Common Mode Filter for USB 3 Interfaces

This paper presents an integrated common mode filter (CMF) for USB 3 interfaces that are operated in Super-Speed mode. For the protection of sensitive USB ports against Electrostatic Discharge (ESD) this filter device includes an advanced low-voltage triggered semiconductor controlled rectifier (LVTSCR). Compared to conventional ESD protection diodes, LVTSCR offer a very low junction capacitance (only 0.20 pF) and a low clamping voltage due to the low dynamic resistance (0.14 Ohm). The manufacturing process combines planar layers of copper and polyimide on top of bipolar silicon wafer. The integrated Cu-air coils obviate the need for additional ferrites for common mode suppression. The -3 dB passband frequency for the differential mode is 6 GHz, while the common mode exhibits a strong attenuation of frequencies above 1 GHz (> 15 dB). A distinctive notch characteristic for the common mode is visible between 2 GHz and 3 GHz (typ. > 30 dB). This makes the device suitable for applications where USB 3.1, Gen 1 and 2.4 GHz WIFI (802.11 b/g/n) are implemented in close vicinity. Due to the small size (0.8mm by 1.2 mm) the device is beneficial in portable applications like smartphones.