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.

Popular Forums

Product Forums

Application Forums

Design Center Forums

Highlighted Webinar

Places

Libraries

Upcoming Learning & Events

Challenge Yourself!

What's Brewing

Places

Resources

Visit ContentZone

The Latest Read

New Release

Recent Technical Insights

Products Mentioned

Thread Details

Transformer and Transistor

Thread Summary