BUCK降压电路的设计与仿真【自动化毕业论文开题报告外文翻译说明书】.zip
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自动化毕业论文开题报告外文翻译说明书
Buck电路开题报告
Buck电路
书与开题报告
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BUCK降压电路的设计与仿真【自动化毕业论文开题报告外文翻译说明书】.zip,自动化毕业论文开题报告外文翻译说明书,Buck电路开题报告,Buck电路,书与开题报告
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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 control 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 analysis. 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 customer, 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 models. 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 stability Fig. 1 gives a quick illustration of at least one definition 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 saturation 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 illustrates 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 this requirement and yet still become unstable with a large-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. We 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 uncontrolled 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 becomes 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 system 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 feed forward.With feed forward control, a control signal is developed directly in response to an input 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, and feed forward control cannot cause instability. It should be clear that feed forward control will typically 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 reference 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 (actually 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 to y/u=1/H Fig. 3. Flow graph of feedback controlNot only is this result now independent of G,it is also independent 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 circuit (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 that G _ H and GH _ 1 over as wide a frequency range as possible without incurring instability. We can make a further refinement to our generalized 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 functions 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 CONVERTER 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 the input voltage to establish an output DC voltage lower than the input value. The transfer function for this stage is defined by tON=switch on -timeT = repetitive period (1/fs)d = duty cycle Fig. 5. The buck converter.Since we assume that the switch and the filter components are lossless, the ideal efficiency of this conversion process is 100%, and regulation of the output voltage level is achieved by controlling 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 the diode when the switch is open. The analysis presented in this topic will emphasize CCM operation because it is in this mode that small-signal stability is generally more difficult to achieve. In the discontinuous conduction mode (DCM), there is a third switch condition in which the inductor, switch, 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 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 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 harmonics 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 same small-signal behavior. Thus, without much loss in generality, 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 lower threshold. By definition, this is a 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. 中文翻译： 设计稳定控制回路引言：本主题的目的是为设计者提供有关环路补偿技术的实用总结，该技术可应用于开关电源的反馈控制。采用自上而下的系统方法作为基本反馈控制理念的第一步，并产生逐步的设计过程，最初应用于简单的降压稳压器，然后扩大到其他的拓扑结构和控制算法。样本设计并采用数学仿真CAD证明，说明了增益和相位裕度及其对性能分析的影响。一、简介： 电源解决方案的稳定性往往是设计过程中具挑战性的方面之一。当你把精心设计线路板在给老板或客户展示时出现严重的振荡，没什么比这令人不安的，但为了避免这种不幸的事情要做一些许多设计师都认为比较困难的数据分析。设计工程师所采取的路径往往强调的是削减和尝试在实验室或计算机模拟寻找数值解的基础上复杂的数学模型的经验测试。虽然这两种方法的反馈理论基本可以理解，通常会允许一个可以接受的补偿网络的定义与最小的计算工作量。二、稳定性的定义：图1 稳定的定义 图1给出了至少一个稳定的定义的快速说明。在其最简单的条件下，系统是稳定的，如果受到来自某些方面的扰动，其响应结果就是扰动消失。请注意，在任何实际的系统中，不稳定不能导致一个完全无界的响应，因为系统将达到饱和的水平，或系统损坏。大多数情况下，在开关调节器中的振荡可以改变零和100%之间的占空比，而这可能无法防止故障，它会最终限制一个不稳定系统的响应。 另一种可视化稳定性的方法如图2所示。虽然这样的图形说明了系统的稳定性的概念，它也指出，我们必须进一步区分大信号和小信号稳定。虽然小信号稳定是一个重要的和必要的标准，一个系统可以满足这个要求的同时仍然可能是不稳定的一个大信号扰动。重要的是，设计师要记住，我们所能做的所有的增益和相位计算只能保证小信号的稳定性。这些计算都是只适用于线性系统，而一个开关调节器是一个非线性系统。我们利用小信号扰动在大信号工作点进行分析解决这一难题，在我们的设计方法的探讨中将进一步澄清这个区分。 图2 大信号与小信号稳定 三、反馈控制原理：一个不受控制的电压源（或电流，或功率）被应用到我们的系统的输入，期望的输出电压（或电流，或功率），将非常好地控制。我们控制的基础是某种形式的参考值，输出值和参考值之间的任何偏差都成了一个误差。在反馈控制系统中，负反馈是用来把这个误差减少到一个可以接受的值的，因为我们要花费的努力去实现接近0。通常，然而，我们也希望迅速减小误差，但与反馈控制是系统的响应速度和系统稳定性之间的权衡。反馈网络越是灵敏，就越是增大不稳定的风险。在这一点上，我们也应该提到，有另一种控制方式-前馈控制，前馈控制，一个控制信号直接传送响应输入变化或扰动。前馈没有反馈精准，因为输出值不会被检测，但是，没有延迟等待输出误差信号的传递，和前馈控制不能导致不稳定。应该清楚的是，前馈控制通常是不足够的电压调节器的唯一控制方法，但它通常是和反馈一起使用，以提高调节器的响应动态输入变化。反馈控制的基础是在图3所示的目标是输出跟随参考值，外部扰动影响（如输入电压的变化）将减少到可以接受的水平在无反馈的输出的情况下，输出的传递函数Y/U=G，我们可以表达输出 asy= Gu 添加反馈（实际上是减法的反馈信号） y = Gu - yHG 输出传递函数的引用成为 y/u=G/1+GH 如果我们假设GH _ 1，那么整体的传递函数简化为Y / U = 1 /H图3 反馈控制流图不仅是这样的结果是独立的G，还独立于
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