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  • IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 3, MAY/JUNE 2013 1383

    A Gallium Nitride Switched-Capacitor Circuit UsingSynchronous Rectification

    Mark J. Scott, Student Member, IEEE, Ke Zou, Jin Wang, Member, IEEE, Chingchi Chen, Ming Su, and Lihua Chen

    AbstractThe physical characteristics of gallium nitride (GaN)make it theoretically superior to silicon (Si) in such aspects asthe temperature of operation, switching speed, breakdown voltage,and efficiency. While much research has been conducted on GaNdevices, the discussion of third-quadrant operation is limited.Furthermore, the merits of GaN transistors, particularly their fastswitching speed and low on-resistance, make them suitable forswitched-capacitor circuits. This paper demonstrates the abilityof a GaN transistor to function as a synchronous rectifier in aswitched-capacitor circuit. A 500 W GaN-based voltage doublercapable of achieving zero-current switching is presented withsupporting experimental results. This circuit achieves peak effi-ciencies of 97.6% and 96.6% while switching at frequencies of 382and 893 kHz, respectively.

    Index TermsDCDC converter, gallium nitride (GaN),HEMTs, resonant power conversion, switched-capacitor circuit,zero-current switching (ZCS).

    I. INTRODUCTION

    POWER SWITCHING devices created from wide bandgap(WBG) devices are actively being researched to realizethe next generation of power conversion hardware [1][14]. Inparticular, gallium nitride (GaN) and silicon carbide (SiC) haveseveral properties that offer advantages over existing silicon(Si) technology. For instance, the bandgaps (Eg) of both GaN(3.44 eV) and SiC (3.26 eV) are about three times higher thanthat of Si (1.12 eV). This enables WBG devices to operateat higher temperatures when compared to their Si counter-parts [1][5]. Furthermore, the critical electric fields of GaN(3.0 MV/cm) and SiC (3.26 MV/cm) are an order of magni-tude greater than that of Si (0.3 MV/cm), and this translatesinto higher breakdown voltages for similar drift region spac-ing (i.e., gate-to-drain spacing) or lower specific on-resistance

    Manuscript received February 13, 2012; revised May 13, 2012; acceptedJuly 7, 2012. Date of publication March 27, 2013; date of current versionMay 15, 2013. Paper 2012-IPCC-052.R1, presented at the 2011 IEEE EnergyConversion Congress and Exposition, Phoenix, AZ, USA, September 1722,and approved for publication in the IEEE TRANSACTIONS ON INDUSTRYAPPLICATIONS by the Industrial Power Converter Committee of the IEEEIndustry Applications Society. This work was supported by the NationalScience Foundation through Project 1054479.

    M. J. Scott and J. Wang are with the Department of Electrical and ComputerEngineering, The Ohio State University, Columbus, OH 43210 USA (e-mail:scott.585@osu.edu; wang@ece.osu.edu).

    K. Zou was with the Department of Electrical and Computer Engineering,The Ohio State University, Columbus, OH 43210 USA. He is now with FordMotor Company, Dearborn, MI 48126-2798 USA (e-mail: kzou2@ford.com).

    C. Chen, M. Su, and L. Chen are with Ford Motor Company, Dearborn,MI 48126-2798 USA (e-mail: cchen4@ford.com; msu7@ford.com; lchen68@ford.com).

    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.2013.2255252

    for devices of comparable voltage rating [1][5]. Both GaN(2.0 107 cm/s) and SiC (2.5 107 cm/s) also have a highersaturation velocity when compared to Si (1.0 107 cm/s) andshould therefore be able to reach higher switching speeds [2].

    Given these considerations, WBG devices should enablethe design of power conversion hardware that achieves higherpower densities and better efficiencies over those createdwith Si. Two separate demonstrations of conversion hardwarebased on GaN devices have been able to achieve efficien-cies exceeding 99%: a 760 W boost converter [6] and a900 W three phase inverter [7]. In addition, a 5 kW SiCphotovoltaic inverter has been demonstrated that realizes upto four times greater power density over similar rated Siimplementations [8].

    An emerging application for these materials is in renewableenergy systems where aims to improve the efficiency are beingheavily pursued [9]. Photovoltaic (PV) panels and fuel cells,which are ubiquitous in these systems, operate at voltage levelsin the range of 2045 V, which is more favorable to currentlyavailable GaN devices. Additionally, volume is a premium inhardware such as microinverters; the smaller footprint of theEfficient Power Conversions (EPC) device has an advantageto that of other commercially available WBG devices. Whatis more, the EPC-1010, which has the highest voltage ratingamong currently available EPC devices, has a typical on-resistance of 18 m [10], and this is four times lower than thatof available SiC devices [11]. For these reasons, GaN devicesare being investigated. Already, EPCs devices are being evalu-ated in PV applications designed for microconverters [12] andmicroinverters [13], [14].

    One of the challenges presented in grid-tie applications is theneed for high-boost-ratio converters to interface low-voltagedc sources, such as PVs and fuel cells, with inverters usedin these systems [15]. An approach to achieving this highboost ratio is to cascade a voltage multiplier with a boostconverter to improve efficiency [16]. In addition, switched-capacitor multipliers have been used to generate the dc link forrenewable energy applications [14]. This paper presents a high-frequency GaN-based switched-capacitor voltage doubler to beused in these types of circuits.

    Switched-capacitor circuits using GaN devices are one areain need of investigation. In high-speed circuits, magnetic com-ponents are becoming the major road block. While new GaNdevices can switch at 1 MHz and above with reduced switchingloss, it is difficult to find suitable magnetic cores, particularlyfor high-power applications. Thus, switched-capacitor circuitsare an attractive application for WBG devices [17], [18].While there are many efforts in new topologies and controls

    0093-9994/$31.00 2013 IEEE

  • 1384 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 3, MAY/JUNE 2013

    TABLE IDEVICE PARAMETERS FOR TECHNOLOGY EVALUATION

    for switched-capacitor circuits, adapting GaN devices in thesecircuits has not yet been widely studied.

    The third-quadrant operation of GaN transistors and theirapplication in synchronous rectification (SR) are another areain need of further research. In [7], third-quadrant operation(i.e., SR) was demonstrated for the commutation current ina GaN-based three-phase inverter circuit, but the discussionof this mode of operation was limited. The doubler presentedin this paper relies on the third-quadrant capabilities of theGaN device to improve the efficiency of the circuit. Thus, thismechanism should be better understood.

    The structure of this paper is as follows. Section II comparesthe merits of GaN against those of Si. Section III provides anoverview of the third-quadrant operation and presents the IVcurves for reverse current flow. In Section IV, the analysis ofa soft-switching modular switch-capacitor voltage doubler isdiscussed. The design of a GaN switched-capacitor circuit andits performance are shown in Section V. An eGaN FET fromEPC (EPC-1010 [10]) is used in this research.

    II. GALLIUM NITRIDE COMPARISON

    A. Device Model

    Several models have been proposed to evaluate GaN tran-sistors in various applications [2], [19], [20]. From a circuitdesigners perspective, a normally off GaN transistor is quitesimilar to a Si MOSFET [2]. Both are voltage control de-vices that inherently have parasitic capacitances between theterminals. Each device is able to conduct third-quadrant cur-rent; however, the mechanism is different as is explained inSection III. In addition, the on-resistance in both devices hasa positive temperature coefficient. These similarities enable theuse of many of the same equations used when evaluating SiMOSFETs.

    B. Comparison Among Competing Technologies

    Table I compares the EPC-1010 against Si MOSFETs fromInternational Rectifier and Infineon to evaluate the theoreticaladvantages of using GaN devices in a high-power switched-capacitor application. The basis for this comparison was thatthe device be rated for 200 V. From there, a device with eithersimilar current rating (IRFU13N20D [21]), comparable sizepackage (BSZ12DN20NS3 [22]), and/or better on-resistance(RDS_ON) (IPB107NA [23]) was selected.

    Table I summarizes the parameters that are relevant to theproposed topology [18]. Several of the values were not given

    TABLE IIPOWER LOSS BREAKDOWN

    directly in the data sheet. For example, the output capacitanceand forward voltage are estimated from the figures in the datasheet. Reverse recovery charge is calculated based on the linearinterpolation of the data provided. The switching speeds arecalculated from the switching times given in the data sheet andthe voltage level at which the test was performed. EPC does notdirectly publish switching speeds for the EPC-1010. Therefore,the value in [24] was used for this comparison.

    A breakdown of the different loss mechanisms is given inTable II. The conduction losses are determined by multiplyingthe RDS_ON by the rms current. Both the output capacitance(COSS) and the gate charge (QG) are used to evaluate theswitching losses. Third-quadrant operation is evaluated byconsidering the forward voltage (VF ) and reverse recoverycharge (QRR). For the following analysis, it is assumed thatthe operating voltage is 55 V, the switching frequency is400 kHz, and the rms current is 6 A.

    The fact that the EPC device does not have a QRR associatedwith it is apparent. This ends up being the dominant formof loss for each of the Si devices. Even if thes