Gbit Ethernet protection circuit approaches the ideal

What if your Gigabit Ethernet protection circuit response resembled that of an ideal diode? Asks Len Stencel, applications manager for semiconductor products at Bourns. 

Gigabit Ethernet (GbE) is a balanced high-speed communication standard that is widely used around the world. Until recently, the majority of its applications were in non-hostile environments. New applications in industrial control and smart grid systems not only require error free communication but also protection against damage that can be caused by transient events such as indirect lightning strikes and power surges.

Transient voltage suppressor (TVS) diodes can limit the peak voltage seen by the physical layer (PHY) device, but can they keep this voltage level low enough to protect the PHY when subjected to very high surge currents?

In addition, how can designers be confident that the protection design will meet their requirements in a production environment with respect to the variation of the clamp performance of the TVS diode as well as the variation in the sensitivity of the protected device? 

What is needed for a robust design is a protection circuit whose response resembled that of an ideal diode. What would make a notable and advantageous design would be if this robust protection circuit did not prevent the application from meeting the performance requirements of the overall design. 

Response of a Typical TVS Diode Compared to the Ideal Diode

The graph in Figure 1 compares the response of an actual 5V TVS diode to that of an ideal diode. The curve labeled “New Approach” will be addressed later in the article. 

As the current through the TVS diode increases the voltage across the diode increases as a function of the dynamic resistance of the device. The higher the diode current, the higher the clamp voltage and the more its response deviates from that of an ideal device. 

The voltage across the 5 V diode can become quite high, reaching 14 V at a current of 16 A. This can represent significant stress to a device operating from a 5 V or lower supply voltage. Satisfying the need for a design that more closely resembles the ideal diode is the Bourns® transient current suppressor (TCS) family of devices.


Figure 1: Actual vs. Ideal Diode Comparison

A New Approach to Circuit Protection

Transient current suppressor (TCS) technology is designed to be used in conjunction with an overvoltage protector (such as a TVS diode) and the ESD protection circuit of a typical semiconductor device.

When protecting, this device will limit current to a specific level that is almost independent of the voltage across the device. This new approach to circuit protection enables engineers to design a protection circuit that closely resembles that of an ideal diode.

Now that we the  performance level of the protection circuit design against high power surge events has been established, the next step is to review the actual performance of this new approach in a GbE application.

Lightning protection for GbE using TCS technology

The TCS device-based protection design for a GbE application was tested with a differential surge event per GR1089-CORE-ISSUE6 for a port type 4 application. This specification requires the design to protect against potential damage from an 800V/100A combination wave (1.2/50?s voltage, 8/20?s current waveform) surge.

The protection circuit evaluation board, which contains the Ethernet transformer, the TVS diode and the TCS™ device, was connected directly to the Ethernet PHY inputs of a production Ethernet router that was purchased at a local electronics store.

For a common mode surge event of 800V, the common mode rejection characteristics of the transformer will prevent any of the surge energy from transferring from the line side to the PHY side of the transformer. Therefore, the common mode test will not be covered in this article.

Under a differential surge event, the saturation characteristics of the Ethernet transformer provide some level of protection, typically reducing the peak current level by approximately a factor of 4 to 5 and also reducing the surge duration seen by the physical layer (PHY) device to less than 5?s. 

The waveforms shown in Figure 2 show the surge voltage (light and dark blue traces) and the current levels (green trace) that the PHY is subjected to during the surge test. Note that the peak current (magenta trace) flowing through the line side of the transformer is just over 100A.

After the initial ringing of the surge, the differential voltage across the PHY inputs (red trace) is clamped at 5.2 V. The current into the PHY is limited to approximately 275 mA after the initial peak of 571 mA.  An estimate of the energy that the PHY absorbed during this event is ~3 micro-joules (275mA x 5.2v x 2 ?s).

For comparison purposes, a design using only the 5 volt TVS diode connected across the line pair was also tested.

When the TCS device is used the voltage and current stress imposed on the PHY are reduced by more than 50% and 90% respectively.  The TCS design reduces the energy that must be absorbed by the PHY by almost 95%.
Less stress

The stress on the PHY was reduced by more than an order of magnitude compared to a TVS diode only design, yielding a protection solution that can be put into production with confidence.

The next question is: how does it affect high-speed transmission performance?

The signal template and amplitude tests per IEEE802.3 were performed on the circuit design as well as the design with the TCS device replaced with a short circuit. Both designs passed all the required template and amplitude tests. The amplitude results at points A and B of the test waveform (Figures 40-19 in the IEEE802.3-2008 specification) for both circuits are summarized and compared in Table 2. Note that the loss due to the series impedance of the TCS device is less than 0.2 dB.

This is equivalent to less than 1m of CAT5 cable.

The new approach for circuit protection design using TCS device technology allows the design of a circuit whose response closely resembles that of an ideal diode. 

Bourns conducted a thorough GbE application evaluation under lightning surge conditions, and found that the TCS device was able to reduce the stress on the protected device (the Ethernet PHY IC) by almost 95% while still meeting the demanding high-speed transmission performance requirements for this application.

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