新型电子课件第5章.ppt

1、CHAPTER 5 SCR in DC and AC Circuit 5.1 SCR introduction The silicon-controlled rectifier (SCR) is made by bonding four layers of semiconductor material together to form a p-np-n junction. 1. the construction,2. Schematic symbol and working principles . Identifying the SCR 1) SCRs are known as the wo

2、rkhorses of industrial electronics because of their ability to control hundreds of volts and amps. 2) SCRs are also found in smaller sizes with ratings of less than 1amp at only a few volts 3) They are also found in many different sizes and styles of cases. SCRs has many same styles as transistors.,

3、. Schematic symbol and working principles,3. Operating Characteristics SCR operates very similar to a relay which is controlled by a set of pushbuttons and is latched with holding contacts. In this circuit the start button is used to turn relay coil CR on.,Once the coil is turned on, the holding con

4、tacts, operated by coil CR, close and maintain a current path around the normally open pushbutton. Once the holding contacts have closed, current continues to flow to coil CR even if the start button is released and the pushbutton is again opened. The start button can control when the relay coil is

5、turned on, but once the relay is energized, the start button has no more control over the circuit.,+,-,In order to turn the relay off, the normally closed stop button must to be pushed to open the circuit to the coil. After the relay coil has been de-energized, CR contacts open and the stop button c

6、an be released.,+,-,继电器是一种电子控制器件，它具有控制系统（又称输入回路）和被控制系统（又称输出回路），通常应用于自动控制电路中，它实际上是用较小的电流去控制较大电流的一种“自动开关”。故在电路中起着自动调节、安全保护、转换电路等作用。 电磁式继电器一般由铁芯、线圈、衔铁、触点簧片等组成的。 只要在线圈两端加上一定的电压，线圈中就会流过一定的电流，从而产生电磁效应，衔铁就会在电磁力吸引的作用下克服返回弹簧的拉力吸向铁芯，从而带动衔铁的动触点与静触点（常开触点）吸合。 当线圈断电后，电磁的吸力也随之消失，衔铁就会在弹簧的反作用力返回原来的位置，使动触点与原来的静触点（常闭触

7、点）吸合。这样吸合、释放，从而达到了在电路中的导通、切断的目的。,4. Operation of the SCR The SCR operates in a manner similar to this relay circuit. The SCR in the figure below will not conduct until switch S2 is closed and current flows through the gate-cathode circuit. when the gate energizes, the SCR conducts through the anode-

8、cathode circuit. After the SCR is turned on by the gate, switch S2 can be opened (removing current from the gate), and the SCR continues to conduct.,When switch S1 is opened, current flow through the anode-cathode circuit is broken, and the SCR turns off. Switch S1 can now be closed and the SCR will

9、 remain turn off until the gate is again used to turn it back on. Notice that the gate operates similar to the start button in the relay circuit. It can turn the SCR on, but not off.,门极电流的作用可以利用正反馈的原理简单地加以说明。当N1P2N2晶体管有控制电流通过时，Ie2就要增加Ib的2倍，而Ie2又是P1N1P2的基极电流。于是Ic1=1Ic2=12Ib，这个Ie1是N1P2N2的基极电流。 这样一来，即使

10、一个很小的基极电流，在正反馈的左右下 ，两个晶体管很快就趋于饱和，从而可控硅导通。 元件导通以后，门极信号可以 去掉。因为此时P1N1P2管供给 N1P2N2管以更大的基极电流，可 控硅继续导通，直到集电极电流 降到维持电流以下，晶闸管关断。 晶闸管就不能维持正反馈过程而 变为关断，此时称为正向阻断。,key parameter gate current( Ig) Definition: the amount of gate current needed to turn the SCR on. The current varies from one device to another. SCR

11、s designed to control small amounts of power require a gate current of only a few micro-amps, while it control large amounts of power require a gate current of several hundred milliamps The current needed to turn the SCR on is only a small amount fraction of the amount of the current that the device

12、 is designed to handle through A and K.,(2) holding current (Ih ) Definition: the amount of current flowing through the ANODE-CATHODE circuit needed to keep the SCR turned on holding current varies from one device to another, the more power an SCR is designed to control , the higher the holding curr

13、ent will be. If the SCR is in on-state and Ih=100ma, when the current through ANODE and CATHODE drops below 100ma, the SCR will turn off.,5. Testing the SCR SCRs can be tested with an ohmmeter by connecting the positive lead to the anode and the negative lead to the cathode. (1) When an ohmmeter is

14、connected across the anode-cathode circuit of the SCR, there is infinite or high resistance. The high resistance value is seen since the SCR is off unless triggered by the gate.,(2) With the lead connected and using a jumper, touch the gate of the SCR to the positive ohmmeter lead. The SCR shows a g

15、reat decrease of resistance. (3) When the gate is disconnected from the positive lead, the SCR may turn off or continue to conduct depending on the device. If it has a small holding current, the ohmmeter may supply enough current to keep the SCR turn on. If the ohmmeter cannot supply enough holding

16、current, the SCR will turn off.,6. SCR in DC circuit The SCR is the workhorse of industrial electronics because it can control large amount of power. This is because it has only two states of operation, on or off. In the off state of the circuit, the full 200V is dropped across the SCR. As there is

17、no current, there is no power being dissipated in the form of heat.,When the switch S1 closed, the SCR turns on and there is about 20amps of current flows. In the on state, the SCR has a voltage drop of about 1V. Then the SCR dissipate 20 watt of heat. Heat sinking components is needed.,+,-,7. APPLI

18、CATIONS: A good example of how an SCR can be used in a DC circuit can be seen in figure below. This is a battery charger that turns the power applied to the battery off when the battery reaches full charge. The circuit is to be used to charge a 12 lead-acid battery.,The transformer is a step-down tr

19、ansformer with a secondary voltage of 28 volts. Notice that the secondary of the transformer has been center tapped. the two diode, D1 and D2, form a full-wave rectifier with the center tap of the transformer forming the positive output lead.,In this circuit, the two-diode type of a rectifier will p

20、roduce a peak DC voltage of 19.8 volts(14*1.414) When a battery is in a low-charge state, the terminal voltage is low, which permits the power supply to furnish current to the battery.,As the state of charge of the battery increases, its terminal voltage will increase to a maximum of 14 volts. The a

21、mount of charging current will be much higher during the period of time the battery is in a low state of charge. As the state of charge of the battery increases, the amount of the charging current will decrease to a very small amount.,The amount of gate current supplied to the SCR is determined by t

22、he amount of resistance connected in the gate circuit, and the difference in voltage between the power supply and the terminal voltage of the battery. When the battery is in a low-charge state, the difference in voltage is great enough to permit enough gate current to flow to turn the SCR on.,Once t

23、he SCR has been turned on by the gate, it will remain on due to the relatively high-charging current. As the state of charge of the battery increase, two things happen at the same time. The charge current decreases The terminal voltage of the battery increases,As the terminal voltage of the battery

24、increases, there is less difference in voltage between the power supply and the battery, which cause the gate current supplied to the SCR to decrease. If the variable resistor connected in the gate circuit of the SCR is adjusted properly, the amount of gate current supplied to the SCR will drop belo

25、w the level needed to turn it on when the terminal voltage of the battery reaches about 13.8 volts.,If the SCR has been selected correctly, the charging current at this point will have dropped below the holding level of the SCR. This permits the SCR to turn off and stop conduction of the charging cu

26、rrent completely.,8. GTO (Gate-Turnoff) SCR The gate-turnoff SCR is also known by the name gate controlled switch (GCS). The gate-turnoff SCR operates in a fashion similar to a common SCR in that it is turned by a gate pulse that is positive with respect to the cathode. Unlike the SCR, however, it c

27、an be turned off with a gate pulse that is negative with respect to the cathode.,A commonly used schematic symbol for the GTO is shown in figure below. Although the GTO can be turned off with a negative current pulse on the gate, it requires from 10 to 20 times more negative gate current to turn it

28、off than it does to turn it on. The greatest advantage of the GTO over a common SCR is its ability to control power in a direct current circuit. Since the anode-cathode current does not have to drop below a holding current level to turn it off, it is much easier to control than an SCR.,5.3 THE SCR i

29、n AC circuit When connecting an SCR in an AC circuit, the first thing to remember is that the SCR is a rectifier and the output of the SCR is pulsating direct current. The only difference between an SCR and a common rectifier diode is that the SCR can be triggered to turn on at a specific point in t

30、he waveform.,1. DC and AC connections when the SCR is connected into a DC circuit, the gate can turn the SCR on, but cannot turned it off. The current through the anode-cathode circuit of the SCR must drop below the holding current level before the device will turn off. The same is true for an SCR c

31、onnected into an AC circuit. The gate still controls the turn on of the SCR. The anode-cathode current must drop below the holding current level for the device to turn off.,the main difference in operation is caused by the AC power itself. The gate turns the SCR on, but the AC waveform dropping back

32、 to 0 volts will turn it off. It is therefore necessary to retrigger the gate of the SCR for each half cycle of alternating current conducted.,2. Operation of the circuit In this circuit, the gate of the SCR is connected through a resistor and diode directly to its anode. when the AC voltage applied

33、 to the anode rises in the positive direction, current flows through the gate-cathode section of the SCR.,when Ig reaches the trigger point (assume 5ma for this SCR), the SCR fires and conducts through the anode-cathode section. It conducts as long as the AC voltage remains in the positive direction

34、 and the current is above its holding current level. When the AC voltage drops to zero and begins to increase in the negative direction, the SCR remains turned off. When the voltage applied to the anode again becomes positive the gate will trigger the SCR on again.,3. Adding the variable Resistor Wh

35、y use an SCR if it does the same thing as a simple junction diode? The SCR can be controlled as to when it turns on. In the figure below, A variable resistor has been added to the gate circuit of the figure. Assume that the gate current must reach a 5ma level before the SCR will fire.,By adjusting t

36、he variable resistor, it is possible to determine how much voltage must be applied to the gate before a 5ma current flows through the gate circuit. By adjusting the resistor to a higher value, it is possible to keep the SCR from firing until the AC voltage has reached its peak value in the positive

37、direction.,In this way the SCR will fire when the AC voltage is at its positive peak. With this setting of the resistor, the SCR will drop half the voltage and the load will drop the other half.,Reducing the resistance By reducing the resistance of the gate circuit, the gate current reaches 5 ma soo

38、ner, and the SCR fires earlier in the AC cycle. This causes less voltage to be dropped across the SCR and more to be dropped across the load. 缺图。,If the gate resistance is reduced even more, the 5 ma of gate current will be reached even sooner during the cycle and the SCR will fire earlier. Still le

39、ss voltage is dropped across the SCR and more voltage is dropped across the load.,4. Problems met on the way to full control There is a problem with this type of control, however, the SCR only controls half of the positive cycle of alternating current applied to it. The latest that the SCR can be fi

40、red is when the positive half cycle has reached 90 This permits the SCR to control only half of the AC positive wave; half voltage is applied to the load when the SCR initially fires. If the load resistor is a light bulb, the bulb will burn at half brightness when it is first turned on. The SCR circ

41、uit controls the light bulb from half brightness to full brightness,5. Application The circuit shown in figure below is an example of how an SCR can be used in an AC circuit. In this circuit, the SCR is used as a touch controller. When the touch plate is touched by a person, the SCR turns on and per

42、mits current to flow through the load.,The SCR will remain turned on as long as the person continues to touch the plate. This circuit can be used when it is necessary to have a switch with no moving or mechanical contacts. Notice that a neon lamp has been connected in series with the gate of the SCR

43、. The neon lamp is a gas-filled tube and depends on ionization of the gas for operation.,Most devices that operate by ionization have a characteristic known as negative resistance. This means that the device will start conduction or turn on at one voltage and continue conduction at a lower voltage. For example, assume that this lamp requires a potential of 15 volts across it to ionize or start conduction. Once the gas in the tube has ionized, it will continue to conduction. Once the gas in the tube has ionized, it will continue to conduct at a much lower vol