电气、自动化控制电路设计

1、山东建筑大学毕业设计外文文献及译文电气、自动化专业中英文对照论文翻译外文文献：Designing Stable Control LoopsThe objective of this topic is to provide the designer with a practical review of loop compensation techniques applied to switching power supply feedback control. A top-down system approach is taken starting with basic feedback con

2、trol concepts and leading to step-by-step design procedures, initially applied to a simple buck regulator and then expanded to other topologies and control algorithms. Sample designs are demonstrated with Math cad simulations to illustrate gain and phase margins and their impact on performance analy

3、sis. I. INTRODUCTIONInsuring stability of a proposed power supply solution is often one of the more challenging aspects of the design process. Nothing is more disconcerting than to have your lovingly crafted breadboard break into wild oscillations just as its being demonstrated to the boss or custom

4、er, but insuring against this unfortunate event takes some analysis which many designers view as formidable. Paths taken by design engineers often emphasize either cut-and-try empirical testing in the laboratory or computer simulations looking for numerical solutions based on complex mathematical mo

5、dels. While both of these approach a basic understanding of feedback theory will usually allow the definition of an acceptable compensation network with a minimum of computational effort.II. STABILITY DEFINEDFig. 1. Definition of stabilityFig. 1 gives a quick illustration of at least one definition

6、of stability. In its simplest terms, a system is stable if, when subjected to a perturbation from some source, its response to that perturbation eventually dies out. Note that in any practical system, instability cannot result in a completely unbounded response as the system will either reach a satu

7、ration level or fail. Oscillation in a switching regulator can, at most, vary the duty cycle between zero and 100% and while that may not prevent failure, it wills ultimate limit the response of an unstable system.Another way of visualizing stability is shown in Fig. 2. While this graphically illust

8、rates the concept of system stability, it also points out that we must make a further distinction between large-signal and small-signal stability. While small-signal stability is an important and necessary criterion, a system could satisfy thisrt quirement and yet still become unstable with a large-

9、signal perturbation. It is important that designers remember that all the gain and phase calculations we might perform are only to insure small-signal stability. These calculations are based upon and only applicable to linear systems, and a switching regulator is by definition a non-linear system. W

10、e solve this conundrum by performing our analysis using small-signal perturbations around a large-signal operating point, a distinction which will be further clarified in our design procedure discussion。Fig. 2. Large-signal vs. small-signal stabilityIII. FEEDBACK CONTROL PRINCIPLESWhere an uncontrol

11、led source of voltage (or current, or power) is applied to the input of our system with the expectation that the voltage (or current, or power) at the output will be very well controlled. The basis of our control is some form of reference, and any deviation between the output and the reference becom

12、es an error. In a feedback-controlled system, negative feedback is used to reduce this error to an acceptable value as close to zero as we want to spend the effort to achieve. Typically, however, we also want to reduce the error quickly, but inherent with feedback control is the tradeoff between sys

13、tem response and system stability. The more responsive the feedback network is, the greater becomes the risk of instability. At this point we should also mention that there is another method of control feedforward.With feed forward control, a control signal is developed directly in response to an in

14、put variation or perturbation. Feed forward is less accurate than feedback since output sensing is not involved, however, there is no delay waiting for an output error signal to be developed, andfeedforward control cannot cause instability. It should be clear that feed forward control will typically

15、 not be adequate as the only control method for a voltage regulator, but it is often used together with feedback to improve a regulators response to dynamic input variations.The basis for feedback control is illustrated with the flow diagram of Fig. 3 where the goal is for the output to follow the r

16、eference predictably and for the effects of external perturbations, such as input voltage variations, to be reduced to tolerable levels at the output Without feedback, the reference-to-output transfer function y/u is equal to G, and we can express the output asy = GuWith the addition of feedback (ac

17、tually the subtraction of the feedback signal)y = Gu - yHGand the reference-to-output transfer function becomesy/u=G/1+GHIf we assume that GH _ 1, then the overall transfer function simplifies toy/u=1/HFig. 3. Flow graph of feedback controlNot only is this result now independent of G,it is also inde

18、pendent of all the parameters of the system which might impact G (supply voltage, temperature, component tolerances, etc.) and is determined instead solely by the feedback network H (and, of course, by the reference).Note that the accuracy of H (usually resistor tolerances) and in the summing circui

19、t (error amplifier offset voltage) will still contribute to an output error. In practice, the feedback control system, as modeled in Fig. 4, is designed so thatG _ H and GH _ 1 over as wide a frequency range as possible without incurring instability. We can make a further refinement to our generaliz

20、ed power regulator with the block diagram shown in Fig. 5. Here we have separated the power system into two blocks the power section and the control circuitry. The power section handles the load current and is typically large, heavy, and subject to wide temperature fluctuations. Its switching functi

21、ons are by definition, large-signal phenomenon, normally simulated in most stability analyses as just a two states witch with a duty cycle. The output filter is also considered as a part of the power section but can be considered as a linear block. Fig. 4. The general power regulatorIV. THE BUCK CON

22、VERTER The simplest form of the above general power regulator is the buck or step down topology whose power stage is shown in Fig. 6. In this configuration, a DC input voltage is switched at some repetitive rate as it is applied to an output filter. The filter averages the duty cycle modulation of t

23、he input voltage to establish an output DC voltage lower than the input value. The transfer function for this stage is defined bytON=switch on -timeT = repetitive period (1/fs)d = duty cycleFig. 5. The buck converter. Since we assume that the switch and the filter components are lossless, the ideal

24、efficiency ofThis conversion process is 100%, and regulation of the output voltage level is achieved bycontrolling the duty cycle. The waveforms of Fig.6 assume a continuous conduction mode (CCM)Meaning that current is always flowing through the inductor from the switch when it is closed,And from th

25、e diode when the switch is open. The analysis presented in this topic will emphasizeCCM operation because it is in this mode that small-signal stability is generally more difficultto achieve. In the discontinuous conduction mode (DCM), there is a third switch condition in which the inductor, switch,

26、 and diode currents are all 5-4 zero. Each switching period starts from the same state (with zero inductor current), thus effectively reducing the system order by one and making small-signal stable performance much easier to achieve. Although beyond the scope of this topic, there may be specialized

27、instances where the large-signal stability of a DCM system is of greater concern than small-signal stability. There are several forms of PWM control for the buck regulator including, Fixed frequency (fS) with variable tON and variable tOFF Fixed tON with variable tOFF and variable fS Fixed tOFF with

28、 variable tON and variable fS Hysteretic (or “bang-bang”) with tON, tOFF, and fS all variable Each of these forms have their own set of advantages and limitations and all have been successfully used, but since all switch mode regulators generate a switching frequency component and its associated har

29、monics as well as the intended DC output, electromagnetic interference and noise considerations have made fixed frequency operation by far the most popular. With the exception of hysteretic, all other forms of PWM control have essentially the samesmall-signal behavior. Thus, without much loss in gen

30、erality, fixed fS will be the basis for our discussion of classical, small-signal stability. Hysteretic control is fundamentally different in that the duty factor is not controlled, per se. Switch turn-off occurs when the output ripple voltage reaches an upper trip point and turn-on occurs at a lowe

31、r threshold. By definition, this isa large-signal controller to which small-signal stability considerations do not apply. In a small signal sense, it is already unstable and, in a mathematical sense, its fast response is due more to feed forward than feedback.REFERENCES1 D. M. Mitchell, “DC-DC Switc

32、hing Regulator Analysis”, McGraw-Hill, 1988,DMMitchell Consultants, Cedar Rapids, IA, 1992(reprint version).2 D. M. Mitchell, “Small-Signal Mathcad Design Aids”, (Windows 95 / 98 version), e/jBLOOM Associates, Inc., 1999.3 George Chryssis, “High-Frequency Switching Power Supplies”, McGraw-Hill BookC

33、ompany, 1984.4 Ray Ridley, “A More Accurate Current- Mode Control Model”, Unitrode SeminarHandbook, SEM-1300, Appendix A2.5 Lloyd Dixon, “Control Loop Design”, Unitrode Seminar Handbook, SEM-800.6 Lloyd Dixon, “Control Loop Design SEPIC Preregulator Design”, Unitrode SeminarHandbook, SEM-900, Topic

34、7.7 Lloyd Dixon, “Closing the Feedback Loop”, Unitrode Seminar Handbook, SEM-300.中文翻译：控制电路设计摘要：本篇论文的写作目的，是为给设计师们提供一个实际性的说明，那就是线性补偿技术在电源转换与电流反馈操作中是如何应用的。一个组织管理严密的系统电路需要一开始就有一个基础的电流反馈操作理论的支持，并且通过一步步的设计步骤，从初步阶段应用到一个简单升压调节器，然后再扩展到其他的拓扑学与算数控制学中去。matchad模拟器也验证了设计样本中幅相裕度整定在分布设计中是存在的，并且还影响着实验的分析报告。一、简介：验证所提

35、议的电源供给解决方案的稳定性，一直就是电路设计过程中一个极具挑战性的方面。最让你感到窘迫的，并不是你最为得意之作的电路板正在实验的重要阶段中，被突然闯入的无序振荡所打乱，而是你实验恰恰验证了许多电路设计者感到最为头疼的数据分析。电路设计师常常强调，在实验室里要注重切换实验的实用价值，或者是以复杂的数学模式为电脑集成系统所需要的数据处理。然而这两者的方向都是以电路设计的前提为基础。于是，对反馈原理最基本的理解将帮助我们去定义接受性补偿网系统的最小值计算范围。二、稳定性的界定：图1 稳定的定义 图1直接展示了至少一个关于稳定性的界定。用最简洁的术语来说，如果一个电路系统是稳定的，就算被从某些来源说

36、产生的微扰所压制时，返回的微扰的也将会一并抵消。需要注意的是，在任何实用电路中，不稳定性不会导致一个完全无束缚的反应，这就如同电路既会达到饱和状态也会处于缺损状态一样。正在调节器转化过程中的振荡极有可能在零和百分之一百间的负荷周期中波动，并且这种变化不可能阻止失败，它将最终制约不稳定电路的回流电。图2 展示的是另外一个设想的稳定性。尽管该图形象地展示了电路稳定性的观点，但与此同时，也指出了我们必须将大信号的稳定性与小信号的稳定性严格区分开来。然而小信号的稳定性是一个非常重要和非常需要的判断标准，一个电路也可以满足这个要求，并且会与一个大信号的微扰一起变得不稳定。重要的是，电路设计师们需要记得，

37、所有我们可能执行的幅相裕度整定计算仅仅只是确保了小信号的稳定性。这些计算结果主要依靠并且只适用于线性电路，和一个转换调节器被定义为非线性的电路。我们通过用围绕小信号直流工作点周围小信号的微扰，来演算我们的分析结果，去解决这个迷团。这之中的具体差别将会在接下来的设计过程的有关探讨来说明。图2 强信号和弱信号三、反馈电流控制原理：展示的是一个最基本的调节器，在这里，不受控制的电压来源（或者电流，或者功率）将会被应用到电路的输入，且在输出过程中被这个不受控制的电压（电流或者功率）的预期值完全的掌控。电流控制的基础是一些基准电压的结构，任何在输出电流和基准电压之间的偏差都是会导致电路的错误。在一个反馈

38、操作电路中，负反馈回流电是用来减少在可接受的标准内这种错误就如我们希望能从一开始付出努力，一直坚持到最后能成功一样。然而，按照典型的案例来说，我们也希望让错误不会那么快的发生，但是回流电控制电路本身就存在着频率响应与电路稳定性的互换。回流电路的频率响应越多，不稳定的危险性就越大。在这一点上我们应该注意，另外一个控制方法前反馈。通过前反馈的控制，一个控制信号将被直接地发展到去回应一个输出波动或者微扰中。前反馈没有回流电那么精准，因为检测输出电流不是那么复杂难懂，然而，无法否认的是，等待一个输出电流的错误信号会被发现，而且前反馈控制无法产生不稳定性。需要清楚表明的是，典型的前反馈控制将不像只有一个

39、电压调节器的控制线路那么有效，但是前反馈的控制经常被用于和反馈一起去加快调节器对动态输入变动的响应频率。图3中的电流图阐述了反馈控制的基础，目标就是为了输出功率能跟着可以预测的基准电压，为了将外部微扰的影响，如同输出功率的变动一样，能会被减少到输出功率所能接受的等级上。图3 反馈控制流图如果没有反馈电，基准电压到输出功率的转换函数y/u就跟G是一样的，我们可以这样表达输出功率：y=Gu另外反馈电流（实际上是反馈信号的减法）：y = Gu - yHG之后r基准电压与输出功率的转换函数：Y=Gu=1 + GH如果我们假设GH=1，那么整体的转换函数就是： y/u=1/h这个函数不仅使得G现在成为独立，它还使所有的电路参数都变得独立，这这可能会影响G（供给功率、温度、元件公差，等等）并且被只被回流电路H（并且，理所当然的，被基准电压作用）所代替来决定它。值得一提的是，H的准确性（通常称为电阻的公差）和电路的总和（错误放大补偿功率）将继续造成输出电流的错误。在实际中，反馈控制电路，如图4的模型所示，如此设计是为了使G ：H和GH=1的振动频率能越大范围越好并且不会产生任何不稳定性。我们可以进一步的改良概括功率调节器就像图4所见到的一样。在这里我们有单独分开的功率系统进去到两个板块功率段和