1 Introduction

AC-DC USB power delivery (PD) charger require higher efficiency with compact size, and this presents a significant challenge for power supply design. For power applications below 100W, the quasi-resonant (QR) flyback is especially popular due to improved efficiency and reduced switching losses, achieved by leveraging valley switching techniques. With advancements in semiconductor technologies such as GaN, the flyback topology continues to achieve higher power density and efficiency, making this an option for applications such as USB PD chargers and other compact power supplies. Figure 1-1 shows TI’s GaN integrated flyback with self-bias VCC UCG28826 application.

Figure 1-1 Simplified Schematic of AC/DC Flyback Converter using UCG28826

For most flyback controllers, the VCC pin provides the bias to the device, which powers the internal references, gate driver, regulators, control circuits and protection features. An extra rectification circuit through the auxiliary winding is required to offer VCC bias. As a result, the VCC bias obtained using this method is proportional to the output voltage. This consequently raises two issues: auxiliary winding increases the volume of the circuit and applications that require wide output voltage range require an internal and external power conversion stage to reduce to VCC range, increasing external components and reducing efficiency.

Self-biasing VCC circuitry is an option for the problems mentioned above. By adopting a capacitor at the VCC pin, the energy stored in both the switch node and the transformer can be harvested as power supply for the controller. Using this method, auxiliary winding is no longer required. This method leverages proper configuration of external capacitor to achieve accurate voltage sensing and output voltage. Additionally, the elimination of the auxiliary winding not only simplifies transformer manufacturing but also streamlines the EMI design process.

Figure 1-2 VCC Self-Bias