电气工程及其自动化专业英语第三章section_3-1

1、Chapter 3Power Electronic Technology Section 1 Semiconductor Switchesn Textn New Words and Expressionsn Exercisesn Endn Transition of part of speechSection 1 Semiconductor Switches Semiconductor switches are very important and crucial components in power electronic systems. These switches are meant

2、to be the substitutions of the mechanical switches, but they are severely limited by the properties of the semiconductor materials and the process of manufacturing. Section 1 Semiconductor SwitchesSwitching losses Power losses in the power electronic converters are comprised of the switching losses

3、and the parasitic losses. The parasitic losses account for the losses due to the winding resistances of the inductors and transformers, the dielectric losses of capacitors, the eddy and the hysteresis losses. The switching losses are significant and can be managed. They can be further divided into t

4、hree components: (a) the on-state losses, (b) the off-state losses and the losses in the transition states. Section 1 Semiconductor SwitchesOn-State Losses The electrical switches conduct heavy current and have nonzero voltage across the switch in the on-state. The on-state power losses are given by

5、 fsononiuP (3-1) The uson and if are respectively the switch voltage in the on-state and the forward current through the switch. For example, the typical power diodes and the power transistors have nearly 0.5 to l volt across them in the on-state. The forward currents can be hundreds to thousands of

6、 amperes. The on-stateSection 1 Semiconductor Switchespower losses are very significant. Off-State Losses The electrical switches withstand high voltages and have nonzero leakage current through the switch in the off-state. The off-state power losses are given by rsoffoffiuP(3-2)Section 1 Semiconduc

7、tor Switches The usoff and ir are respectively the reverse bias voltage in the off-state and the reverse current through the switch. For example, the typical power diodes and the power transistors have high reverse voltages in hundreds to thousands of volts and microamps to milliamps through them in

8、 the off state. Transition-State Losses The practical switching devices have limited capabilities of rate of voltage transition and the rate of current steering. These nonabrupt transition ratesSection 1 Semiconductor Switchesgive rise to power losses in the switching devices. We will examine these

9、switching losses in two cases separately: the inductive and capacitive loads.Switching with Inductive Load The inductor is assumed to be large so that the current through it in steady state is nearly constant Io. Assume that initially the switch is off. The inductor current is +Io and freewheels thr

10、ough diode V1. When the switch is turned on, the current through the switch begins to build up linearly (an assumption) to +Io while the diode V1 is still on. Section 1 Semiconductor SwitchesThe on diode has zero voltage across it (an ideal diode), hence, the voltage on the switch is held constant a

11、t +Us. When the current buildup is over, the diode Vl ceases to conduct and the voltage on the switch ramps linearly (again an assumption) down to zero. When the switch is turned off, the voltage begins to build up linearly to +Us while the diode V1 is off. While the diode is off the current through

12、 the switch equals the inductor current, which is constant Io. After the switch voltage reaches zero, Section 1 Semiconductor Switchesthe current through the switch begins to decrease b e l o w I o , a s t h e remaining current is now steered through the diode V1, which has now turned on. The curren

13、t through the switch ramps down to zero ultimately. Switching waveforms with inductive load are shown in Fig.3-1. Fig.3-1 Switching waveforms with inductive load Section 1 Semiconductor Switches The switching losses are given by s2off1off2on1onossw21fttttIUP(3-3) The switching power losses increase

14、linearly with the switching frequency like in the resistive case but about six times more. The upper bound on the switching frequency is also about half. 2off1off2on1onmaxs1ttttf(3-4)Section 1 Semiconductor SwitchesSwitching with capacitive load The capacitor is assumed to be large so that the volta

15、ge through it in steady state is nearly constant Uo. Assume that initially the switch is on, hence, the current through the switch is IS. The capacitor voltage is Uo, the voltage across the switch is zero and the diode V1 is reverse biased. When the switch is turned off, the switch voltage begins to

16、 ramp up to + Uo while the diode V1 is still off. During this buildup, the current through the switch is held constant at IS . When the voltage buildup isSection 1 Semiconductor Switchesover, the diode Vl begins to conduct and the voltage on the switch is clamped at Uo, and the current through the s

17、witch ramps linearly (again an assumption) down to zero. When the switch is closed, the current begins to build up linearly to IS while the diode V1 is still on. The voltage on the switch remains clamped at UO. After the switch current reaches IS, the diode turns off and the voltage on the switch be

18、gins to ramp down to zero. Section 1 Semiconductor Switches The switching power losses in the case of capacitive load also have similar dependence as in the case of inductive loads. The switching losses can be usually minimized in two ways: divert the energy from the switch to a loss or non-loss cir

19、cuit or switch at either zero current or at zero voltage. The first is called snubbering and the later is known as zero-voltage and zero-current switching. ReturnNew Words and Expressionssemiconductor n. 半导体power electronicn. 电力电子lossn. 损耗，损失parasitic a. 寄生的，附加的windingn. 绕组，线圈 一圈，一转 transformer n 变压

20、器，变换器，变量器dielectric n. 介质，电介质，绝缘材料；a. 绝缘的，介电的，介质的，不导电的 New Words and Expressionseddyn. 涡流，旋涡； a. 涡流的，涡旋的；v. 涡流，起旋涡hysteresis n. 磁滞，滞后，迟滞on-state a. 通态的，接通的，开态off-state a. 关态的，断开的，断态的microamp n. 微安milliamp n. 毫安steering n. 转向，操纵，控制，调整 New Words and Expressionsbuildupn. 形成，产生，出现 增加，增大，上升 建造，建起rampv. 斜

21、变，倾斜，直线上升waveform n. 波形clampv. 箝位，定位，使固定snubbering n. 缓冲zero-voltage 电压过零的zero-current 电流过零的 ReturnTransition of Part of Speech 词类转换是指把原文语言中的某一词类转译为汉词类转换是指把原文语言中的某一词类转译为汉语的另一词类。语的另一词类。一、转换为汉语动词一、转换为汉语动词 1、名词转换成动词、名词转换成动词 英语中表达动作概念的动名词、具有动作意义的抽英语中表达动作概念的动名词、具有动作意义的抽象名词、由动词派生出来的名词、动词加名词构成的象名词、由动词派生出来的

22、名词、动词加名词构成的固定短语、介词加名词构成的固定短语等，往往可以固定短语、介词加名词构成的固定短语等，往往可以根据具体情况译成汉语动词。根据具体情况译成汉语动词。 (1) The application of electronic computers makes for a tremendous rise in labor productivity. 使用电子计算机可以大大提高劳动生产率。 Transition of Part of Speech(2) Computers can provide analyses of every operation in a factory. 计算机能对

23、工厂的每道工序进行分析分析。(3) Primary forces have certain valuations that must be considered in any design. 在任何设计中，对于数值能确切计算计算的主要作用力都必须加以考虑。(4) Integrated circuits are fairly recent development. 集成电路是近年来发展起来发展起来的。 Transition of Part of Speech2、形容词转换成动词、形容词转换成动词 英语中某些由动词转换来的形容词、同介词搭配构成句子表语或定语的形容词、与连系动词一起构成复合谓语的形

24、容词，通常可译成汉语中的动词。(1) Copper wire is flexible. 铜线容易弯曲弯曲。(2) Television is different from radio in that it sends and receives pictures. 电视不同于不同于无线电在于能发送并接收图像。 Transition of Part of Speech(3) If extremely low-cost power were ever to become available from large nuclear power plants, electrolytic hydrogen

25、would become competitive. 如能从大型核电站获得获得成本极低的电力，电解氢的竞争能力就会增强。(4) Heat is a form of energy into which all other forms are convertible. 热是能的一种形式，其他一切能的形式都能转化转化为热能。 Transition of Part of Speech3、介词转换成动词、介词转换成动词 英语中的介词或介词短语在许多情况下可以译成汉语的动词，尤其是当它们用作表语或状语时。(1) The letter E is commonly used for electromotive

26、force. 通常用字母E表示表示电动势。(2) Except for atomic energy, all forms of energy used by man are from the sun. 除原子能之外，人类所利用的一切形式的能量都来来自自太阳。Transition of Part of Speech(3) So long as we have means of producing heat we can keep the steam engine at work. 只要能产生热，就能使蒸汽机做做功。 Transition of Part of Speech4、 副词转换成动词副词

27、转换成动词 英语中有些作表语的副词或复合宾语中的副词，往往可译成汉语的动词。(1) The electric current flows through the circuit with the switch on. 如果开关接通接通，电流就流过线路。(2) Open the valve to let air in. 打开阀门，让空气进入进入。(3) We are through with our test report. 我们的试验报告写写完了。 ReturnExercises I. Choose the best answer into the blank 1. The switching

28、 losses are comprised of the on-state losses, and the losses in the transition states. A. the parasitic lossesB. the dielectric losses C. the off-state lossesD. the hysteresis losses2. The typical power transistors in the on-state have . A. 0.1 VB. 0.7 V C. 2VD. 5V Exercises I. Choose the best answe

29、r into the blank 3. The power transistors, as electrical switches, withstand voltages in the off-state. A. low forwardB. high forward C. low reverseD. high reverse4. The rate of voltage transition and the rate of current steering can produce in the switching devices. A. the switching losses B. the p

30、arasitic losses C. power losses D. the dielectric losses Exercises II. Answer the following questions according to the text 1. Which losses are power losses in the power electronic converters comprised of?2. How to determine the off-state power losses in the electrical switches?3. How do the switching power losses change with the switching frequency?4. How many ways to minimize the switching losses are there? Exercises III. Translate the following into Chinese Diodes in power electronic applications carry high currents, withstand high reverse volt