I wanted to share the results of some new data regarding the potential benefits for PoDL magnetics that are possible if the MDI return loss low frequency corner requirement is relaxed. The graph below summarizes the return loss data vs. OCL for a family of coupled inductors in a 0.09” by 0.06” by 0.059” package:
Currently there is a proposal to relax the return loss low frequency corner from 10MHz to 20MHz in order to accommodate coupled inductors with 1uH OCL, but I also wanted to explore the potential benefits of a more aggressive spec relaxation from 10MHz to 50MHz. The new data (yellow trace) is for a 470nH OCL coupled inductor which would be consistent with an 18dB return loss low frequency corner at 50MHz. Although the symbol droop would increase by ~2% with respect to the 20MHz corner case, the return loss at 600MHz is improved from 11dB to 14dB .
Additional benefits of relaxing the corner frequency to 50MHz from 20MHz include a reduction in DCR from 0.165 ohms to 0.116 ohms max and an increase in saturation current from 0.42A to 0.56A. Recall that coupled inductors also offer superior matching when compared to discrete inductor solutions, e.g. +/-3% vs. +/-10%, and this is an important consideration when trying to engineer a PoDL solution that is compliant with a 55dB MDI mode conversion loss requirement at 10MHz. Thanks.
Regards,
Andy Gardner
Staff Design Engineer
Linear Technology Corporation
IEEE802.3bu task force editor