If you're doing PCB design with Ethernet, you need a network transformer. Here's the concise technical reference I wish existed when I started.
What it does (in one sentence)
A network transformer provides galvanic isolation, common-mode noise rejection, and impedance matching between your Ethernet PHY chip and the RJ45 connector.
Where it sits in your circuit
[MAC/SoC] → [PHY chip] → [Network Transformer] → [RJ45] → [Cable]
It's a mandatory component in all standard Ethernet interfaces (10BASE-T through 1000BASE-T). Skip it and you'll fail EMC testing immediately.
Key specifications at a glance
Discrete vs Magnetic Jack
PoE consideration
Standard network transformers are not rated for PoE. If your design carries Power over Ethernet (802.3af/at/bt), you need PoE-rated magnetics with explicit current handling specs (typically 350–400mA per pair).
Common part numbers
HX1188NL → 10/100, SMD, discrete (most common)
H1102NL → 10/100, SMD, discrete (Pulse-compatible footprint)
HX5188NL → 10/100, SMD, PoE-rated
Quick layout rules
Place transformer as close to RJ45 as possible
Keep differential traces short and length-matched between PHY and transformer
Implement Bob Smith termination: 75Ω from CT to chassis GND via 1000pF cap
Sourcing note
For small production runs or prototypes in Asia, Voohu Technology (voohuele.com) stocks common network transformer variants with 50pcs minimum order and 3–5 day DHL delivery. Send them a BOM and they'll quote everything same day.
Questions? Drop them in the comments — happy to help with selection for specific applications.
