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Implement two-stage protection in GbE applications

Posted: 20 Mar 2013 ?? ?Print Version ?Bookmark and Share

Keywords:gigabit Ethernet? transient voltage suppression? Ethernet PHYs?

Low voltage GbE communication circuits may also benefit from protection offered by the low resistance and fast response provided by current limiting devices with TCS technology. The typical response time of the TCS current limiting operation is less than 50 ns, which meets the parameters for protecting against standard surge test waveforms including: 1.2/50?s (voltage), 8/20?s (current) combination wave test; 10/700?s voltage waveform test; 2/10?s voltage waveform test.

Bourns also executed a series of surge tests with these test waveforms on the circuit in figure 3. A very fast rising transition from zero current at rates greater than 5 kV/?s may cause the current to overshoot the nominal current limit by a small amount for a very short interval with negligible let-through energy. The circuit in figure 3 performed well under each of these tests with two different TVS devices, providing further merit to the two-stage solution.

The current suppression has a fold-back characteristic, reverting to a low current level after the device is triggered and then slowly increasing as the voltage across the device rises. This characteristic of how the current through the device increases as the voltage across it rises is analogous to how the clamp voltage of a TVS diode increases as the current through the device increases. In a GbE application, the voltage across the current suppressor would be in the range of 10 to 20 V depending on the TVS device used. Implementing this solution, the increase in the current through the TCS device was determined to be less than 50 mA above the fold-back current level when the voltage across the TCS device measures 20 V. The first stage clamp voltage level no longer needs to be critically chosen to match the protected device, and its clamping characteristic may be much softer than a single stage TVS diode would need to be. The voltage across the clamp voltage can continue to rise, and the differential voltage across the TCS becomes the maximal limitation of the design. It must stay within the 40 V breakdown voltage limit of the TCS device, which is many times greater than the PHY voltage. It is much easier to implement this protection solution, and many forms of a voltage clamp can be used.

When even a simple, relatively high resistance signal diode is chosen as the voltage clamp, the composite behaviour of current suppression with voltage clamping closely matches that of the ideal "brick wall" the TVS diode could not achieve independently. The surge tests conducted by Bourns indicated that the two-stage GbE protection circuits reduced the energy that the Ethernet PHY must absorb by more than 90% during a surge transient compared to the single-stage TVS diode design. The voltage at the inputs to the PHY is determined by its response to the limited current that passes through the TCS device. It remained below 6 V with the two-stage circuit under test. At this voltage the current remained below 300 mA as well. This is significantly less than the 3.2 A to 4.4 A the PHY must sink or source in a single-stage TVS diode design.

Benefits of a two-stage solution
It is apparent that simply using a TVS diode to protect an interface may not be effective in protecting the driving device, especially in low voltage and high frequency GbE-based applications. The limitations of a TVS diode-only solution stem from its single-stage protection functionality. Even the best voltage limiting device does not prevent current flow into the protected device, which often cannot withstand the levels of current present while the clamping voltage is at its peak.

Conversely, a two-stage protection design that employs TCS technology allows the use of a less rigid clamp device without compromising the level of protection provided. As demonstrated in the GbE application surge testing performed by Bourns, pairing the devices creates what essentially is equivalent to an ideal brick wall protection solution. When coordinated with appropriate over-voltage protection and the ESD structure of the protected device, TCS technology significantly enhances the protection level achievable by a TVS diode alone.

As circuit technology advances, older protection solutions are becoming less effective in preventing high energy surge damage and TCS technology developed by Bourns provides a viable alternative. The dramatic reduction in stress on the PHY helps provide an exceptionally reliable design that minimises design concerns regarding the variation in robustness of Ethernet PHYs in a production environment. Combining the advantages of voltage and current suppression devices in a two-stage circuit protection solution makes possible a high performance and cost-effective solution to the transient electrical surge problems that continue to challenge today's more complex applications.

About the author
Andy Morrish is the Chief Technology Officer of Semiconductor Products at Bourns Inc.

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