DP83848i transformerless with small capacitors

remy charreyre

Other Parts Discussed in Thread: DP83848I

Hello,

I am working on a board with a lot of PHY (DP83848i) and I am trying to cut the price down by replacing transformers by capacitors. However, due to safety constraints, I have to use smaller Y capacitors value (<33nF) than in the AN-1519. To be honest I didnt fully understand the Appendix A: Calculating the Minimum recommended Capacitance and therefore I am wondering if I could customize the solution by adding resistors?

Thank you,

Rémy

over 12 years ago

0 John Hamilton1 over 12 years ago

TI__Intellectual 1885 points

Hello Remy,

Could you share some more information regarding your project?

Such as:

How many DP83848I PHYs are on your board?

Also, what safety constraints are of concern to you?

Thanks,

John

0 remy charreyre over 12 years ago in reply to John Hamilton1

Prodigy 30 points

John,

There are 7 PHY and 3 switch from Micrel (I cant change that, I'm not the designer), which is transformerless capable too. All of these are using many transformers. They are big and cost money.

So we thought we could replace them by capacitors, which are much smaller and cheaper. I actually started to test with 100nF and so far it is working just fine. However I have been told recently to use Y2 capacitors. I cannot provide you the design of the board but it is seperated in sections which are supposed to be independent (it's a "safety board", I'm working in a railway company). Sadly these Y2 capacitors are expensive (100nF are around 1€). That is why I was wondering if I could use smaller ones (~20nF) somehow?

Thanks,

Rémy

ps: I included the tests I have done so far. It's very basic but I'm still working on it.6646.transformerless_basic_test.zip

0 remy charreyre over 12 years ago in reply to remy charreyre

Prodigy 30 points

no news means no solution? Well, I can try by myself by I would like to have an official opinion on this!

0 John Hamilton1 over 12 years ago in reply to remy charreyre

TI__Intellectual 1885 points

In the App Note AN-1519 the minimum value of 33nf is specified in order to remain compliant with IEEE and ANSI standards.

The App Note AN-1519 also recommends that the PHY be operated in Forced 100MB mode, From the waveforms provided

in your transformerless_basic_test.zip file I see that some of your tests are in 10MB mode, not recommended.

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DP83848i transformerless with small capacitors