Design of a Flying-Capacitor-Based Voltage Divider LLC Resonant Converter for USB-C PD Charger in Aircraft Applications

Abstract

Portable electronic devices have grown dramatically at an incredible speed and include many new and sophisticated functions. Thanks to the advent of universal serial bus (USB) power delivery (PD) specifications, power delivery is increased as is the number of applications that can be powered using a USB cable in aircraft. However, the combination of the wide-voltage-gain operation required by the USB PD specification, high step-down ratio, and the lower-pressure environment associated with aircraft puts a lot of stress on converter optimization. This paper presents the DC-DC stage of an integrated, highly compact USB type-C PD charger for aircraft. First, a flying-capacitor-based voltage divider (FCVD) switching bridge is proposed to replace the conventional switching bridge in an LLC converter to improve its efficiency. Second, a printed circuit board (PCB) winding planar transformer is adopted to increase the power density. Gallium nitride devices are used for both the FCVD and the synchronous rectifiers, further shrinking the size and reducing the loss. Critical areas of the converter prone to a high electrical field (E-field) are identified and preemptively addressed with the use of the E-field control methodology that follows the Paschen-curve-based insulation coordination. The prototype is built and tested, and it is verified that the unit successfully operates in rated conditions to achieve a power density of 73.2 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and an efficiency of over 90% at all nominal outputs. A high-altitude partial discharge test is also conducted, showing that the proposed prototype meets the partial discharge inception voltage (PDIV) at 10,000 ft.