrodes - anode (A) cathode (C) and control electrode (G). It has a four-layer structure in which the core is a P-type conductor and an N-type conductor overlapping, and there are three PN junctions. The thyristor is very different in structure from the silicon rectifier diode with only one PN junction. The four-layer structure of the thyristor and the reference of the gate electrode have laid the foundation for its excellent control characteristics of "controlling the big with the small". When applying thyristor, as long as a small current or voltage is applied to the control electrode, a large anode current or voltage can be controlled. A thyristor element with a current capacity of several hundred amperes or even thousands of amperes. Generally, the thyristor below 5 amperes is called low-power thyristor, and the thyristor above 50 amperes is called high-power thyristor.

Voltage measurement method

Why does the thyristor have the controllability of "controlling the big with small"? Below we use Chart-27 to briefly analyze the working principle of the thyristor.

First of all, the first, second and third layers from the cathode can be viewed as an NPN transistor, and the second, third and fourth layers constitute another PNP transistor. Among them, the second and third layers are overlapped and shared by two tubes. When a positive voltage Ea 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-to-shoot of BG1), BG1 will generate a base current Ib1, which is Amplified, BG1 will have a collector current IC1 amplified by a factor of β1. Because the BG1 collector is connected to the BG2 base, IC1 is again the base current Ib2 of BG2. BG2 sends the collector current IC2, which is amplified by β2 compared to Ib2 (Ib1), back to the base of BG1 for amplification. This cycle of amplification until BG1 and BG2 are completely turned on. In fact, this process is a "triggered" process. For the thyristor, the trigger signal is added to the control electrode, and the thyristor is turned on immediately. The turn-on time is mainly determined by the performance of the thyristor.

Once the thyristor is triggered and turned on, due to the cyclic feedback, the current flowing into the base of BG1 is not only the initial Ib1, but the current amplified by BG1 and BG2 (β1*β2*Ib1) This current is much larger than Ib1, enough to keep BG1 on continuously. At this time, even if the trigger signal disappears, the thyristor still remains on, only when the power supply Ea is turned off or Ea is reduced, so that the collector current in BG1 and BG2 is less than the minimum value for maintaining conduction, the thyristor can be turned off. Of course, if the polarity of Ea is reversed, BG1 and BG2 will be in the off state due to the reverse voltage. At this time, even if a trigger signal is input, the thyristor cannot work. Conversely, Ea is connected to positive, and the trigger signal is negative, and the thyristor cannot be turned on. In addition, if the trigger signal is not added, and the forward anode voltage is too large to exceed a certain value, the thyristor will also be turned on, but it is already in an abnormal working situation.

The controllable characteristic of the thyristor, which is controlled by the trigger signal (small trigger current) (large current in the thyristor), is an important feature that distinguishes it from ordinary silicon rectifier diodes.

The three electrodes of an ordinary thyristor can be measured with a multimeter ohm gear R×100 gear. As we all know, there is a PN junction (a) between thyristor G and K, which is equivalent to a diode. G is the positive electrode and K is the negative electrode. Therefore, according to the method of testing diodes, find out two of the three poles, and measure the Its forward and reverse resistance, the resistance is small, the black test lead of the multimeter is connected to the control pole G, you can use the teaching board circuit just demonstrated. Turn on the power switch S, press the button switch SB, the light bulb is good if it emits light, and it is bad if it does not emit light.

Measurement methods

The method to identify the three poles of the thyristor is very simple. According to the principle of the PN junction, you only need to measure the resistance value between the three poles with a multimeter.

The forward and reverse resistances between the anode and the cathode are more than a few hundred thousand ohms, and the forward and reverse resistances between the anode and the control electrode are more than a few hundred thousand ohms (there are two PN junctions between them, and the direction On the contrary, so the anode and the control electrode are not connected forward and reverse) [1].

There is a PN junction between the control electrode and the cathode, so its forward resistance is in the range of several ohms to several hundred ohms, and the reverse resistance is larger than the forward resistance. However, the characteristics of the gate diode are not ideal, the reverse direction is not completely blocked, and a relatively large current can pass through. Therefore, sometimes the measured reverse resistance of the control electrode is relatively small, which does not mean that the control electrode characteristics are not good. . In addition, when measuring the forward and reverse resistance of the control electrode, the multimeter should be placed in the R*10 or R*1 block to prevent the reverse breakdown of the control electrode if the voltage is too high.

If it is detected that the cathode and anode of the element are short-circuited in the forward and reverse directions, or the anode and the control electrode are short-circuited, or the control electrode and the cathode are reversely short-circuited, or the control electrode and the cathode are open-circuited, the element is damaged.

The thyristor is the abbreviation of the thyristor rectifier, which is a high-power semiconductor device with a four-layer structure with three PN junctions. In fact, the function of thyristor is not only rectification, it can also be used as a non-contact switch to quickly turn on or off the circuit, realize the inversion of direct current into alternating current, and convert alternating current of one frequency into another frequency of alternating current, etc. Like other semiconductor devices, thyristor has the advantages of small size, high efficiency, good stability and reliable operation. Its appearance has brought semiconductor technology from the field of weak current to the field of strong current, and has become a component that is eagerly used in industry, agriculture, transportation, military scientific research, as well as commercial and civil electrical appliances.

Major ManufacturersEdit

Main manufacturer brands: ST, NXP/PHILIPS, NEC, ON/MOTOROLA, RENESAS/MITSUBISHI, LITTELFUSE/TECCOR, TOSHIBA, JX, SANREX, SANKEN, SEMIKRON, EUPEC, IR, JBL, etc.

TerminologyEdit

IT(AV)--on-state average current

VRRM--reverse repetitive peak voltage IDRM--off-state repetitive peak current

ITSM--one cycle non-repetitive surge current in on-state

VTM - on-state peak voltage

IGT - gate trigger current

VGT--gate trigger voltage

IH - holding current

dv/dt--critical rate of rise of off-state voltage

di/dt--critical rate of rise of on-state current

Rthjc--junction-to-case thermal resistance

ⅥSO--module insulation voltage

Tjm--rated junction temperature

VDRM - off-state repetitive peak voltage

IRRM - reverse repetitive peak current

IF(AV)--forward average current

PGM - peak gate power

PG----gate average power

Further reading[edit]

1. Noise level on the gate

In an electrically noisy environment, if the noise voltage on the gate exceeds VGT and there is enough gate current to excite the positive feedback inside the thyristor (thyristor), it will also be triggered to turn on.

When applying and installing, first make the connection outside the gate as short as possible. When the connection cannot be very short, stranded wire or shielded wire can be used to reduce the intrusion of interference. A 1KΩ resistor is added between G and MT1 to reduce its sensitivity, and a 100nf capacitor can also be connected in parallel to filter out high-frequency noise.

2. About the rate of change of the conversion voltage

When driving a large inductive load, there is a large phase shift between the load voltage and current. When the load current crosses zero, the triac (thyristor) begins to commutate, but due to the phase shift, the voltage will not be zero. Therefore, the thyristor (thyristor) is required to quickly turn off this voltage. If the change of the commutation voltage exceeds the allowable value at this time, there is not enough time for the charge between the junctions to be released, and the triac (thyristor) is forced back to the conducting state.

In order to overcome the above problems, an RC network can be added between the terminals MT1 and MT2 to limit the voltage variation to prevent false triggering. Generally, the resistance is 100R, and the capacitance is 100nF. It is worth noting that this resistor cannot be omitted.

3. About the rate of change of the conversion current

When the load current increases, the frequency of the power supply increases or the power supply is non-sinusoidal, the rate of change of the conversion current will increase, which is most likely to occur in the case of an inductive load, which can easily lead to damage to the device. At this time, an air inductor of a few millihenries can be connected in series in the load loop.

4. About the SCR (thyristor) open circuit voltage change rate DVD/DT

When a high-speed changing voltage less than its VDFM is applied across the triac (thyristor) in the off state, the current of the internal capacitor will generate enough gate current to turn on the thyristor (thyristor). This is especially serious at high temperatures, in which case an RC snubber circuit can be added between MT1 and MT2 to limit VD/DT, or a high-speed thyristor (thyristor) can be used.

5. About the continuous peak open circuit voltage VDRM

In the case of abnormal power supply, the voltage across the thyristor (thyristor) will exceed the maximum value of the continuous peak open circuit voltage VDRM, at this time the leakage current of the thyristor (thyristor) increases and breaks down. If the load can tolerate a large inrush current, then the local current density on the silicon chip is very high, making this small part turn on first. cause the chip to be burned or damaged. In addition, incandescent lamps, capacitive loads or short-circuit protection circuits will generate high inrush currents. At this time, filters and clamping circuits can be added to prevent spike (burr) voltages from being applied to the triac (thyristor) [2] .