SNVAA25: Differential mode inductor computation

Hi Archie,

The differential mode inductor attenuates the differential mode signal and allows DC to pass, so you can think of it as a second order low pass filter inductor, but with strict requirements from the 10Base-T1L standard. 

Your signal frequency is 10kHz.

The standard limits you to a maximum droop of 10% between 133.3ns and 800ns for a test signal of the PHY. IEEE and various magnetics manufacturers typically recommend 0.47uF capacitors as the best compromise between droop and return loss requirements. These are also used in my circuit. Now you just need the inductance.

The DMI is then chosen to not saturate for your supported PoDL class - as an example, for class 12, your inductor rated current must be higher than 632mA. In addition, your inductance must be large to attenuate the low frequency signal. For this, I would recommend an LC filter design app note, like:

https://www.ti.com/lit/an/slaa701a/slaa701a.pdf?ts=1653394617288&ref_url=https%253A%252F%252Fwww.google.com%252F 

In our testing, a 2x250uH inductor with 0.265mOhm of DCR gave us good results for class 12. Various magnetics manufacturers have tested solutions for particular PoDL classes and you can find them on their websites. 

Please let me know if you require any additional information.

Best regards,
Daniel Waleniak