When using devices to suppress transient interference in the system, the prerequisites need to be clear about the use occasion and the test specifications. Continuing with the last push, first analyze the "GB/T 17626.5 Electromagnetic Compatibility Test and Measurement Technology Surge (Impact) Immunity Test". The specification stipulates that the voltage level, open circuit voltage and short circuit current test waveforms of different shapes are not tested:
For the port connected to the symmetrical communication line, a combination wave generator with an open-circuit voltage waveform of 10/700μs (wavefront time 10μs, half-peak time 700μs) is used. The corresponding short-circuit current waveform of this generator is 5/320μs.
For the ports connected to the power line and the short-distance signal interconnection line, a 1.2/50μs combined wave generator is used. The corresponding short-circuit current waveform of this generator is 8/20μs.
The two waveforms are not contradictory in themselves. The linearity of the value is due to the fact that the effective output impedance of the generator is fixed at 2×(1±10%)Ω, but the appearance of the two is mutually opposite. The determination of the output waveform of the surge generator during the test depends on the input impedance of the load device port. When the port of the device under test as a load presents high impedance (such as the port has no protection circuit, or the protection circuit is disconnected or not working), the judgment standard is the prescribed open-circuit voltage waveform; when the port of the device under test as a load presents low impedance (For example, when the protection circuit is short-circuited), the judgment standard is the specified short-circuit current waveform.
The peak current tolerance of the varistor is indeed very large, but its clamping effect is relatively poor, and the clamping voltage will be relatively high; at the same time, the heat caused by the clamping absorption will damage its own structure and the number of surge impacts. The increase in leakage current will increase.
The gas discharge tube is the representative of the "switch-type device" among the two major types of lightning surge suppression devices. The parasitic capacitance of the discharge tube itself is small, which can be regarded as a low-capacitance symmetrical switch. When the voltage across the discharge tube is higher than the breakdown voltage, the switch is opened to directly discharge the surge energy to the ground.
Gas discharge tube (GDT), also known as ceramic gas discharge tube, is a sealed gas discharge device composed of a metal motor and a ceramic shell. There are diodes and a tertiary tube, and it is generally used for overvoltage protection to the ground. Specifically, it uses metallized ceramic insulating shell and electric welding technology to seal a small amount of gas (mainly rare gas neon and argon) inside. By changing the internal gas pressure, the composition of the motor coating material and the electrode spacing, the DC breakdown voltage, impulse breakdown voltage, power frequency current capability and life of the GDT can be changed.
The working process of the gas discharge tube can be explained as: A. When the voltage applied to both ends of the gas discharge tube exceeds the breakdown voltage, the gas in the discharge tube is ionized and the discharge tube starts to discharge. The voltage at both ends of the discharge tube drops rapidly to the glow discharge voltage, and the current in the tube starts to rise (70~150V, related to the nature of the tube, the same below). B. The current in the tube further increases, and the rare gas inside the discharge tube enters the discharge state. At this time, the voltage across the tube is very low (10~35V), and this state will remain for a period of time. C. When the current flowing through the GDT drops to the voltage that maintains the discharge state (10~100mA), the discharge stops, the discharge ends, and the original voltage value is restored.
During the discharge process, the energy consumed by the rare gas discharge is very small, and the gas discharge tube itself consumes very little energy, and the energy is basically discharged to the ground. Because the gas discharge requires a process, the corresponding time of the gas discharge tube is relatively long, generally several hundred ns or even μs.
The working process of the gas discharge tube is: when the voltage exceeds the breakdown voltage, the gas discharge tube discharges and discharges the surge energy directly to the ground, and when the tube current is lower than the discharge current, the discharge ends. The following parameters are often considered for selection:
1) DC spark-overvoltage: The breakdown voltage measured when the system is working at a low rise rate (test waveform dv/dt≤100V/s) voltage waveform, generally within a voltage range. Represents the normal working range of the protected system.
2) Impulse (or surge) breakdown voltage (Impulsespark-over voltage): The breakdown voltage measured when the system is operating at a high rate of rise (1kV/μs, or with 100 V/μs, 5kV/μs) voltage waveform , Is generally a voltage range, which represents the breakdown voltage during general protection.
3) Nominal discharge current (Service life):
① Pass the rated effective value of 50Hz AC current. It is usually specified that the maximum current effective value can be passed through 10 discharges (each discharge time is 1s), and the maximum current effective value can be passed through a single use (discharge 0.18s, continuous 9 cycles).
②The rated discharge current of 8/20μs waveform is usually specified for a single use to pass the maximum discharge current value, and 10 discharges to pass the maximum current.
4) Insulation resistance (Insulationresistance): When the discharge tube is not on fire, the insulation resistance value of the discharge tube. Generally, 50V (DC) is used for the test of 90V and 150V discharge tubes; the test voltage of other specifications is 100V (DC). The insulation resistance is required to be 1~10GΩ.
5) Capacitance: The capacitance between the electrodes of the discharge tube is generally 2~10pF.
As a transient interference suppression protection device, the gas discharge tube selection must also ensure that the access circuit can clamp the surge voltage, and it must also ensure that it does not affect the normal working process of the circuit. Generally speaking, there are several points:
1) The normal working voltage of the protected electronic equipment must be lower than the minimum value of the DC breakdown voltage of the gas discharge tube, and there is a certain margin.
2) The gas discharge tube has strong absorption capacity, but the absorption speed is very low (0.1~0.3μs), which is suitable for the first primary absorption of large surge energy, or used with varistor.
3) The gas discharge tube cannot extinguish the arc immediately after the surge overvoltage has passed, especially when the protection circuit is powered by a low-impedance power source (such as a 50Hz AC power grid), because the discharge tube is also low-resistance after the arc is started. The partial pressure is uniform, the arc extinguishing cannot be realized immediately, and the actual flowing current may exceed the limit many times. If the freewheeling time is too long, it is prone to overload and burst. For the high impedance of DC power supply, there is no such problem. Therefore, the impedance characteristics of the port network should be analyzed when using it.
4) When soldering the moving pins, you should also fix the pins first to prevent damage to the internal structure of the GDT during the operation.
5) The length of the electrode lead also has a great influence on the voltage limit. Due to the parasitic inductance and resistance of the leads, the transient high current will cause instantaneous high voltage, which is not conducive to equipment protection; PCB traces also need short and wide traces to make the circuit cross-sectional area as large as possible.
1) Single use: discharge the surge energy on the line to the ground. Generally, a single gas discharge tube is used between the line and the ground. Two diodes or one triode can be used, but in fact, two diodes are not as good as one triode. First of all, the volume of two gas discharge tubes is larger than the circuit volume of a single triode. Secondly, the structural characteristics of the two diodes cannot be exactly the same, the discharge must be sequential, and transient differential mode overvoltage will occur; and regardless of whether there is a difference between the two sides of the triode, when one of the sides starts to discharge, the free electrons generated by the ionization of the internal gas will be It spreads rapidly and causes another part to discharge rapidly, causing the two lines to discharge energy at the same time.
Note: When directly using the gas discharge tube for common mode surge suppression, ensure that the residual voltage of the gas discharge tube discharge is greater than 1.7 times the pressure difference between the lines to ensure that the gas discharge tube will not be affected by both ends after absorbing the surge. The voltage is too high to cause freewheeling.
In order to prevent the current from continuing after the gas discharge tube is discharged, when the voltage difference between the line and the ground is large, the discharge tube can also be connected in series with a varistor and other devices to limit the freewheeling. Although this makes the surge suppression methods of the two input lines different, it can make the current flowing through the gas discharge tube less than the sustaining current of the discharge tube after the surge, instead of connecting the gas discharge tube directly in series between the high-voltage line and the ground.
2) Used in conjunction with varistor: due to the residual voltage of the discharge tube, if you choose a discharge tube with a higher residual voltage (to meet 1.7 times the maximum input voltage), the ignition breakdown voltage of the discharge tube will also be very high, which may For some devices that require relatively low protection voltage, it cannot provide overvoltage protection. For this reason, a discharge tube with a relatively low residual voltage can be used in series with a varistor, which can reduce the threshold of surge voltage and at the same time, the discharge tube is opposed to the voltage. The varistor also plays a certain protective role (the voltage applied to both ends of the varistor is relatively small).
In addition, due to the large parasitic capacitance of the varistor itself, it has a large leakage current, and the leakage current of the gas discharge tube is very small, so that the system has almost no leakage current. When there is no transient voltage, GDT separates the MOV from the system, so that the branch leakage current is extremely low. When the voltage is over, the voltage drop of the discharge tube is very low. At this time, the discharge tube discharges, and the varistor is mainly used at this time. After the interference has passed, the cut-off state is restored to separate the varistor from the ground. If the varistor fails in the process of absorbing the surge energy, it can also be separated by a gas discharge tube.
In addition, in the communication circuit, the varistor must be used in series with the discharge tube, because the distributed capacitance of the varistor is very large (a few hundred PF to a few thousand pF), while the distributed capacitance of the discharge tube is very small (only a few pF) ), will not cause a short circuit to the signal.
3) Due to the differences in parameters during manufacturing, it is best not to use gas discharge tubes in parallel