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Electronics Production | July 04, 2011

Common mode choke filtering improves CMRR in ethernet transformer applications

Common mode chokes filter common mode EMI currents without causing signal degradation and without derating under high currents. These components are applied to pairs of conductors – supply and return.

The addition of a common mode choke before the connector filters these common mode currents while allowing the desired signals to pass unaffected. The result is effective communication between devices, reliable product and system operation and product EMI regulatory compliance. Differential Mode and Common Mode Explanation Common mode chokes are the ideal components for EMI filtering of power and signal lines. These components withstand high DC currents without degradation of filtering performance that can occur with differential mode filters like small chip beads. Stable common mode chokes allow most signals to pass unaffected, yet filter the noise (EMI) from these circuits. Most well-designed power and signal circuits present no EMI that is caused from intended currents. As an example, at 30 MHz, a pair of traces or wires 1 m long, separated by 1.3 mm, requires over 20mA differential current imbalance to exceed 100mV/m radiation 3 m away. However, unintended, unforeseen common mode currents can exceed 100mV/m radiation with only 8mA common mode current flow. Suppression of these tiny common mode currents is often crucial to assuring EMI regulatory compliance and reliable product performance. Accompanying the rapid increase of electronic circuit speed, the signal frequency in a circuit is already higher to the EMI noise frequency range. The EMI design becomes a critical issue. The traditional parallel signals possess some disadvantages and cannot normally work in the high frequency. Featuring high anti-interference with low EMI, series signals like USB, SATA, PCIe, etc. are popular alternative for replacing parallel signals which are utilized in current designs. Differential Mode and Common Mode Signal The differential signal has high anti-interference and low EMI performance. It is now widely used in the high speed digital systems like USB, SATA, and HDMI, etc. Common mode and differential mode currents and voltages are always defined relative to the ground. Generally, a signal contains both differential and common mode signals. Diagram 1: Differential and Common Mode Signal Diagram 1 shows a clock driver output of a differential signal to a receiver which contains a common mode signal in the same time. Below is the current and voltage equation of common mode and differential mode signals. Common mode currents flow with the same voltage and direction, and go to ground by the parasitic capacitances between the lines and ground. There is no common mode current flow through the load of the receiver because there is no voltage difference on the load. Diagram 2 and Diagram 3 show a differential mode signal which contains useful information. The current phase difference is 180°, where the current fully flows through the load, and where there is no current going to the ground. Diagram 2: Differential Mode Signal Diagram 3: Phase Different of the Currents and No Current Flow to Ground Because field strength in the two lines creates equal and opposing electromagnetic fields, they tend to cancel each other out and reduce crosstalk and EMI emission. Common Mode Choke Differential mode current, flowing in opposite directions through the choke windings, creates equal and opposite magnetic fields, represented by the black lines in Diagram 4. Cancelling each other out results in the choke presenting zero impedance to the differential mode signal, which passes through the choke un-attenuated. Diagram 4: Common Mode Choke Common mode current, the unintended component of signal current, is the main source of EMI and should be suppressed. The common mode current flows in the same direction through each of the choke windings, represented by the red lines in Diagram 4. This creates equal and in- phase magnetic fields which add together and results in the choke presenting high impedance to the common mode signal, which passes through the choke heavily attenuated. Ethernet Transformer Operation Principle Diagram 5 is Ethernet transformer operation principle. The winding polarity of a center- tapped transformer is as shown in the diagram. The transformer is a 1:1 for the signal transfer between two sides. For the differential signal of Tx+ and Tx-, the current Tx+ is equal to Tx- with opposite direction. But for the common mode current, the Icom will flow into the ground through the center-tap because of no impedance in the opposite direction winding of the transformer for the common mode current. So the transformer has two functions: the first is isolation of the input and output for safety purposes. The second is rejection of the common mode signal which may create EMI noise, but does not affect the useful differential signal. Because of the non-ideal isolation and un-balance of the transformer, it still has some leakage when the common mode current flow crosses the transformer. The Igcom represents the leakage current as it crosses the transformer, which may create EMI noise that should be equal to zero under ideal circumstances. Diagram 5: Ethernet Transformer Operation Principle Definition of Common Mode Rejection Ratio (CMRR) Common mode rejection ratio (CMRR) is defined as the rejection ability for common mode noise: CMRR=20 log V2/V1. With a larger CMRR, the noise is attenuated, resulting in the need for larger suppression of the noise. The CMRR test diagram is shown below: Diagram 6: Ethernet Transformer CMRR Test Circuit ----- Author: Laird Technologies at http://www.lairdtech.com Note: All graphs © Laird Technology
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