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 lossesPower losses in the power electronic converters are comprised of the switching losses and
3、 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 thre
4、e components: (a) the on-state losses, (b) the off-state losses and the losses in the transition states.,Section 1 Semiconductor Switches,On-State LossesThe electrical switches conduct heavy current and have nonzero voltage across the switch in the on-state. The on-state power losses are given by,(3
5、-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. The
6、 on-state,Section 1 Semiconductor Switches,power losses are very significant. Off-State LossesThe 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 usoff
7、 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. Transi
8、tion-State LossesThe 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 in t
9、wo cases separately: the inductive and capacitive loads. Switching with Inductive LoadThe 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. When t
10、he 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 curr
11、ent 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 equals th
12、e 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 ramps dow
13、n 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 resistive
14、 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 the sw
15、itch 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 curren
16、t 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 is clos
17、ed, 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 power
18、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 called snu
19、bbering and the later is known as zero-voltage and zero-current switching.,Return,New Words and Expressions,semiconductor n. 半导体 power electronic n. 电力电子 loss n. 损耗，损失 parasitic a. 寄生的，附加的 winding n. 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 and Ex
20、pressions,eddy n. 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,buildup n. 形成，产生，出现 增加，增大，上升 建造，建起 ramp v. 斜变，倾斜，直线上升 waveform n. 波形 clamp v. 箝位，定位，使固定 snubbering n. 缓
21、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 productivity. 使
22、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 development
23、. 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 extreme
24、ly 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、介词转换成动词英语
25、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 we can ke
26、ep 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. 我们的试验报告写
27、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 losses B. the dielectric losses C. the off-state losses D. the hysteresis losses 2. The typical power transistors in th
28、e on-state have .A. 0.1 V B. 0.7 V C. 2V D. 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 forward B. high forward C. low reverse D. high reverse 4. The rate of voltage transition and the rate of curr
29、ent steering can produce in the switching devices.A. the switching losses B. the parasitic lossesC. 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 det
30、ermine 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
31、currents, withstand high reverse voltages and should possess fast switching characteristics. These requirements render power diodes very different than the ordinary signal diodes. It is also difficult to achieve all the above three characteristics in a single device. Hence, several types of power di
32、odes are available suited for particular applications.,Exercises,III. Translate the following into Chinese,The power diode is a high-current, high-voltage diode with medium fast switching characteristics. The Schottky Barrier diode uses a metal-semiconductor junction and has a lower on-state voltage
33、 than the pn diodes. It has the high-current capability and fast switching characteristics but it is a low-voltage device and exhibits a much higher leakage current. These diodes are ideally suited for switch-mode power supplies. The on-state power,Exercises,III. Translate the following into Chinese
34、,losses, which are calculated by multiplying the on-state voltage drop by the on-state current, determine the package size of all diodes.,Return,Chapter 3 Section 1 End,Section 2 The DC-DC Converters,Text,New Words and Expressions,Exercises,End,Transition of part of speech,Section 2 The DC-DC Conver
35、ters,IntroductionThe DC-DC converter converts a DC power source to another DC source with different terminal specifications. The DC-DC converters change DC to AC first and than change AC back to DC. The DC source is often the uncontrolled DC voltage with ripple from AC to DC rectifier.The DC-DC conv
36、erters are widely used in the switch-mode power supplies and DC motor drive applications. Some of these converters, especially in power supplies, have an isolation transformer.,Section 2 The DC-DC Converters,The DC-DC converters are also used as interfaces between the DC systems of different voltage
37、 levels.The step-down and step-up voltage converters are the two basic converter topologies. These are referred as the Buck and Boost converters respectively. It must, however, be kept in mind that a step-down voltage converter is also a step-up current converter and vice versa because the input pow
38、er must equal the output power. The Buck-Boost converter does both stepping up and down,Section 2 The DC-DC Converters,action. The UK converter is a dual of the Buck-Boost converter. All these converters have single, two and four quadrant variations in topologies.The voltage source driven DC-DC conv
39、erts are more popular than the current driven converters, hence, this section will concentrate on the voltage driven converters. All converts use effective filtering on both the input and the output to reduce the ac components from going outside the converters.,Section 2 The DC-DC Converters,Buck Co
40、nverter The Buck converter is a voltage step-down and current step-up converter. The topology is shown in Fig.3-2. The two-position switch is synthesized from a switch and a diode. The switch is turned on for a time periodically at a rate 1/Ts.,Fig.3-2 Buck converter topology,Section 2 The DC-DC Con
41、verters,A prominent application of the Buck converter is a DC regulated power supply in which the output voltage is regulated against the variations in the load resistance and the input voltages. These power supplies are used in computers and portable instruments in the medical and communication fie
42、ld. There are two modes of Buck converter operations in which the output current waveform is either continuous or discontinuous in time over a switching period.,Section 2 The DC-DC Converters,Continuous conduction mode (CCM)The ideal current sink in the output represents infinitely large inductor in
43、 series with the load resistance. This is a hypothetical circuit but its discussion provides a good understanding of the Buck converter operation.The steady-state operation consists of two circuit modes, shown in Fig.3-3.,Fig.3-3 Circuit modes in steady-state operation,Section 2 The DC-DC Converters
44、,As the switch is turned on in mode l, the diode is reverse biased and the current flows through inductor into the voltage sink. After a time , the switch is turned off. The inductor current then freewheels through the diode as shown in the circuit of mode 2. The second mode is terminated at TS when
45、 the switch is turned on again. The current and voltage waveforms are shown in Fig.3-4. The average output voltage is,Section 2 The DC-DC Converters,or,(3-5),Fig.3-4 The current and voltage waveforms,Section 2 The DC-DC Converters,D is called the duty ratio. The duty ratio D is also the turn ratio o
46、f the equivalent DC transformer. The turn ratio of the equivalent dc transformer can be controlled electronically in a range of 01.The DC output voltage is a linear function of the on-time of the switch, the pulse width, and the switching frequency (inverse function of the switching period TS). The
47、output voltage control using pulse width is known as the PWM control. The output voltage can also be controlled by switching frequency. The PWM control is more popular.,Section 2 The DC-DC Converters,If the duty ratio D is made a linear function of uC, a control voltage,(3-6),The output voltage is t
48、hen a linear function of the control voltage. This is also the principle of switch-mode linear amplifier. The gain of this amplifier is determined by the input dc source voltage. Neglecting the power losses in the circuit elements, we could use the equation of the balance of power,Section 2 The DC-D
49、C Converters,(3-7),where IS is the average current from the DC source. Hence,(3-8),Although the DC output voltage is smaller than the DC input voltage, the DC output current is larger than the input DC current (a step-up action in current),Return,New Words and Expressions,ripple n. 波纹，波度，微波 rectifier n. 整流器 isolation transformer 隔离变压器 interface n. 接口 Buck converter 降压式变换器，降压式斩波器 Boost converter 升压式变换器，升压式斩波器 Buck-Boost converter 升/降式变换器，升/降式斩波器 quadrant n. 象限，四分之一圆周，九十度弧,
