Filters with active tuning for power applications The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Phinney, J., and D.J. Perreault. “Filters with Active Tuning for Power Applications.” IEEE Trans. Power Electron. 18, no. 2 (March 2003): 636–647. © 2003 IEEE As Published http://dx.doi.org/10.1109/TPEL.2003.809329 Publisher Institute of Electrical and Electronics Engineers (IEEE) Version Final published version Citable link http://hdl.handle.net/1721.1/86920 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. 636 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 2, MARCH 2003 Filters With Active Tuning for Power Applications Joshua Phinney, Student Member, IEEE and David J. Perreault, Member, IEEE Abstract—Filters for switched-mode power converters have ries- or parallel-tuned resonance and a reduced low-pass charac- traditionally relied on low-pass networks—with corner frequen- teristic [e.g., the buck converters of Fig.2(a) and 2(b)]. Because cies well below the ripple fundamental—to attenuate switching the resonator effectively attenuates the ripple fundamental, an harmonics over a range of frequencies. The filters explored in this report provide extra attenuation at discrete frequencies, easing the accompanying low-pass network can be designed with a higher filtering requirement of accompanying low-pass networks. When corner frequency and smaller reactances. Inasmuch as suitably a converter’s switching frequency is tuned to a filter resonance low-loss reactive components are available in a small volume, using a novel phase-lock control scheme, a resonant filter can active tuning can reduce the overall size and cost of the filter match the ripple-attenuation performance of a low-pass network network as compared to a conventional low-pass design. for less volume, weight, and expense. The design and application of resonant filters and active-tuning control are discussed, and Note that unlike resonant converters—which utilize the experimental results from the input filter and power stage of a switching frequency to control the converter output—the prototype dc-dc converter are presented. approach described here adapts the switching frequency to best Index Terms—Active tuning, electromagnetic interference, fil- utilize the attenuation characteristics of a resonant network. ters, inductors, magnetic devices, phase locked loops, power con- Converter control can be realized with conventional pulse-width version, resonant, resonator. modulation (PWM) techniques, allowing the resonant-filter approach to be applied to a wide range of converter topologies. Furthermore, resonant filters with active-tuning control can I. INTRODUCTION process high power because they modulate a resonance or OW-PASS networks have traditionally been employed to stimulus frequency to maximize the harmonic selectivity of a L attenuate power-converter switching ripple to acceptable passive network: they do not, like active ripple filters ([3]–[5]), levels. Ripple specifications imposed to observe conducted EMI directly drive the waveforms they condition. Using the novel limits or application constraints, however, can result in heavy, phase-lock control scheme described here, such filters can bulky filters which are detrimental to the transient performance realize all the advantages of resonant networks, matching the of a power converter and contribute significantly to its cost. ripple performance of low-pass filters for less volume, weight, Resonant ripple filters offer attenuation comparable to low-pass and expense. networks—for less volume and weight—using the immitance peaking of parallel- and series-tuned circuits (Fig. 1) to intro- B. Organization of the Paper duce transmission nulls at discrete frequencies. Because reso- Section II of this report introduces a simple phase-lock tuning nant networks must typically have high Q to attenuate target system which controls the switching frequency of a power con- harmonics sufficiently,1 they provide only narrow-band atten- verter to operate at the resonant point of a filter. Section III con- uation. Operating conditions and manufacturing variations can siders the application of the phase-lock approach to the both readily cause narrow-band resonators to miss their design fre- the power stage and input filter of a buck converter. Experi- quencies [2] and fail to attenuate the ripple; for this reason they mental results are presented that demonstrate the value of the are rarely employed in switching power converters. approach in reducing the size of passive components. Section IV A. Resonant Filters With Active Tuning considers additional applications and implementations of the phase-lock control system. Finally, conclusions are outlined in The filters described here circumvent this detuning problem Section V. by placing a converter’s switching frequency (or a resonator’s frequency response) under closed-loop control so that resonant II. PHASE-LOCK TUNING attenuation is always maintained. In this paper we consider the case in which the switching frequency of a power converter is To take advantage of high-Q resonant filters, one must ensure controlled to align with the resonant point of a filter having a se- that the converter switching frequency remains aligned with the filter resonance across all component tolerances and operating conditions. Resonant excitation is equivalent to maintaining a Manuscript received October 29, 2002; revised November 1, 2002. This work was supported by the United States Office of Naval Research under ONR Grant resistive phase relationship (0 ) between resonator voltage and N000140010381. Recommended by Associate Editor J. D. van Wyk. current (note the impedance angles in Fig. 1). Because the phase J. Phinney is with the Laboratory for Electromagnetic and Electronic response of a series- or parallel-tuned circuit monotonically Systems, Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: [email protected]; [email protected]). increases or decreases around the 0 tuning point, it can be Digital Object Identifier 10.1109/TPEL.2003.809329 used as an error signal to control for excitation at the point of maximum immitance. The phase-lock tuning system presented 1Some high-power applications use damped, low-Q resonators precisely for their broad attenuation characteristic and insensitivity to detuning, at the ex- in this paper employs this method precisely, feeding back the pense of attenuation performance [1]. phase difference between resonator voltage and current to drive 0885-8993/03$17.00 © 2003 IEEE PHINNEY AND PERREAULT: FILTERS WITH ACTIVE TUNING FOR POWER APPLICATIONS 637 p Fig. 1. Frequency response of second-order tuned circuits, normalized to the natural frequency 3 aIa vg. The impedance magnitude at a single frequency can indicate proximity to resonance (with calibration) but not whether resonance lies above or below the stimulus frequency. The impedance phase, however, increases or decreases monotonically, and its difference from 0 is an error signal indicating the distance and direction to resonance. Fig. 2. Examples of a resonant filters in a buck converter. (a) The series-tuned leg provides a low-impedance current path (i.e. high attenuation) at a discrete frequency. (b) The parallel-tuned resonator presents a high impedance to switching ripple at a discrete frequency. (c) Transfer function: switch drain current to input current or switch source voltage to output voltage. a voltage-controlled oscillator (VCO) toward the resonator’s a90 phase shift in lock, which allows a subsequent phase de- tuned frequency. tector (multiplier 1) to develop zero average output for a 0 V-I A control topology to excite a series resonance at its min- resonant condition in the resonators. By itself, this phase shift imum-impedance point is shown in Fig. 3(a). The dual of this is poor motivation for introducing the complexity of a PLL, as tuning system is shown in Fig.3(b), which drives a parallel reso- the designer could employ a phase detector with 0 offset.2 The nance at its maximum-impedance point (its resistive-impedance more significant function of the inner-loop PLL’s, then, is to re- point). The control circuitry in either case generates the fre- ject harmonics by locking on the fundamental component of an quency command shown at the right. This command specifies input waveform. the fundamental—but not the harmonic content or dc level—of Consider, for example, the parallel-resonator tuning system the sources on the left of the block diagrams. In a power con- of Fig. (3b). The differential amplifier measures the ac voltage verter, the frequency command would represent an adjustable across parallel-tuned tank, a signal with, presumably, a large PWM frequency. fundamental component. The current through the parallel-tuned circuit, however, is dominated by its harmonic content, since the To excite the parallel- and series-tuned resonators at reso- nance, the controller must, in either case, adjust the fundamental 2In fairness to an inner-loop PLL, phase detectors with 0 phase offset (state- drive frequency such that the resonator current and voltage fun- machine detectors, typically) can be confounded by the edge timing of PWM waveforms. Signal conditioning of some sort (a filter or PLL) will probably be damentals are in phase. The inner-loop PLL’s in Fig. 3(a) and necessary to develop a signal with zero-crossings in phase with the fundamental (b) serve two functions in this regard. They provide, first of all, component of such waveforms. 638 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 2, MARCH 2003 Fig. 3. Block diagram of the phase-lock tuning system for series and parallel resonators. In each case, equivalent tuning controls can be implemented by switching the sensing connections, e.g., by phase-locking to the tank voltage rather than the tank current in the lower diagram. Such an exchange, however, would not take advantage of phase-lock loop’s ability to cleanly extract the fundamental component from the signal most dominated by harmonics.