Structure and operating principle of the SCR
pp 144-152
the end
this chapter the student should:
• Does understanding the structure and operating principle of four-layer diode
• Does understanding the structure and operating principle of the controlled rectifier
silicon
• Knows the different types of silicon controlled rectifier
• Knows the basic connections and circuits of silicon controlled rectifier
• Reads and understands technical brochures of the manufacturers
• Does understanding the structure and operating principle of the DIAC and TRIAC
• Can be applied to the above simple electronic
circuits
K E F A § AI O
Semiconductor elements FOUR LAYER
5.1 Semiconductors N-P-P-N
5.1.1 Structure and general features
Semiconductors O P-N-P-N diode P-N-P-N, is a crystal with four rich regions, two N-type and two type P, which alternated sequentially (sch.5.1.1). The thickness and especially the level of enrichment varies in each particular area is very high in areas of extreme and very low in the intermediate-type semiconductor diode N. The N-P-P-N diode called Shockley.
The Descent
The Rise
(A)
D
The Descent
The Rise
Figure 5.1.1 (a) Structure-pass P-N-P-N and (b) circuit diagram
2
The electrode which is connected to the P-type semiconductor extreme rise is called (A). Similarly, the electrode which is connected to the extreme N-type semiconductor called a cathode (K) (sch.5.1.1v).
A key feature of both this structure and all other structures that is based on the phenomenon of internal feedback, feedback, (internal feedback) which shows. The feedback was due solely to their structure, ie the sequence of areas and dimensions and levels of enrichment of these regions. The internal feedback causes these devices to operate in only two stable states, a conductive (ON) and a nonconductive (OFF). In the ON state resistance of the diode is less than 10 O while in the OFF state resistance of the diode ranges from 1 to MO
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100 MO. An additional feature of these provisions is that in both situations constant power consumption is very small, which makes them especially useful in applications to control power.
So when a P-channel N-P-N connected to a source so that the anode connected to the positive pole of the source and the cathode to the negative pole of the source, shows the two stable states conductivity. The polarization is called right. In reverse bias the diode behaves as a typical diode with very low current saturation at high voltages shows the breakdown voltage (VBR) (sch.5.1.2).
Region
proper polarization
Region
reverse
polarization
Figure 5.1.2 Typical current-voltage curve PNPN diode
Because the diode consists of four layers look like it consists of three diodes D1, D2 and D3 (sch.5.1.1) connected in series. The D1 and D3 have the same address and D2 is reverse connected. When connected via the diode impedance at the ends of a source and applied the correct polarity of diodes D1 and D3 properly polarized while the D2 reverse polarized. As a result, the external applied voltage occurs, almost all, at the ends of D2 and the current, which flows through the device is small. Increasing TA-
ing the source has resulted in a small afsisi current provision. When the voltage exceeds a value, called the proper breakdown voltage (firing or breakover voltage) and is denoted by VBO, the current increases sharply and the voltage across the diode defects
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Comfortable almost instantaneously (Sch.5.1.2). In this voltage trigger diode switches from OFF state to ON. The current which corresponds to the trigger voltage is the IBO. The area in which the diode is in ON condition called saturation region and say that this provision has mandalothei (latched).
If you then reduce the voltage across the diode, the device remains in the ON state until the current of the reduced price IH, called the holding current (holding or latching current). The current IH is the minimum current to maintain the status ON. Holding current in the voltage corresponding restraint VH.
5.1.2 Principle of operation
(A)
(B)
NPN
Figure 5.1.3 (a) Equivalent structure of a diode-P N-P-N and (b) equivalent circuit
The operation of the PNPN diode can be explained by reduction of a circuit as in Figure (5.1.3,). The circuit shown in the separation of the PNPN diode into two parts so that one can have a succession of layers and the other PNP NPN (sch.5.1.3a). Each section corresponds to a transistor in PNP and NPN Q1 to Q2. The N-type region which is the base of Q1 is the collector of Q2 and P-type region which is the base of Q2
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is the collector of Q1. Thus the collector current of a "transistor" is a current basis for the other (sch.5.1.3v), creating a loop that starts from the base of Qv is passed from collector to base Qv Q2 and the collector of Q2 and ends again at the base of Q1.
Against this background, the current-voltage characteristic is interpreted as follows. When the voltage across the diode P-N-P-N is 0, the transistors are not leaking from drafts and in a state of OFF. Increasing the voltage across the diode, the diode collector - base of each transistor reverse polarized. Thus, the current, which flows through each diode is the collector current of saturation (reverse bias) which is very small and can not excite the system.
This results in the diode remains in state OFF. The structure of the diode R-S-P-N is such that the transistor characteristics vary with the collector-emitter voltage. So when the voltage across the diode P-N-P-N exceeds the burst-voltage dalismou then the characteristics of transistors and the current of each diode saturation collector - base such as to allow feedback to lead the P-N diode- P-N mode ON.
The return from the ON state to OFF state is possible only when the currents are flowing through the transistors are small and the characteristic of transistors have taken their positions, which are at low voltages. This occurs when the voltage across the diode P-N-P-N and the stream have become smaller voltage restraint and holding current, respectively.
Example 5.1.1
In the figure, the voltage varies from 0 to 20 V. Which trend will begin to pass current from the circuit? When the voltage is 15 V what is the current which passes through the circuit? What is the minimum voltage to zero before the
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current of the circuit; N · consider the negligible voltage drop
ends when the diode is in ON.
Solution
The PNPN diode has the following characteristics: trigger voltage VBO = 10 V and holding current IH = 4 mA.
As a PNPN diode in OFF state does not pass
current, the voltage drop in resistor 1 KO will be zero until
start interlock. So for 0V <V <10 V the current of the circuit is zero. For V> 10 V the current of the circuit will be nonzero. So you start to pass current from the circuit
when the voltage exceeds 10 V.
In the ON state of the PNPN diode has a very small
resistance. Therefore, the current of the circuit will be determined by
1 KO resistance. So, when V = 15 V the current of the circuit will
determined by Ohm's Law
V 15 V
R 1KO
= 15mA
The PNPN diode switches to OFF when the current,
which the leakage current become smaller containment IH.
Because the diode is still in the ON state current of the circuit will be determined by the resistance 1 KO. So the required voltage will be determined by Ohm's law:
V = I · R = (4 mA) x (1 KO) = 4 V
5.2 silicon controlled rectifier (SCR) 5.2.1 Structure and general features
O or silicon controlled rectifier thyristors (Silicon Controlled Rectifier or thyristor, SCR) is a device that four layers. It has the same basic structure with the passage PNPN, just added a third gate electrode (gate), which has been associated with
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Ici Q2 Qi
intermediate-type semiconductor layer P (sch.5, 2.1a) with the result
silicon controlled rectifier having three electrodes, the anode (A)
the cathode (K) and gate (G) (sch.5.2.1g).
(A)
Figure 5.2.1 (a) Equivalent structure, (b) equivalent circuit and
(C) a circuit diagram of silicon controlled rectifier (SCR)
Called controlled rectifier as the reverse bias behaves like a diode and the common good polarization is possible to define and control the trigger voltage current through the gate. The voltage dependence of the trigger current of the gate shown in Figure (5.2.2). A key feature of the provision is that when the current of the gate is 0 then the SCR diode behaves as a P-N-P-N. When a positive current sent through the gate then the trigger voltage decreases and the decrease is greater the larger the current. For large values of gate current (IG3 in fig 5.2.2), the trigger voltage is so small that the SCR shows a current-voltage characteristic similar to that of the diode. In contrast, negative values of the current gate trigger voltage increases. In an SCR holding voltage and holding current are independent from the power gate.
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Figure 5.2.2 Current-voltage characteristic of a controlled rectifier silicon for different values of gate current
5.2.2 Principle of operation
The interpretation of the operation of SCR based on the behavior of the diode R-S-P-N. As mentioned above, the SCR is a four layer device such as diode diode P-N-P-N, which has added a contact in between P-type layer. The equivalent circuit shown in Fig (5.2.1) and (5.2.1,). So when the current of the gate is 0 then the operation of SCR is similar to the diode P-N-P-N, ie, based on the saturation currents (reverse polarity) and change the characteristics of the transistor equivalent circuit. Because between the gate and cathode formed a simple diode, sufficient to apply a voltage of 0,7 V to start the transistor Q2 is conducting and therefore the Q1, ie the SCR.
The presence of the gate allows current injection transistor Q2 in the equivalent circuit. So if fed direct current Ic when the SCR, risks are OFF then it is possible to start the locking process. In the OFF state current of Ic is added to the collector current of transistor Q1, which is the saturation current of the diode collector. The sum of two currents give the current, asis of Q2. So far as Ic will determine the voltage at which the characteristics of transistors such as to start the locking process. It is obvious that the price of the new trigger voltage is less than the
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which corresponds to zero current of the gate. When the current of the gate is too big Q1 conducts "ever" so the trigger voltage is too low and the SCR to be in ON state for very small voltages. If the SCR has a switch to ON, it remains in it even if the reset current of the gate.
Figure 5.2.3 Typical bias circuit an SCR
When the current gate is negative, the total current base of Q2 will be reduced, requiring greater voltage across the SCR to meet the conditions which will result in a state of the locking device. The application of negative gate current to a method of forced transition from one state SCR ON state to OFF.
Channeled to a gate current in an SCR requires the use of voltage source and resistor limit the current gate, after gate between cathode and a diode, which is polarized correctly. A typical circuit is shown in SCR polarization sch.5.2.3. O! RC and RG resistors limit the current of the gate and SCR. So the input voltage to achieve locking SCR calculated from the relationship:
Vi = VT + IT · Rg 5.2.1
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where VT is the voltage trigger (trigger voltage) and IT is the current trigger (trigger current) of ScR. These data provided the technical brochures of the manufacturers. In the circuit of sch.5.2.3 ScR will go into the OFF state when the voltage decreases so that the current, which flows through ScR become less than R <to zero voltage.
Figure 5.2.4 Photo-SCR (a) without and (b) with an adjustable trigger level
