Transformer and Transistor

Question

The datasheet strongly suggests PSMN075-100MSEX for the transistor and a 10/100 Mbps transformer without common mode chokes for isolation. Wurth 7490100143 is given as an example. 
 Both of these components are difficult to source. 
 BUK9M60-100LX from Nexperia seems to be very similar in parameters to PSMN075-100MSEX. Would this be a valid alternative? For the transformer, S558-5999-T7-F from Belfuse is the same footprint, but it has common mode chokes. Why are common mode chokes and issue for this application?

TimW · Accepted Answer

see also: PSE Controller LTC9101 isolation - Q&A - Power Management - EngineerZone)

EHorsma · Answer

Just to add a bit more information to Tim's answer; you should also verify the MOSFET's SOA. Check the datasheet for LTC9102/LTC9103 foldback curves to get an idea of the required SOA and test in your circuit. Alternatively, the SI7322ADN-T1-GE3 and DMT10H015LFG-7 have been tested by ADI and are acceptable substitutes for PSMN075-100MSE with stock at Digikey. And per the linked Q&A thread, keep the chokes on LTC9103-side of the high-speed bus and it should be OK. 
 Best Regards, 
 Eric

EHorsma · Answer

Yes, the concern is in regards to inrush and current limit events.

TimW · Answer

We cannot test all variations of FETs in an application, so we can't say with any certainty that a particular FET would work in an application. That said, the specs look close enough. It's something you'll have to test. As for the transformer, the short answer is: because the LTC9103 uses low‑frequency/DC signaling over the Ethernet magnetics, and a transformer with integrated common‑mode chokes can distort or attenuate that signaling . The disadvantage of using a transformer with common‑mode chokes is reduced signal integrity, timing errors, or outright communication failure of the controller’s isolated interface.

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Transformer and Transistor

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