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I'm working on a PoE based system with the CS8900A as the Ethernet PHY. It is specified in the CS8900A datasheet that the isolation transformer required must be of 1CT: 1.414CT for 5V of supply.

Please let me know, on what basis is the above turns ratio chosen? What are the calculations involved?

Also, what happens if a transformer with a different turns ratio is chosen?

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The turns ratio is based on the design of the transceivers in your PHY. The ethernet specification calls for a +/-2.5V signal level over the twisted pair, so if your transceiver outputs e.g. 3.5Vpp, this needs to be 'transformed up' to 5Vpp with a 1:1.414 turns ratio transformer.

If you don't adhere to this, your device may not operate together with other network equipment on the other side of the cable. In practice, Ethernet is extremely forgiving on the signal level front so you may get away with a 1:1 transformer, but cable length and possibly performance will definitely take a hit.

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Here's a clue: -

enter image description here

It's all about matching impedances to avoid data corruptions due to incorrectly terminated cables. Take a look at the table circled in red.

With a turns ratio of \$\sqrt2\$ impedances are transformed by this squared. So an impedance of 24.3 ohms for Rt becomes transformed to 48.6 ohms. As the CS8900A outputs differentially the impedance seen by the line (RJ45 side) is double this at 97.2 ohms. Given that there may be some series resistance in the driver outputs of about an ohm the 24.3 ohm might be more like 25 ohms and then double this because it is a diff driver then double because the turns ratio is \$\sqrt2\$ and you get 100 ohms.

To match 150 ohms, divide by 4 to get 37.5 and this is pretty near to the 37.4 ohms specified but what about the 1 ohm I used above. Well there are losses in the transformer that are in-effect parallel resistors (core losses due to eddy currents) and these will be more prevalent at higher impedances so I guess, if I new the exact characteristics of the transformer and could be bothered to look at the tech spec of the line driver for the chip I might be able to justify things a tad better. Gut feeling tells me I'm about right!

So what happens on 3V3 supplies? Well the output drive level is going to be smaller for sure so using a transformer to bump up the output voltage makes sense but you've still got to pay head to the impedances. A 1 to 2.5 steps up impedances by \$2.5^2 = 6.25\$ so the 8 ohm specified becomes 16 ohm differential then 100 ohm on the RJ45 side.

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  • \$\begingroup\$ Great Answer! I think different location for Rt and Rr is because one is source termination and another is load-side termination. \$\endgroup\$ – abhiarora Sep 21 '18 at 9:55

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