Measurement method of thyristor-EEWORLD

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Measurement method of thyristor

1. Overview

一种以硅单晶为基本材料的P1N1P2N2四层三端器件，创制于1957年，由于它特性类似于真空闸流管，所以国际上通称为硅晶体闸流管，简称晶闸管T。又由于晶闸管最初应用于可控整流方面所以又称为硅可控整流元件，简称为可控硅SCR。
在性能上，可控硅不仅具有单向导电性，而且还具有比硅整流元件（谷称“死硅”）更为可贵的可控性。它只有导通和关断两种状态。
可控硅能以毫安级电流控制大功率的机电设备，如果超过此频率，因元件开关损髦显著增加，允许通过的平均电流相降低，此时，标称电流应降级使用。
可控硅的优点很多，例如：以小功率控制大功率，功率放大倍数高达几十万倍；反应极快，在微秒级内开通、关断；无触点运行，无火花、无噪音；效率高，成本低等等。
可控硅的弱点：静态及动态的过载能力较差；容易受干扰而误导通。
可控硅从外形上分类主要有：螺栓形、平板形和平底形。

2. Structure and model of thyristor components

1. Structure
Regardless of the appearance of the thyristor, its core is a four-layer P1N1P2N2 structure composed of P-type silicon and N-type silicon. See Figure 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

Figure 1. Schematic diagram and symbol diagram of thyristor structure

2. Model

At present, there are two types of domestic thyristors, namely the new and old standards issued by the ministry. The new models will gradually replace the old models.

Table 1 Comparison of main characteristic parameters of KP type thyristor between old and new standards

parameter	New standard issued by the Ministry (JB1144-75)	Old standard issued by the Ministry (JB1144-71)
Serial number	KP type right silicon controlled rectifier element	3CT series silicon controlled rectifier components
1	Rated on-state average current (IT(AV))	Rated forward average current (IF)
2	Off-state repetitive peak voltage (UDRM)	Forward blocking peak voltage (UPF)
3	Reverse repetitive peak voltage (URRM)	Peak reverse voltage (VPR)
4	Off-state repetitive average current (IDR(AV))	Average forward leakage current (I)
5	Repetitive average reverse current (IRR(AV))	Average reverse leakage current (IRL)
6	On-state average voltage (UT(AV))	Maximum average forward voltage drop (VF)
7	Gate trigger current (IGT)	Gate trigger current (Ig)
8	Gate trigger voltage (UGT)	Control electrode trigger voltage (Vg)
9	Off-state voltage critical rise rate (du/dt)	Limit forward voltage rise rate (dV/dt)
10	Holding current (IH)	Holding current (IH)
11	Rated junction temperature (TjM)	Rated operating junction temperature (Tj)

The current and voltage levels of KP type thyristors are shown in Table 2.
Table 2. Current and voltage levels of KP type thyristors

Rated on-state average current IT (AV) (A)	1，5，10，20，30，50，100，200，300，400，500，600，700，800，100
Forward and reverse repetition Peak voltage UDRM, URRM (×100) (V)	1～10，12，14，16，18，20，22，24，26，，28，30
Average on-state voltage UT(AV)(V)	A	B	C	D	AND	F	G	H	I
Average on-state voltage UT(AV)(V)	≤0.4	0.4～0.5	0.5～0.6	0.6～0.7	0.7～0.8	0.8～0.9	0.9～1.0	1.0～1.1	1.1～1,2

Examples:
(1) KP5-10 indicates a common reverse blocking thyristor component with an average on-state current of 5A and a forward repetitive peak voltage of 1000V.
(2) KP500-12D indicates a common reverse blocking thyristor component with an average on-state current of 500A, a forward and reverse repetitive peak voltage of 1200V, and an on-state average voltage of 0.7V. (
3) 3CT5/600 indicates an old model common thyristor component with an average on-state current of 5A and a forward and reverse repetitive peak voltage of 600V.

3. Working principle and basic characteristics of thyristor components

1. Working Principle
The thyristor is a P1N1P2N2 four-layer three-terminal structure element with three PN junctions. When analyzing the principle, it can be regarded as consisting of a PNP tube and an NPN tube. Its equivalent diagram is shown in Figure 2

Figure 2. SCR equivalent diagram

When a positive voltage is applied to anode A, both BG1 and BG2 are in the amplification state. At this time, if a positive trigger signal is input from control electrode G, base current ib2 will flow through BG2, which will be amplified by BG2, and its collector current ic2=β2ib2. Because the collector of BG2 is directly connected to the base of BG1, ib1=ic2. At this time, current ic2 is amplified by BG1 again, so the collector current of BG1 is ic1=β1ib1=β1β2ib2. This current flows back to the base of BG2, forming positive feedback, which makes ib2 increase continuously. As a result of such a positive feedback cycle, the current of the two tubes increases sharply, and the thyristor is saturated and turned on.
Due to the positive feedback effect formed by BG1 and BG2, once the thyristor is turned on, even if the current of control electrode G disappears, the thyristor can still maintain the on state. Since the trigger signal only plays a triggering role and has no shut-off function, this thyristor cannot be turned off.
Since the thyristor has only two working states, on and off, it has a switching characteristic. This characteristic requires certain conditions to be transformed. The conditions are shown in Table 3.

Table 3. Thyristor turn-on and turn-off conditions

state	condition	illustrate
From off to on	1. The anode potential is higher than the cathode potential. 2. The control electrode has sufficient forward voltage and current.	Both are indispensable
Maintain conduction	1. The anode potential is higher than the cathode potential 2. The anode current is greater than the holding current	Both are indispensable
From on to off	1. The anode potential is lower than the cathode potential . 2. The anode current is less than the holding current.	Any condition can be

2. Basic volt-ampere characteristics

The basic volt-ampere characteristics of thyristor are shown in Figure 3.

Figure 3. Basic volt-ampere characteristics of thyristor

(1) Reverse characteristics When the control electrode is open and a reverse voltage is applied to the anode (see Figure 4), the J2 junction is forward biased, but the J1 and J2 junctions are reverse biased. At this time, only a very small reverse saturation current can flow. When the voltage is further increased to the avalanche breakdown voltage of the J1 junction, the differential J3 junction also breaks down, and the current increases rapidly. The characteristic of Figure 3 begins to bend, as shown in the characteristic OR segment. The voltage URO at the bend is called the "reverse transition voltage." At this time, the thyristor will undergo permanent reverse breakdown.
(2) Forward characteristics When the control electrode is open and a forward voltage is applied to the anode (see Figure 5), the J1 and J3 junctions are forward biased, but the J2 junction is reverse biased. This is similar to the reverse characteristics of an ordinary PN junction, and only a very small current can flow. This is called the forward blocking state. When the voltage increases, the characteristic of Figure 3 bends, as shown in the characteristic OA segment. The bend is UBO, which is called the forward transition voltage.

Figure 4: Anode with reverse voltage

Figure 5: Anode positive voltage

After the voltage rises to the avalanche breakdown voltage of the J2 junction, the J2 junction undergoes an avalanche multiplication effect, generating a large number of electrons and holes in the junction area, with electrons entering the N1 region and holes entering the P2 region. The electrons entering the N1 region recombine with the holes injected from the P1 region through the J1 junction, and similarly, the holes entering the P2 region recombine with the electrons injected from the N2 region through the J3 junction into the P2 region, causing avalanche breakdown. The electrons entering the N1 region and the holes entering the P2 region cannot all recombine, so that electrons accumulate in the N1 region and holes accumulate in the P2 region, resulting in an increase in the potential of the P2 region and a decrease in the potential of the N1 region. The J2 junction becomes forward biased, and as long as the current increases slightly, the voltage drops rapidly, resulting in the so-called negative resistance characteristic, as shown in the dashed line AB segment in Figure 3.
At this time, the three junctions J1, J2, and J3 are all in forward bias, and the thyristor enters the forward conductive state---on state. At this time, its characteristics are similar to the forward characteristics of ordinary PN junctions, as shown in the BC section of Figure 3.
3. Trigger conduction
When a forward voltage is added to the control electrode G (see Figure 6), due to the forward bias of J3, the holes in the P2 region enter the N2 region, and the electrons in the N2 region enter the P2 region, forming a trigger current IGT. On the basis of the internal positive feedback of the thyristor (see Figure 2), the effect of IGT is added to make the thyristor turn on in advance, resulting in the left shift of the OA section of the volt-ampere characteristic in Figure 3. The larger the IGT, the faster the characteristic shifts to the left.

Figure 6: Forward voltage applied to both anode and control electrode

How to use a multimeter to measure the electrodes of a thyristor

1. Detection of unidirectional thyristor
Select the resistance R×1 gear of the multimeter, and use the red and black test pens to measure the forward and reverse resistance between any two pins until a pair of pins with a reading of tens of ohms is found. At this time, the pin connected to the black pen is the control electrode G, the pin connected to the red pen is the cathode K, and the other empty pin is the anode A. At this time, connect the black test pen to the determined anode A, and the red test pen is still connected to the cathode K. At this time, the multimeter pointer should not move. Use a short-circuit wire to short-circuit the anode A and the control electrode G instantly. At this time, the multimeter pointer should deflect to the right, and the resistance reading is about 10 ohms. If the anode A is connected to the black test pen and the cathode K is connected to the red test pen, the multimeter pointer deflects, indicating that the unidirectional thyristor has been broken down and damaged.
2. Detection of bidirectional thyristor
Use the multimeter resistance R×1 gear, and use the red and black test pens to measure the forward and reverse resistance of any two pins respectively. As a result, two groups of readings are infinite. If one group is tens of ohms, the two pins connected to the red and black test leads are the first anode A1 and the control electrode G, and the other empty pin is the second anode A2. After determining the A and G poles, carefully measure the forward and reverse resistance between the A1 and G poles. The pin connected to the black test lead with the relatively small reading is the first anode A1, and the pin connected to the red test lead is the control electrode G. Connect the black test lead to the determined second anode A2, and the red test lead to the first anode A1. At this time, the multimeter pointer should not deflect, and the resistance value is infinite. Then use a short-circuit wire to short-circuit the A2 and G poles instantly, and apply a forward trigger voltage to the G pole. The resistance between A2 and A1 is about 10 ohms. Then disconnect the short-circuit wire between the A2 and G poles, and the multimeter reading should remain at about 10 ohms. Interchange the red and black test lead wires, with the red test lead connected to the second anode A2 and the black test lead connected to the first anode A1. Similarly, the multimeter pointer should not deflect, and the resistance value is infinite. Use a shorting wire to short-circuit A2 and G again, and apply a negative trigger voltage to G. The resistance between A1 and A2 is also about 10 ohms. Then disconnect the shorting wire between A2 and G, and the multimeter reading should remain unchanged, maintaining about 10 ohms. In line with the above rules, it means that the tested bidirectional thyristor is not damaged and the polarity of the three pins is correctly judged.

To detect a higher power thyristor, you need to connect a 1.5V dry battery in series in the black pen of the multimeter to increase the trigger voltage.

Reference address：Measurement method of thyristor

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