PCB-Based High-Power DC/DC Converters with Integrated Magnetics for Battery Charger Applications

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Virginia Tech


Rising fuel costs and concerns about air pollution have significantly increased interest in electric vehicles (EVs). EVs are equipped with rechargeable batteries that can be fully recharged by connecting to an external electrical source. However, the wider adoption of EVs is hindered by the need for an on-board charger system that is both lightweight and efficient. EVs utilize two main charging methods: on-board chargers (OBC) for regular charging and off-board (fast) chargers for quick refills of battery pack. Most EVs currently use 400V battery packs paired with 6.6kW or 11kW OBCs, while larger vehicles with over 100 kWh battery packs employ 16.5kW or 19.2kW OBCs, constrained by household voltage and current limits. Some manufacturers are transitioning to 800V battery packs to lower costs and enhance fast charging capabilities, necessitating the development of 800V OBCs with high efficiency and power density. For household use, EVs can charge via OBC in a grid-to-vehicle transfer and can supply energy back to the home or grid (vehicle-to-grid) for emergency use or to support smart grid functionalities, requiring bidirectional OBCs. Advanced power semiconductor devices have been instrumental in advancing power conversion technology. The introduction of power semiconductor devices based on wide bandgap (WBG) materials marks a revolutionary shift, offering potential improvements over silicon-based devices. These WBG devices are capable of achieving higher efficiency, and higher power density in power conversion at higher operation frequency. Elevating the switching frequency diminishes the voltage-second across the transformer, facilitating the utilization of printed-circuit-board (PCB) technology for the windings as opposed to Litz wire implementations. Compared to traditional Litz wire-based transformers, the manufacturing process is significantly streamlined, and the management of parasitic is considerably more straightforward. Furthermore, the integration of resonant inductors with PCB-based transformer results in a reduction in the overall number of magnetic components and improved power density. This dissertation focuses on the DC/DC conversion stage of a bi-directional battery charger. It aims to achieve high power density and high efficiency using a PCB-based integrated transformer, enhancing manufacturing processes. The dissertation details the specific accomplishments in this area: Firstly, a two-stage on-board charger structure for 800 V battery EVs is proposed. The first stage is a four-phase bridgeless totem pole AC/DC converter working at critical conduction mode (CRM) so that soft switching can be achieved for all the fast switches. The second stage is single phase CLLC (1PCLLC) converter which is attractive due to its less component counts of devices and driver circuits. A novel matrix integrated transformer with controllable built-in leakage inductance for bi-directional 1PCLLC converter was proposed. Integrating three UI-core-based (1UI-based) elemental transformers with non-perfectly interleaved winding structures into one 3UI-based integrated transformer can reduce the core loss significantly with a smaller footprint compared with three EI-core-based integrated transformers. The proposed integrated magnetics can be scalable for higher voltage and higher power converters by assembling more 1UI-based elemental transformers. A SiC-based 1PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 250 W/in3 with maximum efficiency of 98.4%. Secondly, the challenge of increased common mode (CM) noise after adopting PCB-based windings in the design was discussed. The inter-winding capacitors between the primary and secondary windings act as a conduction path for high dv/dt CM noise, which can lead to electromagnetic interference (EMI) issues. To address this, a winding cancellation method for an integrated matrix transformer in a 1PCLLC converter was proposed and validated. This approach was tested in an 11-kW 1PCLLC converter. The EMI measurement results align with the analysis, confirming the effectiveness of the proposed method, which achieved a reduction in CM noise by 17dB. Furthermore, the 1PCLLC converter, incorporating the proposed planar matrix integrated transformer and winding cancellation technique, attained a power density of 420 W/in³ and a peak efficiency of 98.5%. Thirdly, to enhance efficiency further, the 1PCLLC converter is substituted with the proposed three-phase CLLC (3PCLLC) resonant converter equipped with three-phase rectifiers. The 3PCLLC converter becomes more promising for high power applications as its lower RMS current stress and automatic current sharing capabilities. It can achieve soft switching under all conditions. In addition, due to the symmetrical resonant tank, it is more suitable for bi-directional operation. Variable DC-link voltage is adopted so that the DC/DC stage can always work at its optimized point, providing best efficiency for the entire battery voltage. An improved core structure for the three-phase integrated transformer was proposed to reduce the core loss and simplify the magnetic components by integrating three primary resonant inductors, three secondary resonant inductors and three transformers into one magnetic component. A systematic method of converter design which includes the design of integrated transformer, converter loss optimization was adopted to design an 11kW 3PCLLC resonant converter. A SiC-based 3PCLLC converter prototype operating at 250-kHz switching frequency for 11-kW OBC applications was built with the proposed integrated transformer, and it can achieve a power density of 330 W/in3 with peak efficiency of 98.7%. Fourthly, the power level of OBC continues to increase to make up the large capacitance battery pack inside the EVs to relief the concern of mileage range. To address this challenge of higher power, a scalable matrix integrated transformer for multi-phase CLLC converter was proposed. A universal method of integrating magnetizing inductance with built-in leakage inductance based on multiple perfectly coupled transformers (PCTs). The integration of built-in leakage inductance can be achieved by connecting several PCTs using a standardized core type for cost considerations or can be further integrated into a customized core with interleaved magnetomotive force polarities across transformer legs to achieve better flux distribution and smaller core loss. The proposed concept can be applied to single-input single-output, and multiple-inputs multiple-outputs integrated transformer applications. A 3x3 PCTs-based integrated transformer built with PCB windings was designed for a 3PCLLC resonant converter, which integrates three primary resonant inductors, three secondary resonant inductors, and three transformers into one magnetic core to simplify the complexity of the converter. The effectiveness of the proposed concept was demonstrated through a high-efficiency, high-power density 3PCLLC DC/DC converter for an 800V 16.5kW OBC. The designed converter can achieve a power density of 500 W/in3 and a peak efficiency of 98.8%.



single phase, three phase, DC/DC, CLLC, Resonant converter, integrated magnetics, printed circuit board (PCB) transformer, battery charger