电力电子外文翻译.doc
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1、 毕业设计(外文翻译)题 目 电路与功率二极管器件 系 (院) 自动化系 专 业 电气工程与自动化 学生姓名 陈芬 学 号 2007090403 指导教师 刘应乾 职 称 助 教 2011年 6月15日15Electrical Networks and Power Semiconductor DevicesElectrical NetworksAn electrical circuit or network is composed of elements such as resistors, inductors, and capacitors connected together in som
2、e manner. If the network contains no energy sources, such as batteries or electrical generators, it is known as a passive network. On the other hand, if one or more energy sources are present, the resultant combination is an active network. In studying the behavior of an electrical network, we are i
3、nterested in determining the voltages and currents that exist within the circuit. Since a network is composed of passive circuit elements, we must first define the electrical characteristics of these elements.In the case of a resistor, the voltage-current relationship is given by Ohms law, which sta
4、tes that the voltage across the resistor is equal to the current though the resistor multiplied by the value of the resistance. Mathematically, this is expressed as (1-1A-1) where u=voltage,V; i=current, A; R=resistance, .The voltage across a pure inductor is defined by Faradays law, which states th
5、at the voltage across the inductor is proportional to the rate of change with time of the current through the inductor. Thus we have (1-1A-2)Where =rate of change of current, ; L=inductance, H.The voltage developed across a capacitor is proportional to the electric change q accumulating on the plate
6、s of the capacitor. Since the accumulation of charge may be expressed as the summation, or integral, of the charge increments dq, we have the equation (1-1A-3)where the capacitance C is the proportionality constant relating voltage and charge. By definition, current equals the rate of change of char
7、ge with time and is expressed as i= . Thus an increment of charge dq is equal to the current multiplied by the corresponding time increment, or dq=idt. Eq.(1-1A-3) may then be written as (1-1A-4)where C= capacitance, F.Active electrical devices involve the conversion of energy to electrical form. Fo
8、r example, the electrical energy in a battery is derived from its stored chemical energy. The electrical energy of a generator is a result of the mechanical energy of the rotating armature.Active electrical elements occur in two basic forms: voltage sources and current sources. In their ideal form,
9、voltage sources generate a constant voltage independent of the current drawn from the source. The aforementioned battery and generator are regarded as voltage sources since their voltage is essentially constant with load. On the other hand, current sources produce a current whose magnitude is indepe
10、ndent of the load connected to the source. Although current sources are not as familiar in practic, the concept does find wide use in representing an amplifying device, such as the transistor, by means of an equivalent electrical circuit. A common method of analyzing an electrical network is mesh or
11、 loop analysis. The fundamental law that is applied in this method is Kirchhoffs first law, which states that the algebraic sum of the voltages around a closed loop is 0, or, in any closed loop, the sum of the voltage rises must equal the sum of the voltage drops. Mesh analysis consists of assuming
12、that currents-termed loop currents-flow in each loop of a network, algebraically summing the voltage drops around each loop, and setting each sum equal to 0.Power Semiconductor DevicesPower semiconductor devices constitute the heart of modern power electronic appartus. They are used in power electro
13、nic converters in the form of a matrix of on-off switches. And the switching mode power conversion gives high efficiency.Todays power semiconductor devices are almost exclusively based on silicon material and can be classified as follow: Diode Thyristor or silicon-controlled rectifier (SCR) Triac Ga
14、te turn-off thyristor (GTO) Bipolar junction transistor (BJT or BPT) Power MOSFET Static induction transistor (SIT) Insulated gate bipolar transistor (IGBT)MOS-controlled thyristor (MCT) Integrated gate-commutated thyristor (IGCT)DiodesPower diodes provide uncontrolled rectification of power and are
15、 used in applications such as electroplating, anodizing, battery charging, welding, power supplies (DC and AC), and variable-frequency drives. They are also used in feedback and the freewheeling functions of converters and snubbers. A typical power diode has P-I-N stucture, thatis, it is a P-N junct
16、ion with a near intrinsic semiconductor layer (I-layer) in the middle to sustain reverse voltage.In the forward-biased condition, the diode can be represented by a junction offset drop and a series-equivalent resistance.The typical forward conduction drop is 1.0V. This drop will cause conduction los
17、s,and the device must be cooled by the appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small leakage urrent flows due to minority carries, which gradually increase with voltage. If the reverse voltage exceeds a threshold value, called the breakdown voltage
18、, the device goes through avalanche breakdown, which is when reverse current becomes large and the diode is destoroyed by heating due to large power dissipation in the junction.Power diodes can be classified as follows:Standard or slow-recovery diode Fast-recovery diodeSchottky diodeThyristorsThyris
19、tors, or silicon-controlled rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The modern era of solid-state power electronics started due to the intorduction of this device in the late 1950s. The term ”thyristor” camefrom its gas tube equivalen
20、t, thyratron. Often, it is a family name that includes SCR,taiac, GTO,MCT, and IGCT. Thyristors can be classified as standard, or slow phase-control-type and fast-switching,voltage-fed inverter-type. The inverter-type has recently become obsolete.Basically, it is a three-junction P-N-P-N device, whe
21、re P-N-P and N-P-N component transistors are connected in regenerative feedback mode. The device blocks volgate in both the forward and reverse direction (symmetric blocking). When the anode is positive, the device can be trggered into conduction by a short positive gate current pulse; but once the
22、device is conducting, the gate loses its control to turnoff the device. A thyristor can also turn on by excessive anode voltage, its rate of rise (), by a rise in junction temperature, or by light shining on the junctions.At gate current IG = 0, if forward voltage is applied on the device, there wil
23、l be a leakage current due to blocking of the middle junction. If the voltage exceeds a critical limit (breakover voltage), the device switchs into conduction. With increasing magnitude of IG, the forward breakover voltage is reduced, and eventually at IG3, the device behaves like a diode with the e
24、ntire forward bloking region removed. The device will turn on successfully if a minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current is zero and the anode current falls below a critical limit, called the holding current, the
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