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.