1. Electronic equipment lightning surge immunity test standard>
Electronic equipment The national standard for lightning surge immunity test> is IOS17626.5 (equivalent to the international standard IEC61000-4-5).
The standard is mainly to simulate various situations caused by indirect lightning strikes:
(1) When lightning strikes the external line, a large amount of current flows into the external line or grounding resistance, resulting in interference voltage.
(2) Indirect lightning strikes (such as lightning strikes between clouds or within clouds) induce voltage and current on external lines.
(3) Lightning strikes objects adjacent to the line, and a electromagnetic field is established around it, which induces voltage on the external line.
(4) The interference introduced when the lightning strikes the adjacent ground and the ground current passes through the public grounding system.
In addition to simulating lightning strikes, the standard also simulates the interference introduced by switching actions in substations and other occasions (voltage transients caused by switch switching), such as:
• Interference generated when the main power system switches (such as switching of capacitor banks).
• Interference on the same network, when some smaller switches in the vicinity of the equipment trip.
• Switching of thyristor devices with resonant lines.
• Various systematic faults, such as short-circuit and arcing faults between equipment grounding networks or grounding systems.
The standard describes two different waveform generators: one is the The waveform produced by online induction; the other is the waveform induced on the communication line. These two lines are all overhead lines, but the impedances of the lines are different: the surge waveform induced on the power line is narrower (50uS), and the leading edge is steeper (1.2uS); The resulting surge waveform is wider, but with a slower leading edge. In the following, we mainly analyze the circuit based on the waveform induced by lightning strike on the power line, and also briefly introduce the lightning protection technology of the communication line.
When designing the anti-surge of the common mode surge suppression circuit, it is assumed that the two parts of the common mode and the differential mode are independent of each other. HoweverHowever, the two parts are not really independent, since the common-mode chokes can provide considerable differential-mode inductance. This part of the differential-mode inductance can be simulated by discrete differential-mode inductors.
In order to utilize the differential mode inductance, in the design process, the common mode and the differential mode should not be carried out at the same time, but should be done in a certain order. First, common-mode noise should be measured and filtered out. Differential mode rejection network (Differential Mode Rejection Network) can be used to eliminate the differential mode components, so the common mode noise can be directly measured. If the common-mode filter is designed so that the differential-mode noise does not exceed the allowable range, then the mixed noise of the common-mode and differential-mode should be measured. Because the common-mode component is known to be below the noise tolerance, only the differential-mode component is exceeded, which can be attenuated by the differential-mode leakage inductance of the common-mode filter. For low power power systems, the differential mode inductance of the common mode choke is sufficient to solve the differential mode radiation problem, because the source impedance of the differential mode radiation is small, so only a very small amount of inductance is effective.
To suppress the surge voltage below 4000Vp, generally only need to use LC circuit for current limiting and smoothing filtering, and try to reduce the pulse signal to the level of 2~3 times the average value of the pulse signal. Since L1 and L2 have 50 cycles of grid current flowing through them, the inductance is easily saturated. Therefore, L1 and L2 generally use a common-mode inductance with a large leakage inductance.
It is used in both AC and DC. Usually we see it in power supply EMI filters and switching power supplies, but it is rare on the DC side. It can be seen on the DC side in automotive electronics.
The purpose of adding common mode inductors is to eliminate common mode interference on parallel lines (two-wire and multi-wire). Due to the unbalanced impedance of the two wires on the circuit, the common mode interference is finally reflected in the differential mode. It is difficult to filter out by differential mode filtering method.
Where does the common mode inductor need to be used? Common-mode interference is usually electromagnetic radiation, coupled in space, so whether it is AC or DC, if you have long-term transmission, it involves common-mode filtering and you have to add a common-mode inductance. For example: If there are many USB cables, add a magnetic ring to the cable. Switching power supply inlet, AC power is transmitted over a long distance, so it needs to be added. Usually the DC side does not need remote transmission and does not need to be added. There is no common mode interference, adding it is a waste, and there is no gain to the circuit.
The design of the power filter can usually be considered from two aspects of common mode and differential mode. The most important part of the common mode filter is the common mode choke coil. Compared with the differential mode choke coil, a significant advantage of the common mode choke coil is that its inductance value is extremely high, and the volume is small. The design of the common mode choke coil An important issue to consider when a flow coil is its leakage inductance, which is the differential mode inductance. Usually, the way to calculate the leakage inductance is to assume that it is 1% of the common mode inductance, in fact, the leakage inductance is between 0.5% and 4% of the common mode inductance. The effect of this error may not be negligible when designing a choke for optimum performance.
Second, the importance of leakage inductance>
How is leakage inductanceHow is it formed? Tightly wound and fully wound toroidal coil, even if there is no magnetic core, all its magnetic flux is concentrated in the coil core. However, if the toroid is not wound one full turn, or if it is not tightly wound, flux will leak out of the core. This effect is proportional to the relative distance between the turns and the magnetic permeability of the helical tube core. Common mode choke coils have two windings designed so that the currents they flow through are conducted in opposite directions along the coil core, thereby making the magnetic field zero. If, for safety reasons, the coils on the core are not bifilarally wound, then there is a considerable gap between the two windings, which naturally causes flux leakage, that is to say, the magnetic field at various points of interest is not really 0. The leakage inductance of a common mode choke is a differential mode inductance. In fact, the flux associated with the differential mode must leave the core at some point, in other words, the flux forms a closed loop outside the core, not just confined within the toroidal core.