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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?

Adequate circuit protection is important to maintaining and increasing the lifespan of electronics, and especially for high-speed gigabit Ethernet (GbE)-based applications that in many deployments face additional challenges of higher temperatures, continuous operation and compact design. It often is assumed that a low voltage transient voltage suppression (TVS) diode solution will provide sufficient protection; however, even the most complex and expensive diodes have limited effectiveness as a single-stage solution. Regardless of their voltage rating, diodes can expose electronics to excessive voltage levels at the time of the surge.

Diodes are also seen as a costly solution, and present challenging trade-offs when designers must select between features such as optimal size or higher performance. A cost-effective solution is now available that complements a TVS diode with new transient current suppressor (TCS) technology. TCS devices placed in series with GbE electronics will limit the current before a dangerous level is reached. Once the voltage protection device activates, surge voltage is diverted to ground to ensure that the voltage and current remain within safe levels and the electronics are protected.

This article will examine how a combined TVS and TCS solution provides a level of protection similar to the ideal "brick wall" clamp. It will also discuss how optimal and affordable circuit protection for sensitive GbE-based equipment is now possible with the combined benefits of TVS and TCS devices.

Figure 1: Ideal clamp vs tested TVS device.

Protection needs
Due to a wide range of discharge events up to and including surges caused by lightning, equipment interfaces must include circuit protection. Even with ESD-immune optical fibres, transmitting data over long distances uses conventional electrical cabling that forms the interface at either end. Surge protection devices may conduct tens or even hundreds of amps for short durations. The resulting actual peak voltage across the device, and therefore across the line, can be significantly higher than at the onset of breakdown. The circuit is then exposed to the remaining 'let-through' energy.

The goal of the circuit protection design is to imitate an ideal clamp. Such a solution would be implemented with a device that causes no interference to the signal under normal conditions yet prevents the voltage at the interface from ever going beyond a level that may be dangerous to the equipment. It would limit the voltage level instantly. The actual clamping characteristic of a TVS diode is much softer than the abrupt vertical characteristic of the ideal clamp. Instead, the current begins to increase gradually. Significant internal resistance in the TVS diode causes a distinct gradient of voltage increase with current. In order for the clamping not to interfere with the signal, the clamping voltage must make allowances for this soft transition into the clamping behaviour. Thus the onset of the clamping must be set relatively high compared to the ideal characteristic. The ideal clamp and that of a single-stage TVS diode protection solution are shown in figure 1.

TVS diode performance characteristics
TVS development has centred on optimising the device's ability to clamp well without overly degrading signal performance. Since internal resistance is inversely proportional to the junction area, achieving acceptably low peak clamping voltage at high levels of current may require a very large junction. Adequate clamping performance comes with the expense of higher capacitance and leakage effects, and a significant increase in manufacturing cost. Capacitance can limit the data rates of the circuit, and non-linear junction capacitance can cause distortion and noise that also would impact data rates achievable over long distances. Realising the appropriate trade-off while remaining cost-effective is becoming increasingly difficult for higher bandwidth, lower voltage systems.

In an effort to accommodate the lower voltages of current electronic devices, manufacturers have explored alternative construction and design methods. Their offerings have been expanded to include lower voltage TVS diodes. Designers often push for lower voltage TVS diodes to follow driver technology voltage without realising that the TVS voltage may rise to 15-20 V during the peak current of the surge, regardless of the diode's low voltage rating. The problems inherent in achieving ideal TVS characteristics, the growth in high-speed, low voltage applications, and the severe levels of lightning surge and ESD that electronics must withstand are some of the factors that combine to invite a new approach to circuit protection.

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.

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