IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 6, NOVEMBER/DECEMBER 2007 1619Interleaved Zero-Current-Transition Buck ConverterMilan Ilic and Dragan Maksimovic, Senior Member, IEEEAbstract—This paper introduces interleaved zero-current-transition (ZCT) converters where two sets of switches are op-erating out-of-phase and share the load power equally. Turn-ontransitions at zero current and a significant reduction of the lossesassociated with diode reverse recovery are accomplished throughaddition of two small inductors. This paper describes a 30-kW(300 V/100 A) interleaved ZCT buck converter operating at32-kHz effective switching frequency. Losses and efficiency ofthe experimental prototype compare favorably against the stan-dard and interleaved hard-switched buck converters. Constantfrequency operation with low switching losses and low outputcurrent ripple is very well suited for the realization of dc powersupplies in plasma processes.Index Terms—DC–DC power conversion, modeling, soft switch-ing, zero-current transition (ZCT).I. INTRODUCTIONPOWER semiconductor switches in high-power applica-tions are subject to high switching stresses and switchinglosses, which limit the operation to relatively low switchingfrequencies. Various soft-switching techniques have been pro-posed to mitigate these problems [1]–[12]. In particular, zero-current transition (ZCT) [1], [2] and zero-voltage transition(ZVT) [10] techniques incorporate soft-switching functionsinto standard pulsewidth-modulated (PWM) converters, so thatthe switching losses can be reduced, ideally without increasingthe switch-voltage or current stresses.For high-power/high-voltage applications, insulated gatebipolar transistors (IGBTs) are preferred devices. Hence, ZCTtechniques provide better results than ZVT techniques.In the ZCT technique proposed in [1], an auxiliary circuitforces the switch current to zero prior to turn off, thus reducingthe turn-off losses due to the current tailing of the IGBTs.However, the turn-on of the main switch is not affected bythe auxiliary circuit. As a result, the turn-on losses causedby the diode reverse recovery remain significant. With thenewest generation of IGBTs, which have greatly reduced theturn-off times and losses, the majority of losses in high-powerIPCSD-07-046, presented at the 2006 IEEE Applied Power ElectronicsConference and Exposition, Dallas, TX, March 19–23, and approved forpublication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS bythe Industrial Power Converter Committee of the IEEE Industry ApplicationsSociety. Manuscript submitted for review April 30, 2006 and released forpublication June 8, 2007.M. Ilic is with Advanced Energy Industries, Fort Collins, CO 80525 USA(e-mail: [email protected]).D. Maksimovic is with the Colorado Power Electronics Center, Departmentof Electrical and Computer Engineering, University of Colorado, Boulder, CO80309 USA (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TIA.2007.908175Fig. 1. ZCT buck converter proposed in [2].and high-voltage applications are the switch turn-on and dioderecovery losses (more than 75% of the total loss, as discussedin [13]).The ZCT topology described in [1] can be improved to somedegree with a modified control, as described in [12]. With thischange, it is possible to get the auxiliary and main switches toturn on ideally at zero current. Disadvantages of this approachinclude a very high peak current in the auxiliary switch, in-creased conduction losses, and a more complicated control.In the ZCT topology proposed in [2], which is shown inFig. 1, the auxiliary circuit has been modified to reduce thehigh peak current problem and simplify the control timingof the auxiliary switch. The main and auxiliary switches areswitched on and off under zero-current conditions so that thetotal switching losses can be significantly reduced (by around80% compared to the hard-switched case, as described in [2]).These advantages are achieved at the expense of the auxiliarycircuit consisting of a resonant tank, an increased main switchpeak current, and an increased conduction loss: The resonantcurrent flows through the main switch during the turn-off cycle.In addition, a more complicated control circuit is required togenerate the appropriately timed drive signal for the auxiliaryswitch.Interleaving technique has been widely used in power elec-tronics [14]–[24]. In microprocessor power supplies, inter-leaved multiphase converters are commonly used in order toachieve better dynamic performance, lower current ripple, andlower losses per switch for an easier thermal design [14],[15]. In the context of power-factor-correction (PFC) rectifiers,interleaved boost converters have been proposed to reduce inputcurrent ripple, improve power scaling, and reduce switchinglosses [16]–[24]. In [18], it was shown that reverse-recoverylosses can be greatly reduced in the interleaved boost converterwith coupled inductors. Further soft-switching techniques ap-plied to the interleaved boost converters for PFC applicationscan be found in [19]–[22]. It is also well known that the0093-9994/$25.00 © 2007 IEEE1620 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 6, NOVEMBER/DECEMBER 2007Fig. 2. Interleaved ZCT buck converter proposed in this paper.operation in discontinuous conduction mode (DCM) [23] orcritical conduction mode [24] can lead to elimination of diodereverse recovery losses at the expense of increased inductorcurrent ripples.In this paper, the objectives are to facilitate relatively high-frequency operation by addressing the diode reverse recoverylosses in high-power step-down (buck) dc–dc applications. Inparticular, the focus is on plasma power supplies in physicalvapor deposition applications, which require low-ripple,constant-frequency, and constant current operation over awide range of output voltages and power [25], [26]. Wepropose a simple ZCT scheme [26], which addresses theswitch turn-on and diode recovery losses using an interleavedbuck converter configuration with small auxiliary inductors,as shown in Fig. 2. In this scheme, which is similar to theapproaches described in [18] and [20] for the boost PFCrectifier applications, there is no need for a resonant tank withcirculating currents, and the control of the switches is verysimple. Compared with the hard-switched PWM