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Flux-Balance Control for LLC Resonant Converters with Center-Tapped Transformers

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Flux-Balance Control for LLC Resonant Converters with Center-Tapped Transformers energies Article Flux-Balance Control for LLC Resonant Converters with Center-Tapped Transformers Yuan-Chih Lin, Ding-Tang Chen and Ching-Jan Chen * Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan; No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan * Correspondence: [email protected]; Tel.: +886-2-3366-3366 (ext. 348) Received: 9 July 2019; Accepted: 19 August 2019; Published: 21 August 2019 Abstract: LLC resonant converters with center-tapped transformers are widely used. However, these converters suffer from a flux walking issue, which causes a larger output ripple and possible transformer saturation. In this paper, a flux-balance control strategy is proposed for resolving the flux walking issue. First, the DC magnetizing current generated due to the mismatched secondary-side leakage inductances, and its effects on the voltage gain are analyzed. From the analysis, the flux-balance control strategy, which is based on the original output-voltage control loop, is proposed. Since the DC magnetizing current is not easily measured, a current sensing strategy with a current estimator is proposed, which only requires one current sensor and is easy to estimate the DC magnetizing current. Finally, a simulation scheme and a hardware prototype with rated output power 200 W, input voltage 380 V, and output voltage 20 V is constructed for verification. The simulation and experimental results show that the proposed control strategy effectively reduces the DC magnetizing current and output voltage ripple at mismatched condition. Keywords: LLC resonant converter; center-tapped transformer; flux walking; flux-balance control loop; magnetizing current estimation 1. Introduction The LLC resonant converter is widely used in many different applications such as onboard chargers, server power systems, laptops, desktops, photovoltaic regeneration systems. Owing to the characteristics of zero-voltage-switching (ZVS) at the primary side and zero-current-switching (ZCS) Energiesat the secondary2019, 12, x FOR side, PEER high REVIEW efficiency and high power density of the LLC converter are achieved2 [1of– 719]. The half-bridge (HB) and full-bridge (FB) with the center-tapped transformer rectifier topologies sideshown currents in Figure imbalance1 are the will most reflect commonly upon the used magn topologiesetizing inductance of the LLC on resonant the primary converter. side. ThanksTherefore, to the magnetizing center-tapped current transformer, will have only a DC two component, rectifying diodes which are will necessary lead to the at flux the secondarywalking and side possible [8–11]. Withouttransformer the center-tap,core saturation. it would be required to implement a full-wave rectifier. (a) (b) Figure 1. DifferentDifferent topologies of th thee LLC resonant converter ( a) half-bridge and (b) full-bridge. Figure 2b shows the key waveforms of the LLC resonant converter operating below the resonant conditions with the mismatched leakage inductances in the secondary side windings. In Figure 2b, theEnergies current2019, 12waveforms, 3211; doi:10.3390 of the/en12173211 diode 1 and diode 2 are not symmetrical. Thewww.mdpi.com magnetizing/journal current/energies (iLm) will, therefore, contain a DC component. Since the resonant capacitance is in series with the primary side, the resonant current (iLr) will not have a DC component from charge balance concept. Besides, the mismatched condition considers not only the leakage inductances of the secondary side windings of the transformer but also the parasitic inductances of the printed-circuit-board (PCB) traces on the secondary side. (a) (b) Figure 2. Key waveforms of the LLC resonant converter operating below the resonant frequency with the different leakage inductances in the secondary side windings (a) matched leakage inductances and (b) mismatched leakage inductances. To improve the flux walking issue in the LLC resonant converter, improved winding structures for the secondary side were proposed [12–14]. The coupling coefficients between the primary side and two secondary sides were increased to mitigate the flux walking issue [12]. The further improved method is used in the Bifilar winding structure to overcome this problem [13]. The flux distribution of the non-symmetrical structure of the secondary side windings were analyzed in [14]. However, they cannot consider the mismatch problems caused by the PCB circuit traces. According to abovementioned issue, a flux-balance control strategy, which is based on the original output-voltage control loop, is proposed in this paper. The flux-balance control is added to improve the magnetizing current imbalance problem caused by the secondary side mismatches. Besides, the DC magnetizing current of the LLC resonant converter is difficult to sense directly. An indirect method to sense the magnetizing current was used [15], which involved sensing the currents at the primary and secondary side, simultaneously and subtracting them to obtain the magnetizing current. Then, the DC component was obtained using a low-pass filter. This solution, Energies 2019, 12, x FOR PEER REVIEW 2 of 19 side currents imbalance will reflect upon the magnetizing inductance on the primary side. Therefore, the magnetizing current will have a DC component, which will lead to the flux walking and possible transformer core saturation. (a) (b) Figure 1. Different topologies of the LLC resonant converter (a) half-bridge and (b) full-bridge. Energies 2019, 12, 3211 2 of 18 Figure 2b shows the key waveforms of the LLC resonant converter operating below the resonant conditions with the mismatched leakage inductances in the secondary side windings. In Figure 2b, the currentFigure waveforms2a shows the ofkey the waveformsdiode 1 and of diode the LLC 2 are resonant not symmetrical. converter The operating magnetizing below thecurrent resonant (iLm) will,frequency therefore, with contain the matching a DC component. leakage inductances Since the re insonant both secondary capacitance side is in windings, series with where the primaryvgs1 and side,vgs2 arethe theresonant driving current signals (i ofLr) thewill MOSFET not haveQ a1 DCand componentQ2, respectively. from charge Figure 2balancea reveals concept. that the Besides, current thewaveforms mismatched of the condition diode 1 (considersiD1) and diode not only 2 (i Dthe2) areleakage symmetrical, inductances and of energy the secondary flows through side windings the diode of1 andthe transformer diode 2 in the but positive also the and parasitic negative inductance cycles, respectively.s of the printed-circuit-board In Figure2a, the magnetizing (PCB) traces current on the secondarywill have noside. DC component. (a) (b) FigureFigure 2. 2. KeyKey waveforms waveforms of of the the LLC LLC resonant resonant converter converter operating operating below below the the resonant resonant frequency frequency with with thethe different different leakageleakage inductancesinductancesin in the the secondary secondary side side windings windings (a )( matcheda) matched leakage leakage inductances inductances and and(b) mismatched(b) mismatched leakage leakage inductances. inductances. ToHowever, improve practical the flux walking LLC resonant issue in converters the LLC re withsonant center-tapped converter, improved transformers winding contain structures a flux forwalking the secondary issue, which side were causes proposed a larger [12–14]. output Th ripplee coupling and possible coefficients transformer between saturation the primary [12 side–14]. andThis two is because secondary the sides secondary were increa side windingssed to mitigate are usually the flux not walking symmetrical issue [12]. in practicalThe further manufacture. improved methodThe mismatched is used in leakage the Bifilar inductances winding arestructure generated to overcome in the secondary this problem side windings,[13]. The flux which distribution causes an ofimbalance the non-symmetrical in the secondary structure side currents,of the secondary resulting side in a largerwindings output were voltage analyzed ripple. in [14]. The However, secondary theyside cannot currents consider imbalance the willmismatch reflect proble upon thems magnetizingcaused by the inductance PCB circuit on traces. the primary side. Therefore, the magnetizingAccording to current abovementioned will have a DCissue, component, a flux-balan whichce control will lead strategy, to the flux which walking is based and possibleon the originaltransformer output-voltage core saturation. control loop, is proposed in this paper. The flux-balance control is added to improveFigure the2 magnetizingb shows thekey current waveforms imbalance of the prob LLClem resonant caused by converter the secondary operating side below mismatches. the resonant conditionsBesides, with the theDC mismatchedmagnetizing leakagecurrent of inductances the LLC reso innant the secondary converter sideis diffic windings.ult to sense In Figure directly.2b, Anthe indirect current waveformsmethod to ofsense the diodethe magnetizing 1 and diode cu 2rrent are not was symmetrical. used [15], Thewhich magnetizing involved sensing current (theiLm ) currentswill, therefore, at the containprimary a DCand component.secondary side, Since simult the resonantaneously capacitance and subtracting is in series them with to theobtain primary the magnetizingside, the resonant current. current Then, (i Lrthe) willDC notcomponent have a DC was component obtained using from chargea low-pass balance filter. concept. This solution, Besides, the
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