Select Optimal Protection For Your VDSL-Based Telecom Circuits

A variety of over-current and overvoltage products are on the market for protecting VDSL telecommunications circuits. These include transformer line-side capacitors, transformer driver-side diodes, thyristors, gas discharge tubes (GDTs), electronic current limiters (ECLs), and combinations of all of these devices.

To actively protect telecommunication systems, engineers need to understand the surges, the damage thresholds of VDSL drivers, and the performance of the various protection devices. Even with this understanding, though, solutions may not be effective unless the designer has good knowledge of the secondary effects that can arise when protection components interact with standard VDSL interface circuitry.

And while the various protection solutions offer their own pros and cons, only ECL transient blocking unit (TBU) primary- side protection can provide complete protection for wide-bandwidth VDSL.

SURGE THREATS
The level of potential damage a surge in VDSL systems can cause is the result of the surge threat, the method of protection, and the interaction of the protection circuit with the VDSL system. Figure 1 shows a standard DSL protection circuit, a basic design that has been commonplace for years.

This solution provides commonmode protection to the isolation rating of the transformer, typically 1.5 kV rms or 2.5 kV for impulse, and considerable transverse protection for low-frequency transients to the capacitors’ ac-blocking capabilities. Higher-frequency transverse surges transfer with modest efficiency to the secondary as the blocking capacitors charge, allowing current to flow in the primary. These secondary currents are typically less than 10 A and can be mopped up by a secondary clamping bridge to the power rails.

Figure 2 displays a typical method for enhancing surge-protection capability by placing a GDT in front of the standard protection. Some telephone operating companies require protection up to 6 kV, which is above the isolation rating of both the capacitors and transformer, to ensure reliability in some environments. The GDT can be placed across the line to enhance transverse surge handling capacity, or two GDTs can be used, one on each line to ground, to enhance both the transverse and longitudinal capacity of the circuit.

Placing a GDT, several GDTs, or other crowbar device on the line side of the standard protection circuit seems a simple, rugged solution. Theoretically, the system should block voltages up to the transformer’s and capacitors’ isolation ratings and shunt higher voltages to ground up to the lighting capacity of the GDT or crowbar device.

Upon careful analysis, however, the enhancement introduces a significant weakness. In the transverse surge protection, when isolation capacitors charge to a high enough level to operate the GDT, either the line-to-line or one of the lineto- ground GDTs will operate. This causes the blocking capacitor to discharge very quickly through the transformer primary winding. The result is a very high secondary current transient that, if sufficiently severe, could damage driver-side components, an event shown as the red current paths in Figure 2.

In such cases, driver-side surge doesn’t depend on the input surge as much as on the shunt device’s operating voltage. Secondary surge depends on the crowbar voltage of the shunt device, resistive/inductive impedance in the discharge loop, and transformer turns ratio. Although transverse surges aren’t common in twisted-pair systems, they can be serious in severe protection environments, especially when unbalanced primary protection or other unbalanced shunt protection is on the up-stream of the VDSL protection system.

A similar issue can occur on lines with unbalanced primary protection whether there are GDTs or crowbar devices within the protection circuit or not. The effect is similar to that described above. When the second half operates, any change on the blocking capacitor quickly discharges, causing a severe secondary surge.

Engineering effective VDSL protection that stands up against all transverse events depends upon addressing the limitations and weaknesses of the protection within the basic isolation capacitor and transformer. To effectively deal with these limitations, designers can use additional components such as thyristors, metal-oxide varistors (MOVs), or ECLs to limit energy on the line side. Another common option is to insert diodes, thyristors, or ECL-based protection on the driver side to divert coupled energy from the driver.

DAMAGE THRESHOLD OF VDSL DRIVERS
Recognizing the damage thresholds of VDSL drivers is essential for a complete understanding of the system’s protection requirements. The definition of damage is subtle, ranging from latent to catastrophic damage resulting in immediate failure.

Though each driver design is unique, it is generally true that damage to a driver will possibly occur if current greater than 3 to 5 A is present at driver outputs, voltage transients of 1 to 3 V above the supply rail appear on the output, or voltage transients greater than the absolute maximum driver supply occur on the supply rail.

LINE-SIDE PROTECTION
There are three line-side shunt protection options available for enhancing the ruggedness of the standard DSL protection circuit: pre-blocking capacitor shunt protection, post-blocking capacitor shunt protection, and line-side winding ECL protection.

Pre-blocking capacitor shunt protection employs a low-voltage thyristor across the input on the line side of the blocking capacitance (Fig. 3). For International Telecommunication Union (ITU) compliance, the shunt device must be able to handle all K-series transverse test surges and have an operating voltage higher than the power cross test levels. GR-1089 Level 2 compliance also requires line-side overcurrent protection.

This solution simply limits maximum capacitor charge voltage during a transverse surge. Since the turn-on transition time of a thyristor is slow and the conducting impedance high in comparison to a GDT, the peak current in the line-side winding falls even lower. However, secondary protection is necessary since significant current will continue to generate on the line side.

Due to the energy handling requirements of the thyristor in this exposed position, the secondary protection must be large with high capacitance. As a result, this solution generally isn’t an option on VDSL circuits with bandwidths greater than 17 MHz.

POST-BLOCKING CAPACITOR SHUNT PROTECTION
Two variants of post-blocking capacitor protection are in use today: thyristor and MOV based (Fig. 4). In either, protection of the line-side winding is via a shunt device with a low operating voltage, between 30 and 60 V, the exact level selected to prevent interference with the VDSL signal.

Regardless of which option we use, protection devices in wide-bandwidth VDSL designs require low capacitance, typically less than 20 pF, and very low changes in capacitance. As a result, devices with relatively low energy capacity are the only choice.

As low capacitance most often means small die size, and because the discharge current is so high when an upstream GDT operates and discharges the blocking capacitors, this option is only for protecting against normal transverse charging of the blocking capacitors and not fast discharge of the blocking capacitors by an upstream GDT.

The approach does not guarantee coordination between wide-bandwidth VDSL protection and GDT-based primary protection, though. This becomes problematic with some fast-switching GDTs that have a propensity to oscillate under certain transmission line situations.

LINE SIDE ECL PROTECTION
Line-side ECL protection is viable for protecting the line-side winding from excessive current, independently of the generation mechanism (Fig. 5). When line-side winding current reaches the ECL’s operating current, the ECL disconnects. A GDT keeps the voltage across the line-side winding and below the ECL breakdown limit.

The energy handling capability of the GDT within this circuit, coupled with its low capacitance, means this solution can protect against even the most severe GDT discharge of the blocking capacitors on VDSL bandwidth systems. Yet ECL protection introduces a small amount of resistance into the circuit that can affect transmit-power performance, the effects of which are removable with adjustments of the line termination resistance and the receive hybrid.

DRIVER-SIDE PROTECTOIN OPTIONS
Protection solutions finding employment on the driver side of the isolation transformer focus on preventing any coupled surge current from reaching the line driver. Driver-side protection is always necessary except in those situations when line-side ECL protection is on board. Driver-side protection options include clamp, crowbar, and blocking (ECL) protections.

Probably the simplest driver-side protection arrangement is a clamping-diode bridge, which can be seen in figure six. In this scenario, the diodes clamp the transformer’s driver side to the supply rail during a surge event. The termination resistances of R1’, R1”, R2’, and R2” will dampen the secondary current flow and reduce the higher current entering the line driver. Resulting from the diode’s nonideal impedances in the bridge circuit and the impedance of the decoupling circuit, some or all of the termination resistance is the sum of R1 and R2. Therefore, the total value of these resistors must equal this impedance.

Figure 6 shows in red the secondary surge current diverting to the supply rails and away from the line driver using the diode bridge. Typical clamp diodes that are suitable for deployment here include the 1N4148, MMBD2004, and SR70. These devices have a typical surge resistance of 0.65, 0.3, and 0.3 O and a maximum surge capability of approximately 20, 35, and 50 A, both respectively.

These components typically can handle such surges provided the duration is less than 1 µs. But during such events, a significant voltage develops across the diodes’ internal resistance. Depending upon the level of the surge, this voltage may be sufficient to stress the output stage of the driver, which in turn could result in a significant performance degradation or complete failure.

A thyristor-based version uses clamp diodes to protect the driver until the thyristor operates (Fig. 7). Once the thyristor operates, the voltage within the secondary diode is crowbarred, and all stress on the driver dissolves. To prevent the VDSL bandwidth from being restricted, the thyristor capacitance must be small, meaning this solution is workable up to secondary currents of around 50 A. For higher secondary current surges that result from primary- side GDT discharge, the maximum current can exceed the maximum rating of the thyristor.

As a third option, ECL protection involves replacing resistor R2 in Figure 6 either partially or completely with an ECL device (Fig. 8). This is necessary when a clamp or thyristor circuit is insufficient to keep voltage and current at safe levels. The ECL effectively disconnects the driver from the line while the clamp circuit is in operation, ensuring driver protection, despite relatively high voltage drops across the clamp. This allows the use of low-cost generic diodes in the clamp.

At this point, the ECL device, which necessarily resides in the driver feedback loop wherever configuration permits, adds a resistance in series with the driver output. Depending on the resistive budget, we may have to go in and reduce R1 to zero. For specific applications with low turns ratios, for example, the secondaryside ECL will most likely provide very effective protection and have negligible impact on circuit bandwidth.

CONCLUSION
For VDSL protection, designers will have a number of viable options available to them. After careful examination of each design option, the level of protection required by the specific standard, as well as testing real-world scenarios are all equally important for the appropriate selection of a circuit protection solution for a specific surge environment.

Based on these analyses, only one solution can provide complete protection for wide-bandwidth VDSL systems to the full range of expected telecommunication surges: ECL transient blocking unit (TBU) primary-side protection.