PoDL Circuit with Combined DC Coupling and Common Mode Termination
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing Power over Data Lines (PoDL) systems face issues with common mode noise attenuation and mode conversion due to impedance imbalances and reflections in the termination circuitry, particularly when applying DC power to twisted wire pairs.
Innovation Solution
The PoDL circuit incorporates a serially connected common mode choke (CMC) and differential mode choke (DMC) with AC coupling capacitors, along with a pair of inductors or a second CMC, ensuring balanced impedance matching through capacitors and resistors to prevent mode conversion and reflections, while maintaining low impedance for DC power transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If DC power is coupled to the wire pair through AC coupling capacitors and termination circuitry, then DC power transmission is enabled, but common mode noise reflections and mode conversion occur due to impedance imbalances
Solution Approach 1:
The termination circuitry is segmented into separate common mode and differential mode termination components. The common mode termination is implemented through resistors connected from each wire to ground, while differential mode termination is handled separately. This segmentation allows independent optimization of common mode noise suppression without affecting DC power transmission.
Solution Approach 2:
AC coupling capacitors are introduced as intermediary components between the DC power supply and the wire pair. These capacitors block DC from reaching the differential mode termination while allowing AC signals to pass, effectively decoupling the DC power transmission path from the signal path and preventing impedance mismatches.
2Object-affected harmful factors
If common mode choke and differential mode choke are used to attenuate common mode noise, then common mode attenuation is improved, but impedance imbalances cause mode conversion
Solution Approach 1:
The chokes are designed with different local properties for common mode and differential mode signals. The common mode choke has high impedance specifically for common mode currents while presenting low impedance to differential mode signals. This is achieved through specific winding configurations where the magnetic fields of differential signals cancel while common mode fields add constructively, providing frequency-dependent and mode-selective attenuation.
3Reliability
If matched impedance termination is applied to prevent reflections, then signal integrity is improved, but DC power coupling becomes complex
Solution Approach 1:
The AC coupling capacitors serve multiple functions simultaneously: they enable DC power transmission by blocking DC from the differential termination, provide common mode noise filtering, and maintain impedance matching for AC signals. This multi-functionality reduces overall circuit complexity compared to separate dedicated components for each function.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively reduces common mode noise reflections and mode conversion, providing symmetric termination and improved common mode noise attenuation, ensuring reliable data transmission over twisted wire pairs.
Implementation Method 1
The CMC 402 is an in-line transformer with two windings, where each winding is in series with a wire in the twisted wire pair. As shown by the dots on the CMC 402 windings, the windings have the same polarity, so the magnetic fields generated by a differential mode signal are substantially cancelled out. Thus, the CMC 402 presents little inductance or impedance to differential-mode currents. Common mode currents due to noise, however, see a high impedance due to the combined inductances of the windings.
Implementation Method 2
The DMC 410 has windings with opposite polarities (dots on opposite ends). The DMC 410 presents a high impedance to AC differential mode signals while it shunts the common mode signals to the power supply (connected across the Vs and ground terminals) and to the termination circuitry 404 due to its low impedance to common mode signals.
Implementation Method 3
The PHY 206a is connected to a Media Dependent Interface (MDI) connector 208a via a common mode choke (CMC) 402 and AC coupling capacitors C3 and C4.
Data Source
AI summary
In a Power over Data Lines (PoDL) system that conducts differential data and DC power over the same wire pair, various DC coupling techniques are described that improve DC voltage coupling while attenuating AC common mode noise and avoiding mode conversion. A first CMC and AC coupling capacitors are connected in series between a PHY and a twisted wire pair. A DC power supply is DC coupled to the wires via a series connection of a DMC and either matched inductors or a second CMC. Coupled between the DMC and the inductors/CMC is an RC termination circuit comprising a first capacitor coupled to one leg and a matched second capacitor coupled to the other leg. The two capacitors are connected to the same resistor coupled to ground.