Introducing a Two-Stage Approach
Complementing a TVS diode with a device that suppresses transient current can improve the level of protection on a signal line significantly. Taking this two-stage approach addresses both the voltage and current components of a surge without interference.
While the TVS remains responsible for voltage clamping, TCS technology provides a new and unique means to limit transient current. During regular operation, a TCS device acts like a conventional, linear low-value resistor in series with the line. There is no physical connection to ground and thus no internal parasitic capacitance. As with a resistor, the only significant capacitance in a TCS device is that between the physical package and its surroundings. When circuit board layout guidelines are followed properly, the capacitive loading effects are negligible for data rates well into the gigahertz (GHz) region. This makes TCS technology ideal for enhancing the protection of very high speed data buses like those in GbE applications.
The TCS device drastically reduces stress by adding a current limiting stage in series with the protected device to complement the characteristics of the voltage limiting device. When a surge occurs, the voltage at the interface increases, causing an increase in current to flow through the TCS device. As the current limit is reached, the TCS device prevents a further increase in current beyond its rated limits by allowing the voltage across itself to increase, effectively presenting a very high resistance. With the current limited to a constant level, the voltage at the protected device no longer rises, and the voltage stays below a safe operating limit. On the other side of the TCS device, the voltage continues to rise until the activation level of the voltage clamping device is reached. In this way, the two stage TVS and TCS protection system emulates the "brick wall" clamping effect shown earlier in figure 1. Table 1 shows how the TVS diode and TCS device characteristics are complementary.
Testing for GbE Protection
As with many other communications interfaces, specific signal performance characteristics for a GbE signals are associated with a standard.
IEEE Standard 802.3-2008 (Rev. 26 Dec. 2008) is the industry standard for GbE.
Signal template and amplitude tests from this standard pertain to the effect of using the proposed two-stage solution.
Bourns performed these tests per IEEE802.3-2008 on the circuit topologies shown in Figure 2 and Figure 3 in order to compare the respective single-stage and two-stage protection circuits. The tests proved that both GbE circuit protection designs met the signal template and amplitude requirements of IEEE802.3-2008.
The Bourns test also showed that the addition of TCS technology to the circuit protection design had minimal impact on the quality of the test signal compared to the diode-only design.
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 µs (voltage), 8/20 µs (current) combination wave test; 10/700 µs voltage waveform test; 2/10 µs 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/µs 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 behavior 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 overvoltage 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 minimizes 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. For more information about circuit protection solutions from Bourns, visit www.bourns.com.
About the Author
Andy Morrish is the CTO of Semiconductor Products at Bourns, Inc.