专业英语期末考察_开关式电源.doc

嵌入式系统原理与开发期末考查 开关式电源 院（系）名称信息工程学院 专业班级 学生姓名 学号 2012年 6月 8 日电子信息工程专业英语期末考查 第13页开关式电源开关式电源（开关电源，开关电源，或开关）是一种电子电源供应器，包括一个具有电功率转换效率开关稳压器。像其他电源一样，开关电源的权力转移从源，如电源、负载、个人电脑，然而具有转换电压、电流特性。开关电源通常是通常在一个水平不同的输入电压中用来有效地提供一个稳定的输出电压。不像一个线性电源，通过晶体管的全开和截断状态，开关电源不断转换机能之间的低功耗，并把所有的时间都花在高耗能转换（减少能源浪费）。理想情况下，一个开关电源不消耗功。电压调节是由不同比例的开、关时间来实现的。与此相反，一个线性电源输出电压调节是通过不断耗散晶体管的功率来实现的。这种高能量转换效率是一个一个开关式电源的重要优势。开关式电源供应器也可以由于变压器的较小的体积和重量比线性电源供应器更小、更轻。开关稳压器由于具有效率高，尺寸小、重量轻的特性而替代线性稳压器是必然的。然而更为复杂的的问题是他们的开关电流会导致电噪声，如果不仔细抑制和设计，可能会有一个很低的功率因数。 说明：一个线性稳压器所需的输出电压是通过耗散过剩功率提供的。线性调节器通过耗散过剩的电能以热能的形式调节输出电压或电流，因此其输出电压 / 输入电压最大功率效率的自伏差异是浪费的。与此相反，一个开关电源是调节输出电压或电流的理想的开关存储元件，如电感和电容器，有不同的进、出电气配置。理想的开关元件（例如，晶体管以外的活动模式运作）没有阻力时，“关闭”；在没有电流时，“开放”。因此转换器在理论上可以100%有效率的操作（即，所有输入的功率都传递给负载；没有浪费热量）。 升压转换器的基本原理图例如，如果一个直流电源，一个电感，开关，以及相应的电气接地装置等一系列的开关在一个方波中显示出来，从波形上测量的峰峰值开关电压可超过从直流电源过来的输入电压。这是因为电感器响应电流变化诱导自身电压反电流的变化，源电压增加，开关打开。如果一个二极管和电容器结合是放置在平行于开关，电压峰值可以存储在电容器中，并可和电容器用一个直流电源输出电压大于直流电压的驱动电路。这就像一个升压变压器、升压转换器的直流信号。降压升压转换器工作在一个类似的方式中，但会产生一个与输出电压的极性相反的输入电压。其他降压电路的存在是为了提高平均输出电流与电压降。在开关电源中，输出电流取决于输入信号，存储元件和电路拓扑结构，也取决于驱动开关元件的使用模式（例如，脉宽调制与可调占空比）。通常情况下，这些开关波形信号的谱密度集中在相对较高的频率。因此，开关瞬变，就像涟漪，输出波形可以通过滤波器过滤。优点和缺点这种方法的主要优点是效率更高。当它在它的活动区域，开关晶体管的耗散功率小，（即，当晶体管像一个开关时，有一个微不足道的压降或微不足道的电流通过它）。其他优势有较小的尺寸，较轻的重量（从而消除低频变压器，高体重）和由于较高的效率产生的低热量。缺点有更大的复杂性，产生高振幅，高频率能量，低通滤波器一定要避免电磁干扰（电磁干扰），开关频率和谐波频率的纹波电压。低成本数字电源可能使成对的电气开关噪声返回到主电源线，造成干扰的视听设备连接到同一个阶段。不是正确的数字也造成谐波失真。输入整流器阶段交流，半波和全波整流信号如果开关电源有一个交流输入，那么第一阶段是转换输入的直流。这就是所谓的整流。整流电路可以被配置为一个电压倍增器的另外一个手动或自动开关。这一特征是一个功能较大的供应，允许120伏或240伏的电压供应。整流产生一个稳压直流电压，然后将其发送到一个大滤波电容器中。电流通过电源的整流电路发生在周围短脉冲交流电压峰值中。这些脉冲产生强大的高频能量，降低了功率因数。特殊控制技术可以采用开关电源迫使平均输入电流跟随正弦交流输入电压的波形，功率因素得到校正。开关电源的直流输入不需要这个阶段。开关电源设计的交流输入往往可以从一个直流电源直流供电（230伏交流，这将是330伏直流电）因为直流经过整流级不变。不过在尝试此之前最好咨询手册，虽然大部分供应完全可以这样操作即使没有文件中提到。然而，这种类型的使用可能是有害的，因为只有整流阶段使用二极管整流的完整的负载的一半。这可能导致这些组成部分过热，并导致他们过早损坏掉。如果输入范围切换使用，开关整流器阶段，通常被配置为作为一个电压倍增器工作时的低电压（ 120伏交流电）范围和作为一个直整流器工作时的高电压（240伏交流）范围。如果输入范围切换不被使用，那么全波整流通常是用来和逆变器阶段的下游是可以灵活的接受范围广泛的直流电压，然后将采用整流阶段。在高功率数字电源中，某种形式的量程自动切换范围是可以使用的。逆变阶段逆变器的逆变阶段转换成直流，不论是直接输入或从整流器阶段，如上所述，交流运行它时通过一个振荡器，其输出变压器是频率在几十或几百千赫的几个非常小的绕组。频率通常选择上述的20千赫以上，是人类所听不到的。输出电压耦合到输入，因此要非常严格的控制。开关是作为一个多级（实现高增益）的场效应管放大器。MOSFET类晶体管是低电阻和高电流处理能力的晶体管。电压转换器和输出整流器如果输出需要输入隔离，通常情况下是在主电源，倒AC是用来驱动一个高频变压器初级绕组的。这将使转换电压上升或下降到所需的输出级的副绕组。如果需要一个直流输出时，输出的交流电需要整流变压器整顿。输出电压高于十伏特左右，普通的硅二极管被普遍使用。对低电压而言，肖特基二极管是常用的整流元件；它的优势是有比硅二极管（允许损耗低，运行在更高的频率）更快的恢复时间及在较低的电压降下运行。甚至在较低的输出电压下，MOSFET晶体管可能被用来作为同步整流器；相对于肖特基二极管，这些甚至在更低电压降下工作。整流输出平滑滤波器是电感器和电容器组成的。对于更高得开关频率较低的电容和电感元件是必须的。简单的，非隔离电源用一个电感器代替变压器。这种类型包括升压转换器，巴克转换器，和降压升压转换器。这是最简单的一类单输入，单输出转换器。它使用一个电感和一个有源开关。降压转换器能降低于输入电压成正比的比率的导电时间，即总开关周期，称为占空比。例如，一个理想的降压转换器与一个10伏的输入电压，工作在50%工作周期，将产生一个平均输出电压为5伏特的反馈控制回路 ，它是用来调节输出电压的不同占空比来进行补偿输入电压。输出电压的升压转换器总是大于输入电压和输出电压的升压，但可以大于，等于或少于其输入电压的大小。这类转换器有许多的变化和扩展，但以这三种形式为基础，几乎所有的隔离和非隔离直流-直流转换器。通过添加第二个电感的uk和升压转换器是可以实现的，或者，通过添加额外的主动开关，也可以实现各种桥变换器。其他类型的数字电源使用电容二极管电压倍增器代替电感器和变压器。这些都是用来产生高电压，低电流（倍压整流器发电机）的。低电压变量叫做电荷泵。规则反馈电路监视输出电压及比较它与一个参考电压。根据设计/安全要求，控制器可能包含一个隔离机制（如光电耦合器）孤立的从直流输出。开关电源在电脑，电视和录像机这些光耦合器中严格控制输出电压。开环调节器没有反馈电路。相反，他们依赖恒定电压输入的变压器或电感器，并假设输出会是正确的。调节设计补偿阻抗的变压器或线圈。单极的设计也弥补了磁滞的核心。反馈电路需要运行功率才能产生动力，所以需要备用添加额外的非交换电源。变压器的设计开关电源变压器运行在高频率。电源供应造成的规模较小的高频变压器和以前的50 / 60赫兹变压器相比是最节省成本的（和节省空间）。有额外的设计权衡，终端电压的变压器是与产品的核心区，磁通量，和频率成正比的。通过使用更高的频率，核心区（等质量的核心）可以大大减少。然而，较高的频率也意味着过渡期间的开关半导体有更多的能量损失。此外，注重物理布局的电路板是必要的，大量的电磁干扰会变的更明显，核心损失的频率在增加。内核使用铁氧体材料在高频率和高通量密度中具有低损耗。叠层铁芯变压器低频（ 400赫兹）是令人无法接受的有损几个千赫的开关频率的。 功率因数简单的离线开关电源包括一个简单的全波整流器和一个大的储能电容器。这种数字电流吸引交流线路短脉冲时，电源瞬时电压超过该电容器上的电压。对于余下的部分，具有交流周期的电容器提供能量的电源供应。因此，开关电源等基本输入电流具有高谐波含量和相对较低的功率因数。这造成了额外负载实用线，提高了加热建筑布线，该实用变压器和标准的交流电动机，可能导致一些应用的稳定性问题，如应急发电机系统或飞机发电机。谐波可以过滤去除，但过滤器是昂贵的。与位移功率因数创造线性电感或电容负载，是这种扭曲不能纠正的另外一个线性组件。额外的电路需要抵制简短的电流脉冲的影响。将电流调节升压到滤波电路阶段后的离线整流器（电荷存储电容器）可以改善功率因数，但增加了复杂性和成本。2001年，欧盟实施标准/ IEC / EN61000-3-2设置限制输入电流谐波的交流多达第四十次谐波的设备超过75W以上的标准定义了四类设备，取决于其类型和电流波形。最严格的限制（类）被建立了个人电脑，电脑显示器，电视接收机。遵守这些要求，现代开关电源通常包括一个额外的功率因数校正（功率因数校正）阶段。附：英文原文Switched-mode power supplyA switched-mode power supply (switching-mode power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage.Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.Switching regulators are used as replacements for the linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.ExplanationA linear regulator provides the desired output voltage by dissipating excess power in ohmic losses (e.g., in a resistor or in the collectoremitter region of a pass transistor in its active mode). A linear regulator regulates either output voltage or current by dissipating the excess electric power in the form of heat, and hence its maximum power efficiency is voltage-out/voltage-in since the volt difference is wasted. In contrast, a switched-mode power supply regulates either output voltage or current by switching ideal storage elements, like inductors and capacitors, into and out of different electrical configurations. Ideal switching elements (e.g., transistors operated outside of their active mode) have no resistance when closed and carry no current when open, and so the converters can theoretically operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat).The basic schematic of a boost converter.For example, if a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source. This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals. A buckboost converter works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage.In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used (e.g., pulse-width modulation with an adjustable duty cycle) to drive the switching elements. Typically, the spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients, like ripple, introduced onto the output waveforms can be filtered with small LC filters. Advantages and disadvantagesThe main advantage of this method is greater efficiency because the switching transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts like a switch and either has a negligible voltage drop across it or a negligible current through it). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), a ripple voltage at the switching frequency and the harmonic frequencies thereof.Very low cost SMPSs may couple electrical switching noise back onto the mains power line, causing interference withA/Vequipment connected to the same phase. Non-power-factor-corrected SMPSs also cause harmonic distortion.Input rectifier stageAC, half-wave and full-wave rectified signals.If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120 V or 240 V supplies. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. Special control techniques can be employed by the SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage, correcting the power factor. An SMPS with a DC input does not require this stage. An SMPS designed for AC input can often be run from a DC supply (for 230 V AC this would be 330 V DC), as the DC passes through the rectifier stage unchanged. Its however advisable to consult the manual before trying this, though most supplies are quite capable of such operation even though nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only use half of diodes in the rectifier for the full load. This may result in overheating of these components, and cause them to fail prematurely.If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (120 V AC) range and as a straight rectifier when operating on the high voltage (240 V AC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of DC voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used. Inverter stageThe inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz. The frequency is usually chosen to be above 20kHz, to make it inaudible to humans. The output voltage is optically coupled to the input and thus very tightly controlled. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity.Voltage converter and output rectifierIf the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state voltage drops.The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the buck-boost converters. These belong to the simplest class of single input, single output converters which use one inductor and one active switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. For example an ideal buck converter with a 10V input operating at a 50% duty cycle will produce an average output voltage of 5V. A feedback control loop is employed to regulate the output voltage by varying the duty cycle to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the uk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realised.Other types of SMPSs use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents (Cockcroft-Walton generator). The low voltage variant is called charge pump. RegulationA feedback circuit monitors the output voltage and compares it with a reference voltage. Depending on design/safety requirements, the controller may contain an isolation mechanism (such as opto-couplers) to isolate it from the DC output. Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage.Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct. Regulated designs compensate for the impedance of the transformer or coil. Monopolar designs also compensate for the magnetic hysteresis of the core.The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added.Transformer designSMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies result from the smaller size of high frequency transformer compared to the 50/60Hz transformers formerly used. There are additional design tradeoffs.The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area (and so the mass of the core) can be greatly reduced.However, higher frequency also means more energy lost during transitions of the switching semiconductor. Furthermore, more attention to the physical layout of the circuit board is required, and the amount of electromagnetic interference will be more pronounced.Core losses increase at higher frequencies. Cores use ferrite material which has a low loss at the high frequencies and high flux densities used. The laminated iron cores of lower-frequency (400Hz) transformers would be unacceptably lossy at switching frequencies of a few kilohertzPowe