33.5 A 1.25W 46.5%-Peak-Efficiency Transformer-in-Package Isolated DC-DC Converter Using Glass-Based Fan-Out Wafer-Level Packaging Achieving 50mW/mm2 Power Density

Abstract

Power delivering with galvanic isolation is essential to guarantee system safety and reliability in harsh industry environments. However, efficiently transferring power of hundreds of mW across an isolation barrier is challenging for such size- and costconstrained applications. Isolated capacitive power transfer using on-chip capacitors and an off-chip inductor is demonstrated in [1], but it only delivers 62mW power with less1 kV isolation voltage that is limited by the on-chip capacitors. To increase the output power and the isolation voltage, isolated DC-DC converters using silicon-based postprocessed micro-transformers have been reported recently [2] –[4]. In [2], 6-$\mu \mathrm{m}$-thick plated Au are used for both the primary and the secondary coil windings to achieve quality factors of 6.8 at 200MHz, while the efficiency of the converter is lower than 34% with a maximum output power of 0.8W. A performance-enhanced micro-transformer using a magnetic core is proposed in [3] to achieve a peak efficiency of 52% and a maximum output power of 1.1W. However, the fabrication process of such a transformer is complex and the cost is high. In [4], the micro-transformer is formed by using ultrathick metal windings, and high inductances with high quality factors are achieved to allow the converter to switch at 11MHz. However, the topology proposed produces large resonant currents that flow into the coils, degrading the efficiency to 34% with only 165mW output power. Moreover, the abovementioned isolated converters are assembled in a small-outline integrated-circuit (SOIC) 8-lead [2] or 28-lead [3] packages that measure 6mm $\times 10$ mm or 10mm $\times 18$ mm, respectively, resulting in a maximum power density of only 13.33mW/mm2.