Multifrequency Small-Signal Model for Buck and Multiphase Buck Converters

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  • IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 1185

    Multifrequency Small-Signal Model forBuck and Multiphase Buck Converters

    Yang Qiu, Ming Xu, Senior Member, IEEE, Kaiwei Yao, Senior Member, IEEE, Juanjuan Sun, andFred C. Lee, Fellow, IEEE

    AbstractTo investigate the high-frequency behaviors of buckand multiphase buck converters, this paper develops a modelingmethod based on the harmonic-balance approach. Because thenonlinear pulse-width modulator generates sideband components,the sideband effect occurs in a closed-loop converter. Taking thiseffect into account, the multifrequency small-signal model is pro-posed, which is applicable beyond half of the switching frequency.In a voltage-mode-controlled buck converter, the introducedmodel predicts the measured phase delay of the loop gain, whilethe conventional average model fails to explain this phenomenon.Furthermore, this model is extended to the case of the multiphasebuck converter. The influence from the interleaving technique isdiscussed and the frequency-domain characteristics are clearlyexplained. Simulation and experimental results are provided toverify the achievements of the proposed model.

    Index TermsBuck converter, high frequency, multifrequencymodel, multiphase, sideband effect.

    I. INTRODUCTION

    MOORES law has proven true for more than 40 years,and will continue to predict the future of computer pro-cessors. With more transistors and higher processing speeds,the operating voltage of the next generation of processors willcontinue decreasing to avoid overheating the silicon [1]. Mean-while, a more dynamic load is expected with higher speeds.Consequently, many stringent challenges are imposed on thevoltage regulator (VR), which is the dedicated power supply ofthe processor. One matter of particular concern is how to meetthe requirement of fast transient responses with fewer output ca-pacitors. Besides reasons related to cost, this is also necessarybecause of the limited space for VRs in the computer system.

    The multiphase synchronous buck converter, as shown inFig. 1, is widely adopted in the VR design [2], [3]. Many papershave discussed the multiphase VRs transient response andhow to improve it [4][8]. It has been shown that the feedbackcontrol loops bandwidth plays a very important role in thetransient response. With a higher bandwidth, fewer output

    Manuscript received May 18, 2004; revised June 22, 2005. This work was pre-sented at APEC04, Anaheim, CA, February 2226, 2004. This work was sup-ported by Artesyn, Delta Electronics, Hipro Electronics, Infineon, Intel, Interna-tional Rectifier, Intersil, Linear Technology, National Semiconductor, Renesas,Texas Instruments, the Engineering Research Center Shared Facilities, Transim,and the National Science Foundation under NSF Award EEC-9731677.

    Y. Qiu, M. Xu, J. Sun, and F. C. Lee are with the Center for Power ElectronicsSystems (CPES), Virginia Polytechnic Institute and State University, Blacks-burg, VA 24061 USA (e-mail: yqiu@vt.edu).

    K. Yao is with Monolithic Power Systems, Los Gatos, CA 95032 USA.Digital Object Identifier 10.1109/TPEL.2006.880354

    Fig. 1. Multiphase synchronous buck converter.

    capacitors are needed for the required transient performance[7], [8]. Therefore, it is advantageous to push the bandwidth ashigh as possible with a limited switching frequency.

    In the past, most feedback controller designs have been basedon the average model of buck converters. However, because thestate-space averaging process eliminates the inherent samplingnature of the switching converter, the accuracy of the averagemodel is questionable at frequencies approaching half of theswitching frequency [9]. As a result, the relationship betweenthe control-loop bandwidth and the switching frequency has notbeen clearly understood.

    To predict the sub-harmonic oscillations at half of theswitching frequency in peak-current-controlled converters,sampled-data approaches, hybrid approaches, and harmonic-balance approaches have been proposed [9][14]. For thevoltage-mode control, it is found that the average model shouldalso be reexamined if a high bandwidth is desired.

    As an example, Fig. 2 compares the loop gain for a 1-MHzsingle-phase buck converter with voltage-mode control cal-culated from the average model with the loop gain obtainedby using SIMPLIS software. SIMPLIS performs very fastswitching-model small-signal ac analysis based on a schemesimilar to that used in the measurements, except that theswitching ripples are not considered. Hence, the simulated

    0885-8993/$20.00 2006 IEEE

  • 1186 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006

    Fig. 2. Loop gains of a 1-MHz buck converter with voltage-mode control:(a) 100-kHz-bandwidth design and (b) 400-kHz-bandwidth design.

    transfer function from SIMPLIS is almost the same as that fromthe measurement.

    In Fig. 2, in the case with a 100-kHz bandwidth of the voltageloop, the average model agrees with the simulation up to half ofthe switching frequency. However, in the 400-kHz bandwidthdesign, the average model is good only up to 100 kHz, i.e., one-tenth of the switching frequency. The simulation result has 25less phase margin than the average model. This excessive phasedelay would result in an undesired transient or reduced stabilityif a high-bandwidth converter is designed based on the averagemodel, which cannot predict high-frequency behaviors.

    Although the harmonic-balance technique predicts the phasedelays at high frequencies, it is not an efficient means ofextracting physical insights out of the complicated model.In order to simplify the modeling, as well as to investigate

    Fig. 3. Open-loop buck converter with V perturbation.

    the control-loop bandwidth limitations and to improve thecontrol designs, it is essential to have a clear picture of theconverter up to the switching frequency. Therefore, this paperinvestigates the high-frequency characteristics of buck andmultiphase buck converters, and introduces the multifrequencymodel for the high-frequency analysis. First, the nonlinearityof the pulse-width modulation (PWM) scheme is reviewed inSection II. The sideband effect, which results in the discrepancybetween the average model and measurement, is identified forclosed-loop converters. After that, Section III introduces themultifrequency model to predict the system behavior. Withthe Fourier analysis, the relationships among different fre-quency components are derived for the PWM comparator. InSection IV, the proposed model is applied to the multiphasebuck converter. Simulation and experimental results are pro-vided as verification.

    II. SIDEBAND EFFECT OF PWM CONVERTERSWITH FEEDBACK LOOP

    Fig. 3 illustrates the structure when analyzing the responseof an open-loop single-phase buck converter with a perturba-tion at the control voltage, . In the small-signal analysis, itis assumed that the perturbation is small enough not to changethe operating point of the converter. When studying the per-formance at a certain frequency, the perturbation is assumedto be sinusoidal for simplicity. In this paper, the trailing-edgePWM comparator is used as an example for discussion, whilethe same methodology can be extended to the leading-edge anddouble-edge modulations. With a sinusoidal perturbation fre-quency at , the spectra of , and that of the output, , are illus-trated in Fig. 4 [14], [15]. Clearly, the spectrum of consists ofthe dc component, the components at the switching frequency,

    , and its harmonic frequencies. The components at the pertur-bation frequency, and , appear at the comparator outputas well. Meanwhile, because the PWM comparator works like asample-data function, its output, , has infinite frequency com-ponents at , , , , etc. Thesefrequencies are called the sideband frequencies or the beat fre-quencies around , , etc., which do not exist at the input ofthe comparator. Hence, the PWM comparator is a typical non-linear function.

    In this paper, for the sake of simplicity only the perturbationat is considered. With the assumption that the input voltage,

    , is constant, the buck converters phase voltage, , has awaveform similar to . Hence, has the same number of fre-quency components as and there is no additional frequencygenerated. The only difference is that the magnitudes of s

  • QIU et al.: MULTIFREQUENCY SMALL-SIGNAL MODEL 1187

    Fig. 4. Spectra of the PWM comparator: (a) input spectrum and (b) outputspectrum.

    Fig. 5. Frequency-domain representation for the open-loop buck converter withsideband components.

    frequency components are times as high as those of . There-fore, with the constant input voltage assumption, the function ofthe two switches in the buck converter is a typical linear func-tion. Meanwhile, the output filter topology of buck convertersdoes not change during the switch on time and switch off time,so it is also a linear function. All the components at ap-pear at the output voltage, , through the output low-pass filter.In summary, the PWM comparator is the only nonlinearity forthe open-loop buck converter with perturbations at the controlvoltage.

    When analyzing the small-signal stability and the transientperformance of a converter, it is not necessary to include thecomponents at dc, the switching frequency or its harmonics.Only the consequence of the perturbation, i.e., the componentsat the perturbation frequency and the sideband frequencies,needs to be included in the models, which is shown in Fig. 5.When studying the case with below , the lowest sidebandfrequency is . In the conventional average model, it isassumed that the sideband components can be well attenuatedby the low-pass filters in the converter; therefore, only thecomponents are included.

    Clearly, when is higher than 2, this assumption is nolonger valid since is lower than . As an example, Fig. 6shows the waveforms when is 990 kHz for a 1-MHz open-loop buck converter. In this converter, is 12 V, is 1.2 V,

    is 200 nH, is 1 mF, is 80 m , the peak-to-peak valueof the sawtooth ramp, , is 1 V. The sinusoidal perturbation at

    Fig. 6. Simulated V waveform with 990-kHz V perturbation for a 1-MHzopen-loop buck converter.

    Fig. 7. Voltage-mode-controlled buck converter with V perturbation.

    Fig. 8. Sideband effect in a voltage-mode-controlled buck converter.

    has a magnitude of 5 mV. Because 10 kHz is muchlower than , the output filter has more attenuation at thanat . Hence, the 10-kHz sideband frequency component isthe dominant one in the simulated waveforms. Therefore, thesystem performance cannot be reflected by only considering theperturbation-frequency components.

    Nevertheless, in an open-loop buck converter, because thereis only one perturbation frequency at , s component canstill be predicted by the average model. However, for a buck con-verter with closed-loop control, as in Fig. 7, the system becomesmuch more complicated. Fig. 8 illustrates the relationship be-tween the components of and . The generated sidebandcomponent, , is fed back through the voltage compen-sator, , and added to the perturbation sources. Consequently,the control voltage, , includes both the perturbation-frequencycomponent, , and the sideband component, .As is sent into the PWM comparator, it generates theperturbation-frequency component of at the output ofthe comparator as well. Consequently, the component at

  • 1188 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006

    Fig. 9. Space vector representation ofV s f component with sideband effect.

    Fig. 10. Simulated V waveform of a 1-MHz voltage-mode-controlled buckconverter with a 10-kHz perturbation.

    includes two parts, as shown in Fig. 9. The part of is from, and the part of comes from .

    Therefore, unlike the open-loop case, contains not onlythe components, but also the components. Throughthe PWM comparator, the components and the components are coupled. The sideband effect happens inthe closed-loop converters, i.e., the perturbation-frequencycomponents are influenced by the fed-back sideband compo-nents, which are generated by the PWM comparator. In theconventional average model, only the component comingfrom is included. If the influence from the sidebandcomponents, , is so small that it can be ignored, theaverage model might be sufficient. Otherwise, the sidebandfrequencies should be considered to accurately represent thesystem performance in the model.

    To qualitatively investigate the validity of the averagemodel for a buck converter with the voltage-mode control,the switching-model simulation is performed with different

    . A buck converter with the parameters above is used as anexample. The voltage-loop bandwidth is 100 kHz with a 67phase margin. Figs. 10 and 11 compare the simulated wave-forms for the 1-MHz buck converter with 40-mV perturbations.With a 10-kHz perturbation, it is observed that at there exista 1-mV 10-kHz component and a 6.5-mV switching ripple. The990-kHz sideband component can hardly be observed. There-fore, is the dominant input component of the PWMcomparator. In the case of a 990-kHz perturbation, includeslarge magnitude components at both 10 kHz and 990 kHz. The10-kHz and 990-kHz components have similar magnitudes of39 mV and 40 mV, respectively. Therefore, the influence from

    is significant.From these two cases, it can be concluded that the sideband

    effect becomes more significant with a higher and hencelower . The reason for this is that the feedback controlloop (including the power-stage output filter and the compen-sator) functions as a low-pass filter. Therefore, there is suffi-cient attenuation for the sideband components when is low.

    Fig. 11. Simulation of a 1-MHz voltage-mode-controlled buck converter witha 990-kHz perturbation: (a) V waveform and (b) closed-up V waveform.

    When is high, the attenuation is weak for and the side-band effect cannot be ignored. Clearly, the average model is notsuitable for the high-frequency analysis by only considering theperturbation-frequency component and ignoring the sidebandcomponent.

    III. MULTIFREQUENCY SMALL-SIGNAL MODEL

    The harmonic balance method is promising for further studyto explain the sideband effect. However, it is not widely uti-lized by designers because of the large amount of calculations.In a buck converter with ideal components and a constant inputvoltage, the only nonlinear function is the PWM comparator.If the extended describing functions considering the sidebandcomponents can be derived, it is possible to obtain a simplehigh-frequency model including the sideband components.

    Using Fourier analysis, the PWM describing function hasbeen derived considering the same input and output frequency[16]. It is a simple gain as

    (1)

    if 2. Here, is the peak-to-peak value of thePWM ramp. For the relationship between different frequencies,a similar approach is employed in this paper. Fig. 12 illustratesthe input and output waveforms of the modulator. The controlvoltage is

    (2)

  • QIU et al.: MULTIFREQUENCY SMALL-SIGNAL MODEL 1189

    Fig. 12. Input and outpu...

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