Integrated Magnetic for LLC Resonant Converter - CPES

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as Lm. The problems with this implementation are: first,leakage inductance control is difficult. For the LLCresonant converter, Lr value is critical since it willdetermine the operating point. Second, when theresonant inductor Lr is built like this, the leakageinductance will not only exist on the primary side, itwill also exist on the secondary side of the transformer.So the result get from real transformer will be as inFig.3. L lp and L ls have similar values when reflected tothe same side of the transformer.Lr 14uHTa4:1:11Lm60uH02bFig. 2 Magnetic components for Front End DC/DC LLC resonant converterabLlpLm60uHT4:1:1L ls1L ls2Fig. 3 Magnetic structures from Simple Transformer102resonant converter, the resonant inductor Lr has pureAC current through it, so soft ferrite core can be usedfor both inductor and transformer.Fig. 5 shows the discrete design of the magnetic forLLC resonant converter. Two U cores were used tobuild the resonant inductor and gapped transformer.Fig.6 shows the simulation results of flux density in thecore. For each U core, the cross-section area is116.5mm 2 . Design results are as following: nl=12,np:ns:ns = 16:4:4, gap1=1.45mm and gap2=0.5mm.gap1nlInductornsnpTransformergap2Fig. 5 Discrete magnetic design of LLC resonant converterns(a) Inductor(b) TransformerFig. 6 Flux density simulation result(a)Fig. 4 Simulation waveform of output diodes D1 D2 voltage stress(a): with magnetic structure in Fig 2(b) with magnetic structure in Fig3When the leakage inductance exists on the secondaryside, it will increase the voltage stress on secondaryrectifier diode. So higher voltage-rating diodes areneeded, which will increase the conduction loss. Fig 4shows the simulate waveforms of secondary diodes withmagnetic structure in Fig.2 and Fig.3. It can be seen thatwith inductor on the secondary side, the voltage stressof the diodes is much higher. The magnetic structurewith precise control of Lr, Lm, while all resonantinductor are kept on the primary side, is needed.III. INTEGRATED MAGNETIC FOR LLC RESONANTCONVERTERA. Discrete Magnetic Solution for LLC ResonantConverterTo discuss the magnetic integration, it is necessary tohave a discrete design as the reference. In this part, adiscrete design is presented and simulation result isshown to compare with integrated solution. For LLC(b)(a) Inductor(b) TransformerFig. 7 Flux density change with input voltage rangeB. Extraction of Common Structure for IntegratedMagneticIn the past, a lot of research was done in integratedmagnetic design for power converters. Reviewing thosepapers, it can be found that most of them are based onEE core structure. The difference between differentdesigns is the placement of windings and air gaps.In this part of the paper, the general circuit model ofan EE core with four windings is used as a generalstructure as shown in Fig. 8. There are air gaps on eachleg. This is a very commonly used structure, manyintegrated magnetic design for PWM converter alsoused this structure with some change on the air gap orwinding placement [3][4].The reason of choosing this structure for LLCresonant converter is as following:
To integrate two magnetic components, usually threemagnetic paths are needed. In the LLC resonantconverter, although there are three magneticcomponents, Lm and transformer T can be build withinan air-gapped transformer. Therefore only two magneticcomponents: series resonant inductor Lr and gappedtransformer T are needed to be integrated. An EE corestructure will be a reasonable choice.n1*i1n2*i2Base on this circuit model, more integrated magneticstructures will be investigated.C. Integrated magnetic design A for LLC resonantconverterFrom discrete design, just combine them togetherwith an EE core, the two magnetic components can beintegrated into one, as shown in Fig. 11.nlf inpgap1n3*i3gap0n4*i4gap2Fig. 8 general magnetic structures for Integrated magneticThe proposed model is derived based on dualitymodeling theory [1]. Using this theory, the electricalcircuit model of a physical magnetic structure can beobtained. All the component parameters in the modelare related to the parameters of the physical magneticstructure. Fig 9 shows the reluctance model of magneticstructure shown in Fig. 8. Fig. 10 shows its equivalentelectrical circuit model. In the structure, there exist twosets of ideal transformer and three inductors.R1 R2lgap1R1=µ ⋅ Acn1*i1n3*i3R0n2*i2n4*i4lgap2R2=µ ⋅ Aclgap0R0=µ ⋅ AcFig. 9 Reluctance model of general integrated magnetic structureThe turns ratio of the two ideal transformer are same,as in the real physical structure. The three inductorscorrespond to the three air gaps and they are allreflected to first winding n1. They can also be reflectedto other windings as necessary. The values of eachinductor are given in Fig. 13.n3n1:n1L1L0L2n1:n2n42n1L1=R12n1L2=R22n1L0=R0Fig. 10 Circuit model of general integrated magnetic structuregap1InductorLegf cf tnsnsTransformerLeggap2Fig. 11 Integrated Magnetic Designs AE42/21/20 core is used. The cross-section area of is233mm 2 . For the outer legs, the cross-section area is thesame as discrete design. Turn number nl, np and ns isthe same as in discrete design. For this design, theinductor and transformer design is decoupled. Discretedesign procedure still can be used. Fig. 9 shows thesimulation result of for this structure.Fig. 12 Flux density simulation result for Design AIt can be seen from the simulation result that forinductor and transformer leg, the flux density is thesame as discrete design. But for center leg, the fluxdensity is much smaller than the discrete case. This willgreatly reduce the magnetic loss in the big part of themagnetic component.Fig. 13 Center leg flux density for different input voltage
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