Limitations of Traditional Circuit Protection for High Energy Surge Threats

In the quest for higher speed and lower costs, today’s modern electronics use increasingly dense integrated circuit technologies, whose sensitivity to surges creates difficult challenges for designers. Interfaces of equipment can be exposed to a wide range of dangerous surges, including Electrostatic Discharge (ESD) and lightning. Even with ESD immune optical fibers transmitting data long distances, connection from the optical-electrical interface equipment to offices and residential buildings is still mostly through external conventional electrical cabling, exposing this equipment to high energy electrical surges. As technology advances, older circuit protection solutions for the prevention of high-energy surge damage become less effective.

Unlike conventional TVS diode protection, a new solution improves the robustness without the expense of adding capacitive devices that can rob bandwidth and reduce data rate performance. Lower voltage drivers can safely use very low capacitance, higher voltage clamping methods while still achieving protection that is far superior to even multiple stage TVS diode circuits. There are additional advantages of this new two-stage protection scheme.

TVS Diode Basics

To understand the benefits of the new two-stage protection, we will first review TVS diodes that absorb ESD energy at the interface of a design. Fig. 1 shows the basic operation of a TVS diode. Typically, the TVS diode appears as high impedance in the normal range of the signal working voltage that passes over the line. When a surge at the interface exceeds a preset limit, the TVS diode becomes conductive, quickly limiting the voltage from rising above this safe level. The ideal TVS diode in essence, has a “brick wall” clamping electrical characteristic that causes no interference to the normal signal, yet prevents the voltage at the interface from reaching an unsafe level.

A TVS diode supports current flow in the form of avalanche, zener or punch-through breakdown, depending upon the construction of the device. This differs from the abruptly vertical characteristic of the ideal clamping device when we compare it to a typical semiconductor TVS diode. Although the real TVS diode seems highly resistant as the voltage increases, a finite “leakage” current begins to flow near the breakdown voltage of the junction. The larger the junction, the more leakage can occur which becomes troublesome because the current may negatively impact the operation of the circuit, particularly at higher temperatures.

The TVS diode’s actual clamping characteristic is much softer than the abrupt vertical characteristic of the ideal clamp. The current increases gradually as clamping voltage is reached, avoiding the abrupt right angled characteristic. To ensure that the onset of clamping does not interfere with the signal, the clamping voltage must make allowances for this soft transition into clamping behavior, and the onset of clamping must be set higher than the ideal characteristic.

As the current level continues to rise, a significant internal resistance of the TVS diode causes a distinct gradient in the voltage increase. As ESD devices may conduct tens or even hundreds of amps for short durations, the actual peak voltage that may be seen across the device, and therefore across the line, will be significantly higher than the onset of breakdown. The internal resistance is inversely proportional to the junction area. Therefore, achieving acceptably low clamping voltage at high levels of current may require a very large junction, which greatly impacts capacitance, cost and package size.

Ultimately, simply connecting a TVS diode between an interface and ground is ineffective in protecting the device driving that interface. The device will experience the peak voltage developed across the TVS diode, and the voltage may reach 20 V during a discharge of 11 A. This is considerably beyond the capability of many low voltage technologies. Instead of effectively shielding the interface device from the surge, the TVS diode simply diverts a portion of the energy away. This exposes the device to high voltages and currents, which are often termed “let-through energy.” A potentially high level of let-through energy can be a major problem under surge conditions when using conventional TVS diodes for protecting sensitive electronics in harsh environments.

Transient Current Suppressor

The growth in high-speed, low voltage applications that must withstand severe levels of lightning surge and ESD invites an alternative approach to the TVS diode and its inherent problems in achieving ideal characteristics. The basic limitations of the TVS diode stem from it being a single-stage protection device. Even the best voltage limiting device does not prevent current flow into the protected device, and it often cannot withstand the levels of current flow during the time when the clamping voltage is at its peak. The Transient Current Suppressor (TCS™) is a new device that significantly improves the level of protection when used in series with the protected signal line in a two-stage configuration together with a voltage limiting device. Fig. 2 shows the actual detailed characteristics of a TCS.

Under normal operation, and when normal signal current is low, the TCS behaves like a low value resistor. Under surge conditions, when the current is driven above a certain limit, the TCS transitions very quickly into a current limiting state. Fig. 3 shows the TCS configuration and circuit symbol.

The TCS adds a current limiting stage in series with the protected device to complement the characteristics of the voltage limiting device, thereby drastically reducing stress. The voltage at the interface increases when a surge occurs, causing current to flow through the TCS. As the current limit is reached, the TCS prevents further increase in current within its rated limits by allowing the voltage across itself to increase. This effectively presents a very high resistance. Current is thus limited to a constant level, voltage at the protected device no longer rises, and it is kept at a safe level. On the other side of the TCS, the voltage continues to rise until it reaches the activation voltage of the voltage clamping device.

When used with a TCS, 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 (resistive) than a single-stage TVS diode would need to be. The clamp voltage can continue to rise at this stage. However, the maximum limitation of the differential voltage developed across the TCS must stay within its 40 V breakdown voltage limit, a condition that is easier to achieve than using most other forms of voltage clamp.

The Transient Current Suppressor is directly analogous to the TVS diode. The TVS diode limits transient voltages, and the TCS limits transient currents. The characteristics of the TCS, such as the punch-through TVS diode, also exhibit a degree of fold-back, whereby the current drops approximately 30 % from its maximum value as the voltage increases further. This fold-back improves transient power handling in the TCS and minimizes stress in the protected device.

Like a conventional linear low value resistor in series with the line, the TCS operates without physical connection to ground so that internal parasitic capacitance will occur. As with a resistor, the only significant capacitance is due to the capacitance between the body of the TCS and its surroundings. To achieve minimum capacitance, layout must be carefully planned to avoid electrical traces or planes running under the packaged device. In particular, ground and voltage planes should have windows cut out directly beneath the device pads. When designed this way, the capacitive loading effects of the TCS are negligible, even well up into the GHz region. Hence, the TCS is ideal for enhancing the protection of very high-speed data buses.

The speed of the TCS current limiting operation is ideal for protecting against standard lightning surge test waveforms (1.2/50 ms, 10/1000 ms, etc.). Normal response time to achieve limiting operation is less than 50 ns. Occasionally, a very fast rising transition from zero current at rates greater than 5 kV/ms may cause the current to momentarily overshoot the nominal current limit by a small amount. However, the amount of additional let-through energy during this interval is negligible, and there is no impact on the high level of protection afforded by the TCS.

A dual channel TCS contains two well-matched series resistance in one package. For example, the TCS Dual Channel 40 V, 750 mA device with a nominal resistance of 1 W is matched to just within 20 mW. You can connect TCSs in parallel to give even lower resistance, similar to resistors. You can parallel the individual transient current suppressors in a dual TCS to act as a single device with half the resistance and twice the current limit. If two matching, parallel connected TCSs are required, two dual devices can be used, with one TCS from each package connected in parallel to one TCS from the other package. Behavior of the TCS when used with a voltage clamping device is a near perfect match to the ideal “brick wall” clamping electrical characteristic -- even when used with a very basic voltage clamping device, such as a simple, relatively high resistance signal diode clamp.

VDSL Application

The significant effectiveness of the TCS is made clear when comparing its performance side-by-side with a single-stage TVS diode. Fig. 4 shows a basic test circuit representing the VDSL design, using a single conventional TVS diode. In most designs, a single TVS diode will not protect the driver or additional clamping devices, such as TVS diodes or Metal Oxide Varistors (MOVs), used across the transformer on the line side, or across the driver output. This significantly impacts the VDSL performance. MOVs, for example, typically have capacitance close to 100 pF. However, for direct comparison purposes, this simplified test circuit will demonstrate the effectiveness of the TCS protection system.

When lightning surge transient voltages occur on the line, DSL circuits are often exposed to high levels of stress. C1 and C2 block DC bias voltages that may be present on POTS (Plain Old Telephone System) lines. Two capacitors were used in this test circuit in order to withstand the maximum voltage seen during surge. Often, a single higher voltage capacitor is used in the center tap instead; this does not affect the protection performance of the circuit. The surge causes the line side capacitors to charge, following the surge voltage as it rises to the point that the Gas Discharge Tube (GDT) fires. When this occurs, the GDT appears as a switch that has been suddenly closed, and the charged capacitors are switched instantly and directly across the line side winding of the transformer. The capacitor voltage, which is charged by the surge up to 500-1000 V, is then coupled across to the driver side winding. Very high discharge currents flow through the line side winding as the capacitors rapidly discharge through the GDT. As shown in Fig. 5, this induces current flow in the driver side, equal to that in the line side multiplied by the turns ratio of the transformer.

VDSL transformers turns ratio usually ranges from 1:1 up to 1:4.5, depending upon the driver type. In the test circuit, a 1:1.4 ratio was used, which is a typical value for VDSL circuits. In the VDSL circuit, the driver is typically configured for active termination such that its output acts as virtual line termination. A combination of current and voltage feedback is used, so that the driver output impedance matches a significant portion of the reflected line impedance, as transformed through the driver transformer. With a ratio of 1:1.1, the 100 Ω line impedance transforms to the driver side by 1/n2 where n is the turns ratio. In this case, the total equivalent resistance in the driver circuit including the R1 and R2 must equal 100/1.21 = 82.6 Ω. In addition, at 2.3 Ω, the nominal resistance of even the higher resistance TCS is very small in comparison to the driver side impedance, and it has negligible effect on the VDSL signal.

R1 and R2 contribute to the protection by limiting some current between the voltage developed across the TVS diode and the output of the driver. However, their values must be kept relatively low compared to the line termination resistance, so that the amount of signal voltage dropped across them during normal operation is limited. Values typically range from 1 to 5 Ω. The lower value was used in this test. To demonstrate the TCS effectiveness, the 1 Ω resistors were replaced by a dual TCS with a nominal resistance of 1 Ω and a nominal current limit of 1.125 A, as shown in Fig. 6. The TVS diode was replaced by a simple diode clamp using generic diodes clamping to a generic 12 V zener (e.g. BZT52C12). This provided the necessary first stage of voltage clamping. This configuration forms an extremely low cost and very low capacitance clamp. Bias resistors of 10 kΩ between the zener and the supply rails can be used to minimize diode capacitance effects.

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