An 81.0% Peak Efficiency, 1.0W/cm3 Miniaturized 5V/1A AC-DC Converter Using a Highly-Integrated Primary-Side Active Clamp Flyback Controller with Adaptive Frequency and Zero-Voltage Switching

This paper seeks to reduce the physical size of 5W isolated AC-DC converters, which is increasingly important for smart building and internet of things applications that require small devices integrated with wired power. Prior works on primary-side regulation (PSR) flyback converters [1]–[3] (Fig. 1) eliminate signal isolation blocks to increase power density at 5V/1A, shrinking the converter in [1] to $17.496\text{cm}^{3}$ with a peak efficiency of 78.2%. However, PSR converters require lower frequency and larger magnetizing inductance $(\mu \mathrm{m})$ in the trans-former $(> 700\mu \mathrm{H})$ due to switching loss. A flyback with secondary-side regulation (SSR) achieved zero-voltage switching (ZVS) with a volume of 48 $6\text{cm}^{3}$ for 60 W with a smaller $210\mu \text{HL}_{\mathrm{m}}$ (Fig. 1) [4]. Further reduction in the transformer size requires a higher switching frequency, but the efficiency of the SSR is limited by leakage inductance $(\mathrm{L}_{1\mathrm{k}})$ and switching frequency [5]. Active clamp flyback (ACF) systems (Fig. 1) [6], [7] enable up to 220kHz operation with full system using ZVS by reusing the energy in the $\mathrm{L}_{1\mathrm{k}}$. Most prior ACF systems target higher output power (>30W) and have larger volume (e.g., 24.32cm³ [6]). In addition, compared to the SSR, extra components are required for ACF switching operation [4]. There remains a need for 5V/1 A ACF converters with smaller volume from reducing the transformer size and suppressing the number of components.