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CASES
Transient pulse disturbance and suppression methods
Release time:
2014-10-31 00:00
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1 Introduction
There are some short-term high-energy pulse disturbance sources in the electromagnetic environment of the power system. These disturbances have a great impact on the normal operation of relays and devices, and in severe cases, they will damage components, even damage equipment or the entire system. These disturbance sources are called transient pulse disturbance sources. The causes of transient pulse disturbance sources include: lightning discharge, electrostatic discharge, switching action process of power system, etc. Common sources of transient pulse disturbance include electrical fast transient burst disturbance, electrostatic discharge disturbance, surge ( impact ) disturbance and 1MHz (100kHz) burst disturbance, etc. This paper starts from the analysis of the causes of transient pulse disturbances, summarizes the characteristics of various transient pulse disturbances, and proposes some suppression methods.
2 Causes of transient pulse disturbance
2.1 The mechanism of transient pulse Sao generation
During the process of switching off an inductive load circuit, a back EMF is generated across the inductor. According to Lenz's law: This back EMF should be . The counter electromotive force will reversely charge the parasitic capacitance C. As the charging voltage increases, when it reaches a certain value, a breakdown phenomenon will occur between the contacts to form a conductive path. Once a conductive path appears, the capacitor C will start to discharge, causing the voltage to drop. When the voltage drops below the voltage to maintain the contact, the contact will be in an open state again. The above process is repeated until the voltage on the capacitor can no longer cause breakdown between the contacts. When the capacitor cannot be discharged through the breakdown contact, it will be discharged through the inductance loop until the energy in the inductance is exhausted.
In the above process, every time the capacitor C breaks through the contacts, it will reversely charge the power circuit, so a large pulse current is formed on the power circuit. Since the power circuit also has impedance, when the pulse current passes through the power circuit, the A pulse voltage will be formed at its two ends, and other circuits sharing this power circuit will be affected by the pulse voltage. This is the reason for the formation of transient pulse disturbances. As the contact gap changes, the voltage required to break down the contact gap varies. When the contact gap is getting larger and larger, the breakdown voltage is getting higher and higher. Therefore, the voltage on the capacitor C is getting higher and higher. When the voltage required for contact breakdown is higher, the capacitor charging time is longer, and the frequency of the oscillation waveform is lower.
2.2 Generation and characteristics of main transient pulse disturbances
(1) Electrical fast transient burst disturbance
Electrical fast transient burst disturbances are generated when inductive loads are disconnected from the circuit. Its characteristic is that the disturbance signal is not a single pulse, but a series of pulse groups. On the one hand, because the pulse group can produce cumulative effects at the input end of the circuit, the amplitude of the disturbance level may eventually exceed the noise tolerance of the circuit. On the other hand, the period of the pulse group is short, and the interval between each pulse wave is short. When the first pulse wave has not disappeared, the second pulse wave follows closely. For the input capacitor in the circuit, it starts to charge again when the discharge is not completed, so it is easy to reach a higher voltage, which has a great impact on the normal operation of the circuit.
The voltage of the electrical fast transient burst disturbance source depends on the inductance of the load circuit, the load disconnection speed and the tolerance of the medium.
The characteristics of this type of disturbance voltage are: high amplitude and high frequency. When the contacts are disconnected, the current in the inductive circuit tries to continue to pass through, generating a high voltage between the contacts and causing the arc to reignite, which will generate a series of voltage pulses superimposed on the relay and the power supply connected to the device.
Electrical fast transient burst disturbance voltage is mainly common mode voltage. It is transmitted indirectly to other circuits through capacitive coupling, which occurs when an electric field generated by a voltage in one circuit interlinks with a conductor in a second circuit.
(2) Surge ( shock ) harassment
Surge ( impact ) disturbance is the disturbance induced by lightning on the cable, and it may also be generated during the disconnection of a high-power switch. The characteristic of impact ( surge ) disturbance is that the energy is very large. Indoors, the surge ( impact ) voltage can reach 6kV , and outdoors can reach more than 10kV . Surge ( shock ) disturbances are not as frequent as electrical fast transient pulse disturbances, but the hazards caused by each occurrence are very serious, and may even cause damage to circuits and relays and devices. (3) Electrostatic discharge disturbance
Because the human body will generate static electricity under certain environmental conditions. When the human body touches the relay and the device, it will generate electrostatic discharge to the relay and the device. Although electrostatic discharge is also a transient pulse disturbance, its coupling method is different from that of other transient pulse disturbances. Generally, the transient pulse disturbance The coupling methods are conduction coupling, but electrostatic discharge disturbance has radiation coupling as well as conduction coupling. But it should belong to radiation harassment in essence. Electrostatic discharge can cause serious damage and malfunction of relays and devices. There are two ways of electrostatic discharge energy propagation, one is through the surface of the metal body; the other is through the excitation of pulse energy with a certain bandwidth to propagate in space.
(4) 1MHz (100kHz) burst disturbance
For power system measurement relays, protection and automation devices, it is a disturbance of oscillation decay waves. Its generation is also due to the fact that there is also a short discharge process during the opening and closing process of the switch in its auxiliary power supply circuit. Appear.
The coupling mode of 1MHz (100kHz) burst disturbance is electromagnetic disturbance mainly conducted disturbance, which is mainly transmitted directly to relays and devices through various input wires ( conductors ) . At the same time, inductive coupling or magnetic inductive coupling is also generated to transmit disturbance signals.
The pulse group disturbance voltage acting on the product is mainly reflected in two forms: common mode disturbance voltage and differential mode disturbance voltage.
3 Suppression methods of transient pulse disturbance
3.1 Electrical Fast Transient Burst Disturbance
(1) Problems existing in the electrical fast transient burst immunity test process:
① If the tested product does not have good filtering performance at the power supply end, the electrical fast transient burst disturbance signal will enter the circuit of the relay and the device through conduction coupling. Electronic circuits in relays and devices are more sensitive to pulse disturbances. If the electronic circuit contains digital circuits, it is more sensitive to pulse disturbance. The disturbance signal of the electrical fast transient pulse group entering the electronic circuit makes the electronic circuit work abnormally through direct triggering and inductive coupling.
② For the electrical fast transient burst disturbance signal entering the circuit, it can also be coupled to the relay and other sensitive parts of the device through the common impedance of the ground wire of the printed circuit board. Since any ground wire has resistance and reactance, a voltage drop occurs when a current flows through it. For electrical fast transient burst disturbance signals, the current changes extremely fast and contains a large number of high-frequency components. According to the results , it can be known that a potential difference is easily generated on the common ground. When this voltage is below the immunity level of the circuit, no interference will occur. Otherwise, it may cause disturbance to other circuits on the common ground.
③ Due to the unreasonable binding of the relay and the device, when the electrical fast transient burst disturbs the signal, it will also cause disturbance. For example, when the wires of the strong and weak current loops are bound together or the signal line and the strong current power supply are put together, when the disturbance signal passes through the circuit, because the distance between the circuits is too close, they are coupled with each other, resulting in "crosstalk " . " Phenomenon, resulting in abnormal operation of relays and devices.
(2) Method for suppressing electrical fast transient burst disturbance
① Use filters and absorbers for electrical fast transient burst disturbance;
② Reduce the common impedance value of the ground wire of the printed circuit board;
③ Keep the electrical fast transient burst disturbance source away from sensitive circuits;
④ Add anti-harassment instructions to the software;
⑤ Correct use of grounding technology;
⑥ Reasonable wiring, strong current, weak current, and signal wires should be tied separately, and the input and output wires connected to the printed circuit board should be as short as possible.
3.2 Surge ( impact ) harassment
Protection against surge ( shock ) disturbance:
For surge ( impact ) disturbances, components to suppress surges should be used to prevent electromagnetic interference generated by surge ( impact ) disturbances. The components that suppress surge disturbance mainly include lightning arrestors, varistors and transient suppression diodes, which are absorbers for transient transient pulses.
① Lightning arrester: Also known as a gas discharge tube, the tube is filled with a certain type and a certain amount of inert gas. When the surge voltage appears, the inert gas in the tube is ionized to form a short circuit state. So the voltage at both ends of the lightning arrester is rapidly reduced to a very low voltage value, which makes most of the disturbance energy generated by the surge disturbance released. This is how lightning arrestors suppress surge ( shock ) disturbances. The lightning arrester has the characteristics of a strong ability to absorb surge current, it can withstand a large current, and at the same time, the parasitic capacitance generated is very small. In addition, it has high insulation resistance before arcing.
② Varistor: Varistor is a voltage-sensitive device. When the voltage applied to both ends of the varistor is lower than the nominal voltage, its resistance is almost infinite; once its voltage value is exceeded, the resistance of the varistor The value drops sharply, thereby releasing the energy in the surge disturbance, and limiting the amplitude of the surge voltage within a certain range, and the response time of the whole process is ns level. The inductive reactance of the lead wire of the varistor will affect the high-speed response characteristics of the varistor. The longer the lead wire, the greater the additional induced voltage generated by the lead inductance. Therefore, when using leads to install varistors, it is required that the installation leads should be as short as possible.
③Transient suppression diode (TVS) : The full name of the transient suppression diode is a silicon transient voltage absorption diode. It has extremely fast response time and very high surge absorption capacity, and can be used to protect relays and devices or circuits from static electricity, cut off inductive loads, and transient pulse disturbances generated by inductive lightning. Under normal conditions, the TVS diode does not work. When there is a transient pulse disturbance ( including impact surge ) , the avalanche effect of the silicon PN junction in the device will clamp the peak of the transient overvoltage; within the allowable range of the circuit. This is the working principle of TVS diodes.
3.3 1MHz (100kHz) burst disturbance
1MHz (100kHz) burst disturbance is less harmful than electrical fast transient burst disturbance, and its coupling mode includes differential mode coupling in addition to common mode coupling. It is also a source of harassment mainly conducted harassment. Therefore, the method of suppressing 1MHz (100kHz) burst disturbance can follow the method of suppressing common mode disturbance and differential mode disturbance of power supply disturbance.
(1) Suppression method of common mode disturbance
①Adopt floating ground shielding method;
② Use the balanced circuit method;
③ Using isolation transformer and common mode choke coil method;
④ Adopt photoelectric coupling method;
⑤ Using the input filter method
(2) Suppression method of differential mode disturbance
①Using the input method of twisted pair ( so that the interference potential cancels each other out ) ;
②Adopt shielding grounding method ( to suppress the influence of electric field interference ) ;
③The input end is connected to the low-pass filter method ( reducing the input of high-frequency disturbance ) ;
④ Separate various power line input methods ( to reduce the influence of magnetic field disturbance )
3.4 Electrostatic discharge disturbance
The protection of electrostatic discharge disturbance has the following methods:
(1) Design a complete closed metal conductor shell according to the principle of electromagnetic compatibility design, but there may be secondary arcing and conductive coupling between the circuit and the shell, so when designing the shell, redesign it outside the metal shield An insulating shell to strengthen the insulation performance of the metal shell; or use insulating materials in parts of the metal shell ( such as the panel part ) , so that the charged conductor contacts the insulated conductor, and electrostatic discharge will not occur. On the other hand, most shells are designed with holes, vents, screws, etc. on the basis of maintaining integrity. For the openings on these shells, the principle of " replacing one large hole with several small holes " should be followed , which is more beneficial to suppress electromagnetic emissions. When there are gaps on the edge of the shell, an electrical connection should be used between the two gaps to reduce electromagnetic noise.
(2) Design a good grounding system for relays and devices, that is, provide a low-impedance discharge path for electrostatic discharge current, and effectively limit the discharge current in this path.
(3) Filtering is adopted to prevent radiated disturbances from being coupled to relays and devices. Generally, the filter should be a shunt capacitor or a series of inductors, and a hybrid of the above two filters.
(4) Improve the electrostatic discharge immunity of the system through the design of the printed circuit board. The printed line on the printed circuit board can become an antenna for electromagnetic emission during electrostatic discharge. In order to reduce the coupling effect of these antennas, the printed lines on the printed circuit board should be as short as possible, and the area surrounded by the printed lines should be as small as possible. When designing, all components should be evenly distributed throughout the entire area of the printed board to reduce common-mode coupling. The use of multilayer printed circuit boards and grid routing can also reduce coupling and suppress common-mode radiation disturbances.
(5) Shield and filter the cable to prevent the cable from becoming an antenna for receiving electromagnetic disturbance. On the other hand, especially when the cable is connected to the housing, the cable should also provide a low impedance path.
(6) Adding electrostatic discharge suppression measures in the software design of relays and devices is an effective method for serious malfunctions of relays and devices. These actions are " Refresh " , " Check " and " Restore " . The " refresh " process involves periodically resetting to rest and refreshing the display and indicator states. " Check " processes are used to determine whether a program is executing correctly, and they are activated at regular intervals to confirm that the program is performing a certain function. If this function is not implemented, a " recovery " procedure is activated.
4 Conclusion
I hope that readers can understand by analogy and find ways to suppress transient pulse disturbances of other electronic products through their understanding of this article.