How to generate the signal of three-phase inverter, and how to design the circuit that shifts the phase of 50Hz sine signal 120?

Thyristor is the abbreviation of thyratron, and it can also be called silicon controlled rectifier, formerly referred to as silicon controlled rectifier. 1957 general electric company developed the first thyristor product in the world and commercialized it in 1958; Thyristor is a PNPN four-layer semiconductor structure with three poles: anode, cathode and grid; The working conditions of thyristor are: DC voltage is applied, and the gate has trigger current; Its derivative devices are: fast thyristor, bidirectional thyristor, reverse thyristor, light-controlled thyristor and so on. It is a high-power switching semiconductor device, which is represented by "V" and "VT" in the circuit (represented by the letter "SCR" in the old standard).

Thyristors have the characteristics of silicon rectifier devices, can work under high voltage and high current conditions, and the working process is controllable. They are widely used in controllable rectification, AC voltage regulation, contactless electronic switches, inverters, frequency conversion and other electronic circuits.

Types of thyristors

Thyristors can be classified in many ways.

(1) Classification by off, on and control modes.

Thyristors can be divided into ordinary thyristors, bidirectional thyristors, reverse thyristors, GTO, BTG thyristors, temperature-controlled thyristors and light-controlled thyristors according to their turn-off, turn-on and control modes.

(II) Classification by pin and polarity

Thyristors can be divided into diode thyristors, triode thyristors and quadrupole thyristors according to pin and polarity.

(3) Classification by packaging form

Thyristors can be divided into three types according to the packaging form: metal encapsulated thyristors, plastic encapsulated thyristors and ceramic encapsulated thyristors. Among them, metal encapsulated thyristors are divided into bolt shape, flat plate shape, round shell shape, etc. Plastic encapsulated thyristors can be divided into two types: those with fins and those without fins.

(4) according to the current capacity classification

Thyristors can be divided into high power thyristors, medium power thyristors and low power thyristors according to their current capacity. Generally, high-power thyristors are mostly packaged in metal shell, while medium and small-power thyristors are mostly packaged in plastic or ceramic.

(5) Classification by turn-off speed

Thyristors can be divided into ordinary thyristors and high frequency (fast) thyristors according to their turn-off speed.

Working principle of thyristor

In the working process of thyristor T, its anode A and cathode K are connected with power supply and load to form the main circuit of thyristor, and its grid G and cathode K are connected with the device for controlling thyristor to form the thyristor control circuit.

Working conditions of thyristor:

1. When the thyristor bears the reverse anode voltage, the thyristor is in a short circuit state regardless of the gate voltage and voltage.

2. When the thyristor bears the positive anode voltage, the thyristor is turned on only when the grid bears the DC voltage.

3. When the thyristor is on, as long as there is a certain positive anode voltage, the thyristor will remain on regardless of the gate voltage, that is, after the thyristor is on, the gate will lose its function.

4. When the thyristor is turned on, when the voltage (or current) of the main circuit is reduced to close to zero, the thyristor is turned off.

Analyze the working process from the inside of thyristor;

Thyristor is a four-layer three-terminal device, which has three PN junction diagrams: J 1, J2 and JBOY3. The NP in the middle can be divided into two parts to form a composite tube of PNP transistor and NPN transistor. Figure 2.

When the thyristor bears forward anode voltage, in order to make the thyristor conduct copper, the PN junction J2 bearing reverse voltage must lose its blocking effect. The collector current of each transistor in fig. 2 is also the base current of the other transistor. Therefore, when there is enough gate-machine current Ig flowing into two mutually compound transistor circuits, a strong positive feedback will be formed, which will lead to the saturation conduction of the two transistors and the saturation conduction of the transistors.

Let the collector current of PNP tube and NPN tube be Ic 1 and Ic2 be respectively; Emitter currents are Ia and ik respectively; The current amplification factors are a 1=Ic 1/Ia and a2=Ic2/Ik, and the reverse leakage current flowing through J2 junction is Ic0.

The anode current of thyristor is equal to the sum of collector current and leakage current of two tubes:

Ia=Ic 1+Ic2+Ic0 or Ia=a 1Ia+a2Ik+Ic0.

If the gate current is Ig, the cathode current of thyristor is Ik=Ia+Ig.

It can be concluded that the anode current of thyristor is: I = (IC0+IGA2)/(1-(a1+A2)) (1-kloc-0/).

The current amplification coefficients a 1 and a2 of silicon PNP transistor and silicon NPN transistor change sharply with the change of emitter current, as shown in figure 3.

When the thyristor bears the forward anode voltage, but the gate does not, in the formula (1-1), ig = 0, and (A 1+A2) is very small, so the anode current Ia≈Ic0 of the thyristor is in the forward blocking state. When the thyristor is at the positive anode voltage, the current Ig flows in from the grid G. Because large enough Ig flows through the emitter junction of NPN tube, the starting current amplification factor a2 increases, and large enough electrode current Ic2 flows through the emitter junction of PNP tube, and the current amplification factor a 1 of PNP tube increases, resulting in large electrode current Ic 1 flowing through the emitter junction of NPN tube. This powerful positive feedback process is progressing rapidly. It can be seen from fig. 3 that when a 1 and a2 increase with the emitter current (a 1+a2)≈ 1, the denominator in the formula (1- 1) ≈ 0. Thyristors are in a forward conduction state.

In the formula (1- 1), after the thyristor is turned on, 1-(a 1+a2)≈0. At this time, even if the gate current Ig=0, the thyristor can still keep the original anode current Ia on. After the thyristor is turned on, the gate loses its function.

After the thyristor is turned on, if the anode current Ia is reduced below the holding current IH by continuously reducing the power supply voltage or increasing the loop resistance, when a 1 and a 1 decrease rapidly, the thyristor will return to the blocking state.

GTO (Gate Turn-Off Thyristor) is also called gated thyristor. Its main feature is that when the negative trigger signal is applied to the gate, the thyristor can turn off automatically.

As mentioned above, after the ordinary thyristor (SCR) is triggered by the positive gate signal, it can remain conductive even if the signal is removed. In order to turn it off, it is necessary to cut off the power supply, make the forward current lower than the holding current IH, or apply a strong reverse voltage to turn it off. This requires adding a rectifier circuit, which not only increases the volume and weight of the equipment, but also reduces the efficiency, resulting in waveform distortion and noise. Turning off the thyristor overcomes the above defects. It not only retains the advantages of high withstand voltage and large current of ordinary thyristor, but also has self-turn-off ability and is convenient to use. It is an ideal high-voltage and high-current switching device. The capacity and lifetime of GTO exceed that of giant transistor (GTR), but its working frequency is lower than that of GTR. At present, GTO has reached the capacity of 3000A and 4500V V. High-power turn-off thyristors have been widely used in chopping speed regulation, frequency conversion speed regulation, inverter power supply and other fields, showing strong vitality.

The turn-off thyristor also belongs to the four-layer three-terminal device of PNPN, and its structure and equivalent circuit are the same as those of ordinary thyristors. Therefore, the figure 1 only depicts the shapes and symbols of typical GTO products. High-power GTO is mostly manufactured in modular form.

Although GTO and SCR have the same principle of triggering conduction, the principle and mode of turning off are completely different. This is because the common thyristor is out of the deep saturation state after being turned on, and GTO can only reach the critical saturation after being turned on, so the GTO gate can be turned off by adding a negative trigger signal. An important parameter of GTO is turn-off gain βoff, which is equal to the ratio of anode maximum turn-off current IATM to grid maximum negative current IGM, and there is a formula.

βoff =IATM/IGM

Generally, βoff is several times to dozens of times. The larger the value of β-βoff, the stronger the control ability of grid current on anode current. Obviously, βoff is quite similar to the hFE parameter of Changsheng.

This paper introduces the method of judging GTO electrode with multimeter, checking GTO's trigger ability and turn-off ability, and estimating turn-off gain βoff.

1. Determine the electrode of GTO.

Set the multimeter at R× 1 to measure the resistance between any two feet. Only when the black stylus is connected to the G pole and the red stylus is connected to the K pole, the resistance is low, otherwise the resistance is infinite. From this, we can quickly determine the G pole and the K pole, and the rest is the A pole.

2. Check the trigger ability

As shown in Figure 2(a), firstly, connect the black stylus of Table I to the A pole and the red stylus to the K pole, with infinite resistance; Then, the tip of the black watch also touches the G pole at the same time. With the positive trigger signal, the watch hand deflects to the right to a low resistance value, which indicates that GTO has been turned on; Finally, the G pole is disconnected. As long as GTO remains on, it means that the tube under test has the trigger ability.

3. Check ability

Now, the double-table method is used to test the turn-off ability of GTO. As shown in Figure 2(b), the gears and connections in Table 1 remain unchanged. Set Table 2 as R× 10, with the red stylus connected to the G pole and the black stylus connected to the K pole, and apply a negative trigger signal. If the pointer in table I is set to the infinite position on the left, it proves that GTO has the ability to close.

4. Estimate the turn-off gain βoff.

When going to step 3, don't consult Table II first, and write down the forward deflection grid number n1of Table I when g to is turned on; Then connect Table 2 to forcibly turn off GTO, and write down the positive deflection grid number n2 in Table 2. Finally, according to the reading current method, the turn-off gain is estimated as follows:

βoff = IATM/IGM≈IAT/IG = k 1n 1/k2 N2

Where K 1 is the current proportional coefficient of R× 1 in Table I;

K2 —— the current proportional coefficient at R× 10 in Table 2.

βoff≈ 10×n 1/ n2

The advantage of this formula is that it is not necessary to calculate the values of IAT and IG in detail, and the turn-off gain value can be quickly estimated by reading the forward deflection grid number of the corresponding pointer.

Precautions:

(1) It is suggested to connect a 1.5V battery E' in series outside the R× 1 gear to improve the test voltage and current and make GTO conduct reliably.

(2) To accurately measure the turn-off gain βoff of GTO, there must be special test equipment. But in amateur conditions, you can use the above method to estimate. Due to different test conditions, the measurement results are for reference only or as a basis for relative comparison.

Reverse conduction thyristor (RCT) is also called reverse conduction thyristor. Characterized in that a diode is reversely connected in parallel between the anode and the cathode of the thyristor, so that the emitter junction of the anode and the cathode is in a short circuit state. Because of this special circuit structure, it has excellent properties such as high voltage resistance, high temperature resistance, short turn-off time and low on-state voltage. For example, the turn-off time of reverse thyristor is only a few microseconds, and its working frequency reaches tens of kilohertz, which is better than that of fast thyristor (FSCR). The device is suitable for switching power supply and UPS uninterrupted power supply. An RCT can replace a thyristor and a freewheeling diode, which is not only convenient to use, but also can simplify the circuit design.

The symbol and equivalent circuit of the reverse thyristor are shown in figures 1(a) and (b). Its volt-ampere characteristics are shown in Figure 2. It can be clearly seen from the figure that the volt-ampere characteristics of the reverse thyristor are asymmetric, the forward characteristics are the same as those of the ordinary thyristor SCR, and the reverse characteristics are the same as those of the silicon rectifier (only the coordinate positions are different).