Different PHY wiring principles

The following is a detailed analysis of the differences and design considerations between voltage-based PHY and current-based PHY in network transformer applications, combined with actual scenarios and technical requirements:

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1. Differences between voltage-based PHY and current-based PHY

Summary of key differences

• Driving principle
• Voltage-type PHY: directly outputs a specific voltage signal (such as a 2.5V swing).
• Current-type PHY: driven by a current source, the output current is determined by the line impedance and the required voltage.
• Network transformer selection
• Voltage type: Focus on the transformer primary/secondary voltage ratio (e.g. 1:1 or 1:2).
• Current type: The impedance of the transformer needs to be matched (such as 1:1CT, center tap is used for common mode rejection).
• Impedance matching design
• Voltage type: A terminating resistor (such as a 100Ω differential resistor) may be required on the secondary side of the transformer.
• Current type: A matching resistor network (such as a 25Ω resistor in series + a 100Ω resistor in parallel) needs to be set on the PHY side.

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2. Differences in network transformer wiring design

1. Typical wiring of voltage-based PHY

• Wiring diagram:
  
  

PHY TX ± → Transformer primary ± → Secondary ± → RJ45 (center tap connected to filter capacitor + VDD)

 Design points:

• The center tap needs to be connected to the PHY power supply (e.g. 2.5V) through a capacitor (e.g. 0.1μF).

• The secondary side needs to terminate the differential line with a 100Ω resistor to suppress signal reflection.

2. Typical wiring of current-mode PHY

• Wiring diagram:

PHY TX ± → matching resistor → transformer primary ± → secondary ± → RJ45 (center tap connected to common mode inductor)

• Design points:
• The PHY side requires impedance matching through a series resistor (e.g. 25Ω) and a parallel resistor (e.g. 100Ω).
• The center tap should be connected to a common mode inductor or directly to ground (depending on the PHY manual requirements).

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3. Design considerations

1. Notes on voltage-based PHY

• Power supply stability: The center tap power supply needs to be low-noise. It is recommended to use LDO power supply and add decoupling capacitors (such as 10μF+0.1μF).
• Termination resistance accuracy: 100Ω differential resistors must have 1% accuracy to avoid clock jitter caused by signal reflection.
• Signal amplitude debugging: The oscilloscope detects whether the signal swing meets the standard (such as 1V peak-to-peak value) to prevent insufficient PHY driving capability.

2. Notes on current-mode PHY

• Impedance matching network: Design matching resistors strictly according to the PHY manual (example: 25Ω series + 100Ω parallel).

• Current source protection: To avoid output short circuit, PHY may be damaged due to overcurrent.

• Common-mode noise suppression: The center tap increases the common-mode inductance (e.g. 10mH) to improve EMI performance.

3. Common points of attention

• Transformer selection: It must support the operating frequency (10/100/1000BASE-T corresponds to different frequency bands).
• PCB wiring rules:
• Differential lines are strictly equal in length (±5 mil), evenly spaced, and impedance error is controlled to be ≤10%.
• The distance between PHY and transformer is ≤50mm to reduce path loss.
• EMC design:
• Place an isolated ground plane near the network transformer.
• Add TVS diodes to prevent surge damage.

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4. Common Errors and Solutions

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V. Conclusion

• Voltage-type PHY: suitable for low-cost, medium- and low-speed scenarios (such as 10/100M). The circuit design is simple but requires strict voltage regulation.

• Current-type PHY: used in high-speed/high-precision scenarios (such as Gigabit Ethernet), requiring precise impedance matching and noise suppression.

• Core principles:

• Design the network transformer peripheral circuit according to the PHY chip manual.

• Focus on Signal Integrity (SI) and Electromagnetic Compatibility (EMC).

Select the appropriate PHY type based on actual needs and use simulation tools (such as ADS/HFSS) to optimize performance during design.