What is Gas Discharge Tube？
What is a Gas Discharge Tube?The Gas Discharge Tube (GDT) is a type of ceramic gas discharge tube commonly used to safeguard electronic devices from transient voltage damage. A GDT is a product filled with inert gas within a sealed ceramic or glass tube that features an insulating gap.It is widely used for lightning surge protection in devices such as fax machines, network modems, mobile base stations, and cable TV cables.
Operation Principle of Gas Discharge Ttube
A gas discharge tube consists of hot-cathode gas tube and cold-cathode gas tube separated by a certain distance within a ceramic tube filled with an inert gas pressure. Its electrical performance primarily depends on the type of gas, the gas pressure, and the electrode spacing. The gas used is mainly neon or argon, maintained at a specific pressure, and the square electrode surfaces are coated with an emissive material to reduce the electron production energy losses. These measures allow the operating minimum voltage to be adjusted (generally from 70 volts to several thousand volts) and kept within a defined error range. When the broad voltage spectrum across its terminals is below the discharge voltage distribution, the gas discharge tube acts as an insulator, with a resistance greater than 100 megohms.
When the broad voltage spectrum at both ends rises above the discharge voltage distribution, a non-uniform electric field is generated between the electrode material. Under the influence of this electric field, the gas pressure inside the tube begins to ionize. As the applied voltage increases to a level where the field strength between the common electrodes exceeds the insulating strength of the gas, the gap between the electrodes undergoes dielectric breakdown, transitioning from its original insulating state to a conductive state. Once conduction occurs, the voltage between the electrodes of the discharge tube maintains itself at the residual voltage level determined by the discharge arc, which is generally very low, dropping by approximately several tens of volts. When subjected to transient high-energy impacts, the gas discharge tube can convert the high insulation impedance between its electrodes to low insulation impedance within the order of 10^-6 seconds, allowing surge currents of up to several tens of kiloamperes to pass through.
Protective Characteristics of gas discharge tubes
The protective characteristics of a GDT can be explained using its current-voltage characteristic curve. Figure 1 shows the characteristic curve of a two-electrode GDT, symmetrical around the origin, with parameters indicated only in the first quadrant. If a DC voltage is applied to the discharge tube and it exceeds VSDC, the tube components will break down and conduct; for AC voltage, breakdown occurs when the ignition voltage exceeds VS. During the arc discharge phase, the tube's terminal voltage VA changes little with current variation. When the current decreases and turns off, there is a maintaining electrical discharge voltage VH that must not be exceeded; otherwise, the tube components will remain in an arc voltage or glow voltage, failing to turn off completely, which might damage it. Thus, in systems protected by the tube, the working DC supply voltage should be below VH, or the AC voltage waveform below the tube's VH to allow automatic recovery to an off state.
Interpretation of Gas Discharge Tube Symbols
The electrical characteristics of a GDT include several parameters important for ordinary applications:
1.DC Breakdown Voltage: This is the ignition voltage at which a electric discharge begins under a voltage distribution rate lower than 100V/s. Due to the dispersive nature of gas pressure around the nominal average value, manufacturers also provide upper and lower limits for the breakdown voltage.
2.Impulse Breakdown Voltage: Defined as the voltage distribution under specified transient voltage pulse rise rates. The impulse breakdown voltage varies with different rise rates, with manufacturers typically providing values for 100V/μs and 1kV/μs. Some may list values for higher rise rates like 5-kV range or 10kV/μs for reference.
3.Nominal Impulse Discharge Current: The maximum current peak at which the tube's DC breakdown voltage and insulation resistance remain unchanged after a specified waveform and number of pulse currents. Manufacturers often specify this under an 8/20μs impulse current waveform, flowing 10 times, or 10/1000μs waveform flowing 300 times.
4.Maximum Single Impulse Discharge Current: The highest discharge current value for a single 8/20μs waveform impulse.
5.Power Frequency Discharge Current: The maximum current at which the tube's DC breakdown voltage and insulation resistance show no significant change after five power frequency currents. It is crucial where the tube diameter is used in AC supply lines or communication lines prone to induction effects from power lines.
6.Insulation Resistance: Usually starting at several GΩ, this parameter decreases over repeated use, potentially increasing leakage current and noise interference during normal operation.
7.Capacitance Between Common Electrodes: A small parasitic capacitance exists between the metal electrodes, typically 1-5pF, stable across a wide range of frequency , with minimal variance among identical model tube lifetime. Excessive capacitance can negatively impact high-speed signal transmission, deteriorating waveforms.
Gas Discharge Tube Operating Temperature and Influencing Factors
The ambient temperature range of a Gas Discharge Tube (GDT) varies depending on manufacturing technology and design parameters. Typically, the rated operating gas temperature range is between -40°C and +85°C. However, certain models of GDTs can function effectively over a broader gas temperature range, such as from -55°C to +125°C. In practical applications, to ensure the longevity and reliability of a GDT, it is advisable to maintain its operating temperature between -40°C and +70°C. Under normal conditions, the gas temperature generally does not significantly affect the operation of a gas discharge tube, as its discharge phenomenon is primarily related to gas pressure and structure. Nonetheless, in extreme high or low temperature environments, the performance of a gas discharge tube may alter. Therefore, it is crucial to select an appropriate model and parameters based on the actual environment, taking into account factors like gas temperature and humidity. In standard temperature conditions, it is recommended to determine the performance of the gas discharge tube through testing and experimentation to ensure its effectiveness and reliability.
The standard temperature range of a Gas Discharge Tube (GDT) can be adjusted across a broad spectrum, influenced by several factors:
1.Materials of the GDT: Typically made from materials like glass or ceramic, the thermal expansion coefficients of these materials change with absolute temperature fluctuations. Ceramic boasts higher heat resistance, allowing it to withstand elevated temperatures, making it ideal for high-temperature environments. For applications requiring functionality at 125°C, opting for a ceramic-based GDT is advisable. Conversely, in conditions as low as -55°C, ceramics offer better thermal expansion coefficient compatibility compared to glass.
2.Type and Pressure of Gas in the GDT: Different gases exhibit various type of discharge characteristics under different pressures. Generally, inert gases like helium provide superior discharge performance in high-temperature settings, enhancing the stability and reliability of the GDT. For low-temperature applications, it is recommended to fill the GDT with standard gases such as nitrogen. Additionally, as gas pressure can vary in low-temperature scenarios, it's crucial to account for the impact of gas pressure under cold conditions during GDT design.
3.Design Parameters of the GDT: These include control voltage, threshold current, and discharge power. In high-temperature environments, these parameters are subject to change with rising temperatures, necessitating adjustments and compensations to ensure proper GDT operation. When using a GDT in low-temperature conditions, appropriate parameter selection and design are essential.
4.Protection Circuit of the GDT: In cold environments, the electrical properties of a GDT may fluctuate, thus requiring the design of a suitable protection circuit component to guarantee its safe and reliable operation of equipment.
In summary, for a GDT to function correctly, one must select materials, gases, and design parameters suited to both high and low environments while making necessary adjustments and optimizations. Moreover, rigorous testing and validation are imperative in real-world applications to ensure the GDT can operate stably over long periods in varying temperature conditions.
Broad Selection of a Gas Discharge Tube
GDTs are generally used for protection in power and signal interfaces. Key considerations include working voltage drop, protection level, withstand control voltage testing, and signal rate.
● Determine the circuit's working voltage: The DC breakdown voltage of the GDT should exceed the maximum circuit component working value, generally 1.5-2 times the circuit's working control voltage, or 1.4 times the AC supply voltage.
● Determine current capacity per the circuit's protection level: Protection devices should handle at least twice the required current capacity, related to test voltage drop and waveform.
● Identify the device's maximum junction capacitance: For signal interface protection, select devices with minimal junction capacitance for high-speed lines.
● In products undergoing withstand amperes at voltage drop, ensure the GDT’s DC breakdown exceeds the test voltage.
Additionally, note that the follow-on voltage of a GDT is typically tens of volts. If used independently across power source terminals, when the working voltage exceeds this value, the follow-on effect may cause continuous conduction, leading to a short circuit. Therefore, they should be paired with varistors.
Common Applications of Gas Discharge Tubes
Application 1: Basic Lightning Protection for Power and Communication Interfaces
Gas discharge tubes are often used on circuit boards of electrical power modules and external signal ports, particularly in outdoor electronic equipment. Currently, typical built-in lightning protection designs for electrical power modules require up to 5kA, while signal ports such as dry contacts and RS485 interfaces have lightning protection requirements ranging from 2kA to 5kA. Gas tubes are commonly employed for common-mode protection of power source supplies and both common and differential mode protection of signal lines.
Application 2: Combination with Varistors to Form Suppression Circuits
In surge suppression circuits formed by combining gas tubes and varistors, a critical flaw of varistors is their unstable region, which can cause poor performance. Over time, this leakage current increase can lead to overheating and self-destruction. Additionally, the arc voltage continuation phenomenon associated with gas-filled tubes necessitates their series connection with varistors to resolve this issue. However, this introduces a drawback, as the response time becomes the sum of the response times of each electronic component. For instance, if a varistor has a response time of 25ns and a gas-filled tubes 100ns, then the response time of the R2/G/R3 configuration diagrammed previously would be 150ns. To improve the response time, an R1 varistor can be added, achieving a faster response time of 25ns.
Application 3: Use in Integrated Surge Protection Systems
Surge protection systems typically consist of multiple stages, utilizing the characteristics of various surge suppression devices to achieve reliable protection. Gas discharge tubes are generally placed at the line input end, serving as the primary surge protection device to withstand large surge currents. Secondary protection devices use varistors, which respond more quickly within the microsecond range. For highly sensitive electronic circuits, a more refined tertiary protection device like TVS (Transient Voltage Suppressor) can be employed, responding to surge gas discharge tubes voltages in the picosecond range. When surge gas discharge tubes such as ambient light emission, the TVS activates first, precisely controlling momentary overvoltage to a certain level. If the surge current is substantial, the varistor is triggered to dissipate a portion of the surge current. The broad voltage spectrum across the terminals rises until it triggers the type of discharge of the upstream gas discharge tube, diverting the large current to the ground. However, this approach presents a challenge, as activating the TVS first may harm downstream circuits. Therefore, after primary protection, an air-core common mode choke should be connected in series to ensure that the large current is first dissipated through the upstream protection before carrying out fine-tuned downstream protection.
