DEV Community

Mia
Mia

Posted on

Selecting a Network Transformer for Smart Home and IoT Devices

#hardware #iot #electronics #tutorial

Many IoT products use Wi-Fi, Zigbee or Thread, but Ethernet is still common in gateways, alarm panels, video doorbells and building controllers.

The magnetics between the PHY and RJ45 must be selected for signal integrity, isolation, EMC and, when applicable, PoE current.

Start with the Interface 

ethernet_selection:
  phy: LAN8720A
  standard: 100BASE-TX
  active_pairs: 2
  turns_ratio: "1CT:1CT"
  isolation: "1500 VAC"
  poe: false
  temperature: "-40°C to +85°C"
Enter fullscreen mode Exit fullscreen mode

A 10/100BASE-TX device normally uses two active cable pairs. A 1000BASE-T device uses all four pairs and requires four-channel magnetics.

Do not approve a component using the speed rating alone. Verify:

  • PHY reference schematic
  • Internal topology
  • Pinout and polarity
  • OCL test conditions
  • Insertion loss and return loss
  • Isolation voltage
  • Package footprint

OCL Matters

For many 10/100 transformers, a common requirement is:

OCL >= 350 µH
Test frequency: 100 kHz
Test voltage: 0.1 V RMS
Enter fullscreen mode Exit fullscreen mode

The OCL value cannot be compared correctly without its test conditions.

In PoE applications, confirm OCL under DC bias. Excessive current can push the core toward saturation.

PoE Thermal Check

Copper loss is approximately:

def winding_loss(current_a, resistance_ohm):
    return current_a ** 2 * resistance_ohm

print(winding_loss(0.6, 0.35))
# 0.126 W
Enter fullscreen mode Exit fullscreen mode

For four-pair PoE, calculate losses for every powered winding and evaluate temperature rise at the maximum internal enclosure temperature.

Check support for:

802.3af  -> 15.4 W PSE
802.3at  -> 30 W PSE
802.3bt3 -> up to 60 W PSE
802.3bt4 -> up to 90 W PSE
Enter fullscreen mode Exit fullscreen mode

A non-PoE network transformer should not be assumed to handle DC current.

PCB Layout Checklist 

PHY -> transformer -> common-mode network -> RJ45
Enter fullscreen mode Exit fullscreen mode
  • Put the transformer near the RJ45 connector.
  • Route differential pairs as approximately 100-ohm differential traces.
  • Keep pair lengths matched.
  • Do not route switching nodes beneath the magnetics.
  • Preserve creepage and clearance at the isolation barrier.
  • Connect Bob Smith termination according to the PHY reference design.
  • Keep chassis ground separate from signal ground where required.

Temperature Selection

An indoor hub may accept a commercial-temperature component. Outdoor cameras, video doorbells and access controllers often need −40°C to +85°C components.

Use the estimated PCB temperature inside the enclosure, not only room temperature.

Final Checklist 

{
  "speed_verified": true,
  "phy_reference_checked": true,
  "pinout_compared": true,
  "ocl_conditions_verified": true,
  "isolation_verified": true,
  "poe_current_verified": true,
  "thermal_test_completed": true,
  "sample_tested_on_pcb": true
}
Enter fullscreen mode Exit fullscreen mode

Voohu Technology supplies network transformers, LAN magnetics, RJ45 connectors and passive components. Prototype orders can start from 50 pieces, with BOM cross-reference support and typical three-to-five-day DHL delivery, subject to stock and destination.

More information: Voohu Technology

Top comments (0)