1、控制电路设计外文翻译文献控制电路设计外文翻译文献(文档含中英文对照即英文原文和中文翻译)外文：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 feed
2、back 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 performan
3、ce 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 o
4、r 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 mathema
5、tical 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 stabilityFig. 1 gives a quick illustration of at least one def
6、inition 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 reac
7、h 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 graphicall
8、y 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 thisrt quirement and yet still become unstable with
9、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 s
10、ystem. 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 u
11、ncontrolled 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 referen
12、ce 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 bet
13、ween 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 feedforward.With feed forward control, a control signal is developed directly in response
14、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, andfeedforward control cannot cause instability. It should be clear that feed forward control will t
15、ypically 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 foll
16、ow 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 feedba
17、ck (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 toy/u=1/HFig. 3. Flow graph of feedback controlNot only is this result now independent of G,it is also in
18、dependent 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 circ
19、uit (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 general
20、ized 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 func
21、tions 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 C
22、ONVERTER 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
23、 the 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 idea
24、l 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
25、the 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, switc
26、h, 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 specialize
27、d 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 wi
28、th 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 h
29、armonics 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 g
30、enerality, 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 lo
31、wer 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 Swi
32、tching 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 BookCompany, 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 L
