Traditional 802.3 Ethernet systems utilize an isolation transformer between the cable and PHY (physical layer) chipset. In Power over Ethernet (PoE) applications, the center tap of the transformer is used as the delivery and return path of the power. While an isolation transformer can still be implemented in PoDL, it is not required.
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Figure 1 – PoDL solution does not require a transformer for isolation
The schematic in Figure 1 shows that a single common mode choke and two capacitors are all that is required for isolation in PoDL. PoE applications use one bus pair for data delivery and another pair for the power return path. Unlike PoE, PoDL employs differential power injection using a single-pair bus for both the power delivery path and the return path (Figure 2).
Figure 2 – PoDL power injection is differential using a single bus
| Inductors | DCR max. (mOhms) |
Max. Area (mm2) | Isat (A) 30% | Irms both windings | SRF (MHz) | Turns Ratio | Capacitance |
Max Temp (°C) |
AEC Grade | Application |
|---|---|---|---|---|---|---|---|---|---|---|
| LPD4012-333 (33 µH) | 1830 | 16.0 | 0.58 | 0.31 | 12 | 1:1 | 27.4 | 125 | – | For ~0.3A |
| LPD5030-333 (33 µH) | 565 | 23.04 | 0.78 | 0.60 | 16 | 1:1 | 37.5 | 125 | 3 | For ~0.6A |
| MSD7342-333 (33 µH) | 270 | 56.25 | 1.7 | 0.76 | 11 | 1:1 | 32.9 | 125 | 3 | For ~0.75A |
| MSD1260T-333 (33 µH) | 134 | 151.29 | 4.5 | 1.64 | 12.4 | 1:1 | 91.4 | 165 | 1 | For ~1.5A |
The 33 µH Coilcraft components in this table can be used as both the common mode and differential mode filters shown in Figure 2. Specific selections are based on the desired injected current level.
To validate the performance of these components in 10BASE-T1S PoDL applications, the schematic in Figure 2 was implemented. On the PHY path, a Coilcraft 1210CAN-513 was used as a data only common mode choke. On the PSE / PD path, the different components in the table were connected in common mode / differential mode. Return Loss (Sdd11) and Insertion Loss (Sdd21) were then generated between the microcontroller differential pair and the data point differential pair.
