High Frequency Bi-directional DC/DC Converter with Integrated Magnetics for Battery Charger Application

Due to the concerns regarding increasing fuel cost and air pollution, plug-in electric vehicles (PEVs) are drawing more and more attention. PEVs have a rechargeable battery that can be restored to full charge by plugging to an external electrical source. However, the commercialization of the PEV is impeded by the demands of a lightweight, compact, yet efficient on-board charger system. Since the state-of-the-art Level 2 on-board charger products are largely silicon (Si)-based, they operate at less than 100 kHz switching frequency, resulting in a low power density at 3-12 W/in3, as well as an efficiency no more than 92 - 94%

Advanced power semiconductor devices have consistently proven to be a major force in pushing the progressive development of power conversion technology. The emerging wide bandgap (WBG) material based power semiconductor devices are considered as game changing devices which can exceed the limit of Si and be used to pursue groundbreaking high frequency, high efficiency, and high power density power conversion.

Using wide bandgap devices, a novel two-stage on-board charger system architecture is proposed at first. The first stage, employing an interleaved bridgeless totem-pole AC/DC in critical conduction mode (CRM) to realize zero voltage switching (ZVS), is operated at over 300 kHz. A bi-directional CLLC resonant converter operating at 500 kHz is chosen for the second stage. Instead of using the conventional fixed 400 V DC-link voltage, a variable DC-link voltage concept is proposed to improve the efficiency within the entire battery voltage range. 1.2 kV SiC devices are adopted for the AC/DC stage and the primary side of DC/DC stage while 650 V GaN devices are used for the secondary side of the DC/DC stage. In addition, a two-stage combined control strategy is adopted to eliminate the double line frequency ripple generated by the AC/DC stage.

The much higher operating frequency of wide bandgap devices also provides us the opportunity to use PCB winding based magnetics due to the reduced voltage-second. Compared with conventional litz-wire based transformer. The manufacture process is greatly simplified and the parasitic is much easier to control. In addition, the resonant inductors are integrated into the PCB transformer so that the total number of magnetic components is reduced. A transformer loss model based on finite element analysis is built and used to optimize the transformer loss and volume to get the best performance under high frequency operation.

Due to the larger inter-winding capacitor of PCB winding transformer, common mode noise becomes a severe issue. A symmetrical resonant converter structure as well as a symmetrical transformer structure is proposed. By utilizing the two transformer cells, the common mode current is cancelled within the transformers and the total system common mode noise can be suppressed.

In order to charge the battery faster, the single-phase on-board charger concept is extended to a higher power level. By using the three-phase interleaved CLLC resonant converter, the charging power is pushed to 12.5 kW. In addition, the integrated PCB winding transformer in single phase is also extended to the three phase. Due to the interleaving between each phase, further integration is achieved and the transformer size is further reduced.