Application of Gas Discharge Tube in Surge Suppression Circuit

Abstract: The mechanism of surge voltage generation is explained, and the working principle, characteristic parameters and application of gas discharge tube in surge suppression circuit are introduced.

Keywords: surge voltage suppression; gas discharge tube; application

1. Generation and suppression principle of surge voltage

Electronic systems and network lines are often disturbed by transient overvoltages from the outside world. These interference sources mainly include: operating overvoltages caused by switching on and off inductive loads or starting and stopping high-power loads, line faults, etc.; lightning surges caused by natural phenomena such as lightning. This overvoltage (or overcurrent) is called surge voltage (or surge current), which is a transient interference. Surge voltage can seriously endanger the safe operation of electronic systems. Eliminating surge noise interference and preventing surge hazards have always been the core issues related to the safe and reliable operation of electronic equipment. In order to prevent surge voltage from damaging electronic equipment, shunt defense measures are generally adopted, that is, the surge voltage is short-circuited with the ground in a very short time, so that the surge current is shunted into the ground, so as to weaken and eliminate overvoltage and overcurrent, thereby protecting the safe operation of electronic equipment.

2. Classification of surge voltage suppression devices

Surge voltage suppression devices can basically be divided into two types. The first type is the crow bar device. Its main feature is that the residual voltage after the device breakdown is very low, which is not only conducive to the rapid discharge of surge voltage, but also greatly reduces power consumption. In addition, this type of device has a small leakage current and a small capacitance between the device electrodes, so it has little impact on the line. Commonly used crow bar devices include gas discharge tubes, air gap surge protectors, silicon bidirectional symmetrical switches (CSSPD), etc.

The other type is a clamping protector, which means that after the protection device breaks down, the voltage across its two ends is maintained at the breakdown voltage and does not rise any more, thus playing a protective role in a clamping manner. Commonly used clamping protectors are zinc oxide varistors (MOVs), transient voltage suppressors (TVSs), etc.

3. Structure and basic principle of gas discharge tube

The gas discharge tube is sealed with ceramics, and consists of two or more metal electrodes with gaps, filled with inert gas (argon or neon). The basic shape is shown in Figure 1. When the voltage applied to the two electrode ends reaches the point where the gas in the gas discharge tube breaks down, the gas discharge tube begins to discharge, and changes from high resistance to low resistance, so that the voltage across the electrodes does not exceed the breakdown voltage.

(a) BB type (b) BBS type

Figure 1 Basic appearance of a gas discharge tube

4. Parameter comparison between gas discharge tube and other surge suppression devices

1) Arc chopping

It is two electrodes shaped like horns, with a short distance between them. When the potential difference between the two electrodes reaches a certain level, the gap is broken down and sparks are discharged, thereby releasing the overcurrent into the ground.

Advantages: strong discharge capacity, large current capacity (can reach more than 100kA), small leakage current;

Disadvantages: high residual voltage (2-4kV), slow response time (≤100ns), and follow-up current (continuous current).

2) Metal oxside varistor

At a certain temperature, the conductivity of this device increases sharply with the increase of voltage. It is a metal oxide semiconductor nonlinear resistor with zinc oxide as the main component. It is in a high resistance state when there is no overvoltage. Once there is an overvoltage, the voltage is immediately limited to a certain value, and its impedance suddenly changes to a low value.

Advantages: large current capacity, low residual pressure, fast response time (≤50ns), no follow-up current (following current);

Disadvantages: large leakage current and relatively fast aging speed.

3) Transient voltage suppressor

Also known as Zener diode, it is a device specially used to suppress overvoltage. Its core part is a PN junction with a large cross-sectional area. When the PN junction works in an avalanche state, it has a strong pulse absorption capability.

Advantages: low residual voltage, high action accuracy, fast response time (<1ns), no follow-up current (freewheeling);

Disadvantages: poor current carrying capacity and small current capacity, generally only a few hundred amperes.

4) Gas discharge tube

Gas discharge tubes can be used for surge protection in data lines, cable TV, AC power supplies, telephone systems, etc. The general device voltage range is from 75 to 10,000V, with a peak impact current of 20,000A, and can withstand discharges of up to several thousand joules.

Advantages: large flow capacity, high insulation resistance, and small leakage current;

Disadvantages: high residual voltage, slow response time (≤100ns), low action voltage accuracy, and follow-up current (continuous current).

The common feature of various surge suppression devices is that they all present high impedance below the threshold voltage. Once the threshold voltage is exceeded, the impedance drops sharply, and they all have a certain inhibitory effect on the peak voltage. However, each has its own disadvantages. Therefore, according to the specific application, one or a combination of the above devices is generally used to form a corresponding protection circuit. The parameter comparison of various surge suppression devices is listed in Table 1.

Table 1 Comparison of parameters of several commonly used surge suppressors