The most complete summary of switching power supply EMC knowledge
Importance of EMC
As early as the early 1980s, the infant mortality rate in the maternity ward of a hospital in New Jersey, USA was quite high. In the middle of the night, the warning light on the baby monitor kept going off for no apparent reason. The nurses were very annoyed by this, so they turned off the monitor and went back and forth to check one by one.
After some preliminary investigations, the professor found out the truth of the matter. It turned out that the transmitter of a nearby TV station had been approved by the US Communications Commission to increase its output power to a relatively high level after about midnight, but it had to be released at six o'clock in the morning. Point before, or other specified time, restore to the original level. The connecting cables between the nurse's station and each baby's monitor resonated at these interfering frequencies, inducing a voltage that caused the monitor's warning lights to go out. Almost six children died before the hospital discovered the problem.
Here is another example: There was a complaint from a customer that when a switching power supply was turned on in the computer room, the company's 100M speed LAN slowed down and stopped, but the 10M speed network was not affected. Turn off the power and the network returns to normal.
After experiments, it was found that the high-frequency interference signal of the switching power supply was coupled to the network line, causing the network to fail.
History of EMC development: EMC was actually born with the rapid development of the modern electronics industry. By the end of the last century, with the rapid increase of electronic and electrical equipment. EMC has been extended to many fields, it is no exaggeration to say: where there are electronic products, there are EMC problems. Western countries have increasingly stringent requirements for this, and EMC has become one of the trade barriers for electronic products from developing countries to enter the western market.
For enterprises, different EMC design concepts will lead to different costs and waste of time.
The root cause of EMI:
First, the biggest disadvantage of the switching power supply is that the noise voltage generated by the switching action (TURN-ON or TURN OFF) is its source of noise. Because the waveform of the switching action is a square wave, and the square wave contains many high-order harmonics.
Second, due to the nonlinearity of the switching transistor and the reverse recovery characteristics of the diode, the rapid nonlinear change of the current causes noise.
EMI transmission methods and channels:
EMI interference signals can be divided into common mode signals (COMMON MODE) and differential mode signals (DIFFERENTIAL MODE) according to their characteristics.
Common mode signal: the signal that interferes with the signal current and the current direction on the wires of the two loops is the same relative to the ground, called a common mode signal;
Differential mode signal: The current direction of the interference signal current on the wires of the two loops is opposite to the ground, called the differential mode signal.
