MP8030-QJ: Inductor selection for VDD and current route for Flyback in reference design

Reference Designs
1

Hi MPS engineer,
I’m evaluating on PoE PD solution, MP8030-QJ, for delivering 45W to the application, with PD Class 6 (Power rating: 40 to 51W) and PSE 12V/5A selected. Regarding the reference design, I’d like to consult you two questions.
The first is about the inductor selection for VDD. It’s one common mode choke with the parameter shown below.
a. How do you decide on the selection of 1mH?
b. Is it because of EMI concern or voltage spike suppression? I guess this value depends on the schematics design as well.
c. If so, are there any methods or simulation tools that could be used to evaluate on our schematics design accordingly?

Manufacturer/model: WURTH/7448012501
Inductance: 1mH, Rated current: 2.5A, DC resistance: 55m ohm

The second question is about the current route for secondary side in terms of transformer, which is based on flyback topology. Could you provide the detailed explanation about how current goes in the secondary side of the transformer?

In case you couldn't see the attached image, the reference document is EVL8030-QJ-00B -- PRIMARY-SIDE REGULATE FLYBACK EV BOARD

image
image1188×893 102 KB

Thanks for your time to spend on this consultation.

Warm regards,
Bill
2

Hi Bill,

Apologies for the delay in response. Generally, the components are selected for the most optimal performance. The common mode inductor is for improving EMI performance. There is a section on this application note that talks about this under EMI design.
How to Optimize a Power over Ethernet (PoE) PD Design | Article | MPS (monolithicpower.com)
We do not have a simulation tool for this device at the moment. In terms of design, it is recommended to follow the guidelines for component selection and pcb layout from the datasheet or refer to the evaluation board.
Hope this helps.

Best Regards,
Yu