This is how the EMC interference of the switching power supply is generated.adapter wholesaler

　　In the research and development process of switching power supply products, the EMC problem is one of the difficult problems that engineers must overcome, and it is also a thorny problem common to switching power supplies of different powers. EMC is also known as electromagnetic compatibility. If you want to make the designed product meet the electromagnetic compatibility standard, you must figure out where the electromagnetic interference of the switching power supply comes from. This article will briefly summarize and analyze several common ways of electromagnetic interference.

　　Electromagnetic Interference from Switching Circuits

　　In the EMC design of switching power supply, the first thing that engineers need to avoid is the electromagnetic interference problem generated from the switching circuit of the power supply, which is also one of the main interference sources of switching power supply. The switching circuit is mainly composed of switching tubes and high-frequency transformers in terms of structure, so the du/dt generated by it has a large amplitude pulse, a wide frequency band and rich harmonics. The interruption of the power supply voltage will produce the same magnetizing impulse current transient as when the primary coil is turned on. This transient is a kind of conducted electromagnetic interference, which will not only affect the primary of the transformer, but also return the conducted interference to the power distribution system, causing Power grid harmonic electromagnetic interference, thereby affecting the safe and economical operation of other equipment.

　　Electromagnetic interference from rectifier circuits

　　In the EMC design of switching power supply, another large source of electromagnetic interference is the rectifier circuit. In the rectifier circuits of some small and medium power supplies, there will be a reverse current when the output rectifier diode is turned off, and the time it takes to return to zero is related to factors such as junction capacitance. When the rectifier diode in the high-frequency rectifier circuit is forward-conducting, a large forward current flows. When it is turned off by the reverse bias voltage, due to the accumulation of more carriers in the PN junction, the current-carrying For a period of time before the carrier disappears, the current will flow in the opposite direction, resulting in a sharp decrease in the reverse recovery current of the carrier disappearance and a large current change.

　　Electromagnetic interference from high frequency transformers

　　High-frequency transformers will also inevitably generate electromagnetic interference during the operation of switching power supplies. This interference problem is especially common in the product testing process of large power supplies. The high-frequency switching current loop formed by the primary coil, switching tube and filter capacitor of the high-frequency transformer sometimes produces large space radiation, forming radiation interference, which has a great impact on the EMC design of the power supply. If the filter capacity of the capacitor is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be conducted into the AC power supply in a differential mode to form conduction interference.

　　Interference caused by distributed capacitance

　　Distributed capacitance is a very inconspicuous source of electromagnetic interference in the design of switching power supplies and the testing of EMC products. When the switching power supply operates in a high frequency state, the interference caused by the distributed capacitance is very large. On the one hand, the contact area of the insulating sheet between the heat sink and the collector of the switching tube is large, and the insulating sheet is thin. The high-frequency current will flow to the heat sink through the distributed capacitance, and then to the chassis ground, which will cause common mode interference. On the other hand, there is a distributed capacitance between the primary and secondary sides of the pulse transformer, which can directly couple the voltage of the primary side to the secondary side, resulting in common mode interference on the two power lines that are used for DC output on the secondary side.

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