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1、阅读教材380,1 What are the one word answers to the following questions?,It is one of the most popular types of diode. When forward biased with current flowing through the junction, light is produced. The diodes can produce a variety of colours, although the original colour was red.,Light emitting diodes
2、,This type of diode is not the same as the ordinary light emitting diode because it produces coherent light. They are widely used in many applications from DVD and CD drives to laser light pointers for presentations. They also have a limited life.,Laser diode:,It is used for detecting light. It is f
3、ound that when light strikes a PN junction it can create electrons and holes. These diodes are operated under reverse bias conditions where even small amounts of current flow resulting from the light can be easily detected. They can also be used to generate electricity. For some applications.,Photod
4、iode:,It is a very useful type of diode as it provides a stable reference voltage. It is run under reverse bias conditions and it is found that when a certain voltage is reached it breaks down. If the current is limited through a resistor, it enables a stable voltage to be produced. This type of dio
5、de is therefore widely used to provide a reference voltage in power supplies.,Zener diode:,It may be thought of as the normal or standard type of diode in use today. These diodes can come as small signal types for use in radio frequency, or other low current applications which may be termed as signa
6、l diodes.,PN Junction diode,This type of diode has a lower forward voltage drop than ordinary silicon PN junction diodes. At low currents the drop may be somewhere between 0.15 and 0.4 volts as opposed to 0.6 volts for a silicon diode. To achieve this performance they are constructed in a different
7、way to normal diodes having a metal to semiconductor contact. They are widely used as clamping diodes, in RF applications, and also for rectifier applications.,Schottky diodes:,2 what are charge carriers? Identify the charge carriers in metallic substances, semiconducting substances, and conductive
8、liquids.,“Charge carriers“ are any particles possessing an electrical charge. Different types of substances have different charge carriers: Metals: “free“ (conduction-band) electrons Semiconductors: electrons and holes Liquids: ions,3 Which type of material is the best conductor of electricity? Whic
9、h type of material is the worst conductor of electricity?,Metals are the best conductors of electricity. Insulators are the worst conductors of electricity.,4 Describe the difference between an intrinsic and an extrinsic semiconducting substance.,An “ intrinsic“ semiconducting material is absolutely
10、 pure. An “ extrinsic“ semiconducting material has dopant(s) added for enhanced conductivity.,5 In extrinsic semiconductors, what are majority carriers and how do they differ from minority carriers?,N-type semiconductor The number of free electrons The number of holes. Free electrons are majority ca
11、rriers. Holes are minority carriers.,P-type semiconductor The number of free electrons The number of holes, Holes are majority carriers. Free electrons is minority carriers.,6 The characteristic curve for a normal PN junction rectifying diode looks something like this:,Reverse characteristic 反向特性,Fo
12、rward characteristic 正向特性,Barrier potential 死区,Breakdown voltage 击穿电压,Reverse cut-off 反向截止,Reverse breakdown 反向击穿,Reverse saturation current 反向饱和电流,Characteristic curve 特性曲线,Ge-diode 锗管 Si-diode 硅管,7 How is it possible to determine the polarity of a rectifying diode (which terminal is the anode, and
13、 which terminal is the cathode) from its physical appearance?,there is a stripe (similar to a color band on a resistor) closer to one end of the diode than the other!,8 The “1N400x“ series of rectifying diodes are very popular for low-current applications. By “1N400x,“ I mean the 1N4001, 1N4002, 1N4
14、003, . . . 1N4007. Only one parameter differs between these different diode models. What parameter is this?,Only the reverse (or blocking) voltage rating differs between these diode models.,9 Applications of the Diode.,1. Rectifying 整流,2. Amplitude limiting 限幅,Fig.1,3. Clamping 钳位,4. Isolation 隔离,10
15、 What are power supplies?,All electronic circuits need a power source to work. For electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific value.,11 Fill in the blanks.,(1)The AC line is first passed through a low pass filter shown in figure. This
16、eliminates noise on the AC line from bothering the power supply circuits and prevents unwanted signals from the power supply from being transferred back into the AC line where they might interfere with other equipment.,(2)A transformer is commonly used to step the input AC voltage level down or up.
17、Most electronic circuits operate from voltages lower than the AC line voltage so the transformer normally steps the voltage down by its turns ratio to a desired lower level. For example, a transformer with a turns ratio of 10 to 1 would convert the 120 volt 60 Hz input sine wave into a 12 volt sine
18、wave.,(3)The rectifier converts the AC sine wave into a pulsating DC wave. There are several forms of rectifiers used but all are made up of diodes,(4)The rectifier produces a DC output but it is pulsating rather than a constant steady value over time like that from a battery. A filter is used to re
19、move the pulsations and create a constant output. The most common filter is a large capacitor.,(5)The simplest form of rectifier is the half wave rectifier shown. Another widely used rectifier is the bridge rectifier. It uses four diodes.,(6)The regulator is a circuit that helps maintain a fixed or
20、constant output voltage. Changes in the load or the AC line voltage will cause the output voltage to vary. Most electronic circuits cannot withstand the variations since they are designed to work properly with a fixed voltage. The regulator fixes the output voltage to the desired level then maintain
21、s that value despite any output or input variations.,(7)Most modern power supplies also contain one or more DC-DC converters Modern electronics often demand different voltages to function. A DC-DC converter changes one DC voltage to another, higher or lower DC voltage. A DC-DC converter is used with
22、 a power supply to prevent the need for a second AC-DC supply.,12 Main circuits in most power supplies?,13 How does the filter work?,A large capacitor is connected across the load resistor. When the diode conducts, the capacitor charges up to the peak of the sine wave. Then when the sine voltage dro
23、ps, the charge on the capacitor remains. Since the capacitor is large it forms a long time constant with the load resistor. The capacitor slowly discharges into the load maintaining a more constant output.,14 Although not a popular design, some power supply circuits are transformerless. Direct recti
24、fication of AC line power is a viable option in some applications: However, this form of AC-to-DC power conversion has some significant limits. Explain why most power supply circuits utilize a transformer instead of directly rectifying the line power as this circuit does.,Transformers provide voltag
25、e/current ratio transformation, and also electrical isolation between the AC line circuit and the DC circuit. The issue of isolation is a safety concern, as neither of the output conductors in a non-isolated (direct) rectifier circuit is at the same potential as either of the line conductors.,15 Obs
26、erve the following two waveforms, as represented on an oscilloscope display measuring output voltage of a filtered power supply: If both of these waveforms were measured on the same power supply circuit, at different times, determine which waveform was measured during a period of heavier “loading“ (
27、a “heavier“ load being defined as a load drawing greater current).,The left-hand waveform was measured during a period of heavier loading. If it is necessary for us to maintain a low amount of ripple under this heavy loading, what must we change in the power supply circuit?,16 What is ripple?,The ca
28、pacitor does a good job of smoothing the pulses from the rectifier into a more constant DC. A small variation occurs in the DC because the capacitor discharges a small amount between the positive and negative pulses. Then it recharges. This variation is called ripple. The ripple can be reduced furth
29、er by making the capacitor larger. The ripple appears to be a sawtooth shaped AC variation riding on the DC output.,17 What does it mean if a power supply has a DC output with 5% ripple?,This means the peak-to-peak ripple voltage is equal to 5% of the DC (average) voltage.,18 What does it mean if a
30、power supply exhibits 2% voltage regulation?,This means the difference between no-load output voltage and full-load output voltage is 2% of the full-load output voltage.,19 What will be the consequence of one diode failing open in the bridge rectifier of a single-phase power supply?,The (unfiltered) output voltage will be half-wave, not full-wave.,