1.3 Common mode conducted disturbance caused by high frequency transformer
High-frequency transformer is an important component in energy storage, isolation, output and voltage conversion in switching power supply. Its leakage inductance and distributed capacitance have a great influence on the electromagnetic compatibility of the circuit. Since the primary coil has a leakage flux, a part of the energy is not transmitted to the secondary coil, but a decaying oscillation with a peak formed by a capacitor and a resistor in the collector circuit is superimposed on the turn-off voltage to form a turn-off voltage spike. Produces the same magnetizing inrush current transient as when the primary coil is turned on. This noise is transmitted to the input and output terminals to form conducted disturbances, which may cause the switch to break through. In addition, the high frequency switching current loop formed by the high frequency transformer primary coil, the switching tube and the filter capacitor may generate large space radiation to form radiated disturbance.
There is a distributed capacitance between the primary and secondary of the FM transformer of the switching power supply. Interference channels are formed by using a device capacitor (the distributed capacitance of the device to ground) to be equivalent to the entire switching power supply. The common mode interference passes through the coupling capacitance of the transformer, and then returns to the ground through the device capacitor, and a voltage divider composed of the transformer coupling capacitor and the device capacitor is obtained. The high-frequency switching current loop formed by the pulse transformer primary coil, the switching tube and the filter capacitor may generate large space radiation and form radiated disturbance.
1.4 Distribution and parasitic parameters caused by switching power supply noise
The distributed parameter of the switching power supply is the internal factor of most interference. The distributed capacitance between the switching power supply and the heat sink, the distributed capacitance between the primary and secondary of the transformer, and the leakage inductance of the primary and secondary sides are all noise sources. Common mode interference is transmitted through the distributed capacitance between the primary and secondary of the transformer and the distributed capacitance between the switching power supply and the heat sink. The distributed capacitance of the transformer winding is related to the structure and manufacturing process of the high-frequency transformer winding. The distributed capacitance between the switching power supply and the heat sink is related to the structure of the switch tube and the way the switch tube is mounted. The use of shielded insulating gaskets reduces the distributed capacitance between the switch and the heat sink.
Components that operate at high frequencies have high-frequency parasitic characteristics that affect their operating conditions. When the high frequency is working, the wire becomes the emission line, the capacitance becomes the inductance, the inductance becomes the capacitance, and the resistance becomes the resonance circuit. When the frequency is too high, the frequency characteristics of each element undergo a considerable change. In order to ensure the stability of the switching power supply during high-frequency operation, the design of the switching power supply should fully consider the characteristics of the component during high-frequency operation, and choose to use components with better high-frequency characteristics. In addition, at high frequencies, the inductive reactance of the parasitic inductance of the wire is significantly increased. Due to the uncontrollable inductance, it eventually becomes a single emission line, which becomes a source of radiation interference in the switching power supply.
2. Measures to suppress electromagnetic interference
Switching power supplies have two forms of electromagnetic interference: common mode interference and differential mode interference. According to the electromagnetic interference sources analyzed above, combined with their coupling paths, interference can be suppressed from EMI filters, absorption circuits, grounding and shielding, and electromagnetic interference can be attenuated to within the allowable limits.
2.1 AC input EMI filter
Filtering is a method of suppressing conducted interference. Connecting a filter to the input end of the power supply can suppress the noise from the power grid to the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. As an important unit to suppress the conducted interference of power lines, the power supply filter plays an extremely important role in the electromagnetic compatibility design of equipment or systems. The power input terminal usually adopts an EMI filter circuit as shown in FIG. The circuit can effectively suppress low frequency differential mode disturbance and high frequency mode common mode disturbance at the input end of the AC power source. In the circuit, the differential mode capacitors Cx1 and Cx2 (also called X capacitors) connected across the power supply are used to filter out differential mode interference signals. Generally, ceramic capacitors or polyester film capacitors are used, and the capacitance value is usually 0.1 to 0.47F. The common mode capacitors Cy1 and Cy2 (also known as Y capacitor) grounded in the middle of the line are used to short-circuit the common mode noise current, which is usually C1=C2#2200pF. The suppression inductors L1 and L2 are usually taken as 100~130H. The common mode choke L is composed of two coils which are equivalent and wound in the same direction on one magnetic core. The inductance L#15~25mH is usually required. When the load current crosses the common mode choke, the magnetic lines of force generated by the coils connected in series on the live line are opposite to the lines of magnetic force generated by the coils connected in series on the zero line, which cancel each other out in the core. Therefore, even in the case of a large load current, the magnetic core is not saturated. For the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will exhibit a large inductance, thereby attenuating the common mode interference signal.
The stator core is an important part of the stator and the main component of the magnetic circuit of the motor. Different sizes of stator cores have different technological processes and application fields. For example, small-sized stator cores are usually assembled by interlocking, and the outer diameter is usually less than 200mm. Large-sized stators are assembled in different ways depending on the motor design, such as stator core by cleating and staor core by welding.
Stator Core,Motor Core,Stator Lamination,Motor Stator Core
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