This work explores how GaN devices and advanced control can improve the power density of battery chargers for the plug-in hybrid electric vehicle. Gallium nitride (GaN) devices are used to increase switching frequency and shrink passive components. An innovative DC link reduction technique is proposed and several practical design issues are solved. A multi-chip-module (MCM) approach is used to integrate multiple GaN transistors into a package that enables fast, reliable, and efficient switching. The on-resistance and output charge are characterized. In a double pulse test, GaN devices show fast switching speed. The loss estimation based on the characterization results shows a good match with the measurement results of a 500 kHz GaN-based boost converter. Topology selection is conducted to identify candidates for the PHEV charger application. Popular topologies are reviewed, including non-isolated and isolated solutions, and single-stage and two-stage solutions. Since the isolated two-stage solution is more promising, the topologies consisting of an AC/DC front-end converter and an isolated DC/DC converters are reviewed. The identified candidate topologies are evaluated quantitatively. Finally, the topology of a full bridge AC/DC plus dual active bridge DC/DC is selected to build the battery charger prototype for fixed switching-frequency, low loss, and low realization complexity. The DC link capacitor is one of the major power density barriers of the charger, as its size cannot be reduced by increasing the switching frequency. This work proposed a charging scheme to reduce the DC link capacitance by balancing the ripple power from input and output given that the double-line-frequency current causes minor impact to the battery pack in terms of capacity and temperature rise. An in-depth analysis of ripple power balance, with converter loss considered, unveils the conditions of eliminating the low-frequency DC link capacitors. PWM-zero-off charging where the battery is charged by a current at double-line-frequency and DC/DC stage is turned off at the zero level of the waveform, is also proposed to achieve a better tradeoff between the DC link capacitor size and the charger efficiency. The practical design issues are outlined and the solutions are given at different levels of implementations, including the full bridge building block, the AC/DC stage, and the DC/DC stage. The full bridge section focuses on the solution of a reliable driving and sensing circuitry design. The AC/DC stage portion stresses the modulator improvement, which solves the often-reported issues of the current spike at the zero-crossing of the line voltage for the high frequency totem-pole bridgeless converter. In the DAB section, analytical expressions are given to model the converter operation at various operating conditions, which match well with the measurement results. The overall charging-system operation including the seamless transition of bi-directional power flow and the charging-profile control is verified on a laboratory GaN charger prototype at 500 kHz and 1.8 kW with an efficiency of 92.4%. To push the power density, some bulky components including the control board, the cooling system, and the chassis are redesigned. Together with other already-verified building blocks including full bridges, magnetics, and capacitors, a high-density mock-up prototype with 125 W/in3 power density is assembled. / Ph. D.