电气工程专业英语section-3-1

1、Chapter 3 Power Electronic Technology,Section 1 Semiconductor Switches,Text,New Words and Expressions,Exercises,End,Transition of part of speech,Section 1 Semiconductor Switches,Semiconductor switches are very important and crucial components in power electronic systems. These switches are meant to

2、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 Switches,Switching losses Power losses in the power electronic converters are comprised of the switching losses an

3、d 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 thr

4、ee components: (a) the on-state losses, (b) the off-state losses and the losses in the transition states.,Section 1 Semiconductor Switches,On-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、(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 amperes. T

6、he on-state,Section 1 Semiconductor Switches,power 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,(3-2),Section 1 Semiconductor Switches,The us

7、off 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 the off state. Tra

8、nsition-State Losses The practical switching devices have limited capabilities of rate of voltage transition and the rate of current steering. These nonabrupt transition rates,Section 1 Semiconductor Switches,give rise to power losses in the switching devices. We will examine these switching losses

9、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 through diode V1. W

10、hen 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 Switches,The on diode has zero voltage across it (an ideal diode), hence, the voltage on the switch is held constant at +Us. When the

11、 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 the switch equ

12、als the inductor current, which is constant Io. After the switch voltage reaches zero,Section 1 Semiconductor Switches,the current through the switch begins to decrease below Io, as the remaining current is now steered through the diode V1, which has now turned on. The current through the switch ram

13、ps 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,(3-3),The switching power losses increase linearly with the switching frequency like in the res

14、istive case but about six times more. The upper bound on the switching frequency is also about half.,(3-4),Section 1 Semiconductor Switches,Switching with capacitive load The capacitor is assumed to be large so that the voltage through it in steady state is nearly constant Uo. Assume that initially

15、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 ramp up to + Uo while the diode V1 is still off. During this buildup, the

16、current through the switch is held constant at IS . When the voltage buildup is,Section 1 Semiconductor Switches,over, the diode Vl begins to conduct and the voltage on the switch is clamped at Uo, and the current through the switch ramps linearly (again an assumption) down to zero. When the switch

17、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 begins to ramp down to zero.,Section 1 Semiconductor Switches,The switching

18、 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 circuit or switch at either zero current or at zero voltage. The first is ca

19、lled snubbering and the later is known as zero-voltage and zero-current switching.,Return,New Words and Expressions,semiconductor 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 a

20、nd Expressions,eddyn. 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 Expressions,buildupn. 形成，产生，出现 增加，增大，上升 建造，建起 rampv. 斜变，倾斜，直线上升 waveform n. 波形 clampv. 箝位，定位，使固定 snubbering n.

21、缓冲 zero-voltage 电压过零的 zero-current 电流过零的,Return,Transition 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语的另一词类。,一、转换为汉语动词 1、名词转换成动词 英语中表达动作概念的动名词、具有动作意义的抽象名词、由动词派生出来的名词、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 productivit

22、y. 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. 计算机能对工厂的每道工序进行分析。 (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的主要作用力都必须加以考虑。 (4) Integrated circuits are fairly recent develop

23、ment. 集成电路是近年来发展起来的。,Transition of Part of Speech,2、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容词，通常可译成汉语中的动词。 (1) Copper wire is flexible. 铜线容易弯曲。 (2) Television is different from radio in that it sends and receives pictures. 电视不同于无线电在于能发送并接收图像。,Transition of Part of Speech,(3) If e

24、xtremely low-cost power were ever to become available from large nuclear power plants, electrolytic hydrogen would become competitive. 如能从大型核电站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为热能。,Transition of Part of Speech,3、介词转

25、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。 The letter E is commonly used for electromotive force. 通常用字母E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自太阳。,Transition of Part of Speech,(3) So long as we have means of producing heat w

26、e can keep the steam engine at work. 只要能产生热，就能使蒸汽机做功。,Transition of Part of Speech,4、 副词转换成动词 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，电流就流过线路。 (2) Open the valve to let air in. 打开阀门，让空气进入。 (3) We are through with our test report.

27、 我们的试验报告写完了。,Return,Exercises,I. Choose the best answer into the blank,1. The switching 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 losses 2. The typical power transisto

28、rs in the on-state have . A. 0.1 VB. 0.7 V C. 2VD. 5V,Exercises,I. Choose the best answer 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 reverse 4. The rate of voltage transition and the rate o

29、f current steering can produce in the switching devices. A. the switching losses B. the parasitic 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. Ho

30、w 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 voltages and should possess f