With the increasing application of electrical equipment, the electromagnetic noise generated by electronic equipment has become more and more serious, which interferes with the normal operation of electronic equipment, especially for some portable equipment with low power consumption.
There are two media channels for electromagnetic interference. One is that the dynamic change of the working current makes the voltage on the local power grid unstable, which affects the operation of the equipment using the local power grid. This interference is called conducted interference. In addition, the dynamic change of the working current (voltage) in the equipment produces electromagnetic radiation, which also affects the work of other equipment. This interference is called radiation interference.
In addition to artificially produced electrons, electromagnetic noise (interference) sources also include natural phenomena (such as lightning) and other human actions (such as nuclear explosions).
The influence of electromagnetic interference is also great, from the slightest it makes the performance of the equipment not well represented, at the worst it makes the equipment unable to work at all, and electromagnetic radiation may also lead to the leakage of confidential information.
Two effective ways to suppress electromagnetic interference are color power supply filters and shielding devices. The shielding devices are mainly aimed at secondary radiation interference, which not only prevents the leakage of electromagnetic waves from causing new interference sources, but also avoids interference from external radiation. The most basic function of the power filter is to suppress conducted interference, and some varieties can also improve the ability to suppress secondary radio interference. Broadly speaking, the AC stabilized power supply and UPS power supply we use can also be regarded as a kind of power filter, because these devices isolate electronic equipment from the power grid to some extent. The power filters we introduce here are all Attached to electronic equipment as a device that is not very complicated, we often add an RC circuit to the DC power circuit to suppress ripple. The function of the power filter is to suppress interference on the AC power supply. At present, with the improvement of the precision of electronic equipment, the requirements for power supply are also higher and higher. At the same time, the wide application of electronic equipment also requires all electronic equipment manufacturers to make a common commitment to the electromagnetic environment, which leads to the power filter as a A green product has received more and more attention from the society. At present, some World Standardization Organizations and governments of various countries are formulating standards in this area.
2. The composition of the power filter
The power filter is composed of an LC network. Its working principle is to make the impedance of the filter not match the impedance of the interference source, so that the interference signal is reflected back in the direction where the interference source comes in, thereby reducing the influence of the interference source.
Figure 1 is the principle circuit of a power supply filter.
In the figure, L1 and L2 present high impedance to common mode interference signal (asymmetric interference current), and low impedance to differential mode signal (symmetric interference current) and power supply current. It can ensure that the attenuation of the power supply current is small, while suppressing the current noise. Usually the values of L1 and L2 are very small and equal, and they are symmetrically wound on the same spiral tube. In this way, within the normal operating current range, the magnetism generated by the magnetic materials compensates each other to prevent the magnetic flux from being saturated, but for asymmetric interference ( For common mode) signals, the magnetic fields generated by the two coils strengthen each other, and the total inductance presented to the outside is significantly increased, so that the symmetrical interference components are greatly suppressed by L1, L2 and the capacitance CX between the phase line and the neutral line. Up. If you want to further attenuate the symmetrical interference, in addition to using the capacitor CX (due to the leakage current between the phase line and the neutral line. To make it possible to make CX very large), an uncompensated choke can also be used.
Through the above analysis, it can be seen that the power supply filter must be installed at the input end of the power supply when in use, that is, the power supply filter is connected in series between the power grid and the equipment power line.
3. Measurement parameters of power filter
3.1 Insertion loss
Since the power supply filter is connected in series between the power grid and the equipment power line, and the power supply filter is a passive network, it is bound to cause a voltage drop, which leads to insertion loss caused by the power supply filter.
The insertion loss A of the filter is usually measured with a 50Ω resistance when the device is not working, and then an attenuation curve is made. The principle of the measurement circuit is shown in Figure 2. A is defined as the ratio of the applied voltage V0 to the filter output voltage V2.
The typical insertion loss curve of the filter, as required, should be shown in the figure together with the asymmetric insertion loss (measured between the phase line or the neutral line and the ground line). For different interference sources and specific equipment, the actual loss curve may be quite different. Therefore, whether a filter can effectively suppress the interference on a specific power grid can only be determined after actual measurement.
3.2 Working voltage
The working voltage is the stable voltage at which the filter can work safely. Generally, the working voltage of the filter for single-phase power is 250V, and the working voltage for three-phase power is 420V.
3.3 Working current
The allowable working current has a certain relationship with the working temperature. Generally, only the value at room temperature (20°C) is given, and sometimes the value at a higher temperature (40°C or 45°C) is given.
3.4 Leakage current
In the power filter, because there is a capacitor between the phase line and the neutral line, when the power is turned on, the current will flow into the ground through the capacitor, which leads to the existence of leakage current. For safety considerations and other purposes, leakage current must be limited, and various countries have specified the maximum allowable leakage current for all types of equipment.