The noise of power supply is one of the electromagnetic interference (EMI). The spectrum of the transmitted noise is approximately 10kHz~30MHz, up to 150MHz. The power supply's noise, especially the transient noise interference, has the following advantages: fast rising speed, short duration, high voltage amplitude and strong randomness, which is likely to cause serious interference to microcomputer and digital circuits.
The noise of the power supply can be divided into 4 types:
1. the range of change of voltage is too large.
The power supply of the power grid is insufficient, the power supply department takes the step-down power supply, or is in the remote area, and the loss is too much, which leads to the low voltage. There is too little electricity in the power grid, which leads to high voltage, low voltage and low load. The voltage is too high, the service life of the load is shortened, or the load is burned down.
2. waveform distortion (or harmonic Waveform Distortion)
Universal waveform distortion refers to a variety of harmonics of the standard power waveform. The reason for the harmonic generation is rectifiers, UPS power, electronic speed control equipment, fluorescent lamp system, computers, microwave ovens, energy saving lamps, dimmer and other power electronic equipment and electrical equipment in the use of switching power supply or the two generation of the power itself. The harm of harmonics to public power grid mainly includes:
1) make the components of the public power grid produce additional harmonic losses, and reduce the efficiency of power generation and transmission and transformation equipment. When a large number of 3 harmonics pass through the neutral line, they will cause overheating and even fire.
2) affect the normal work of all kinds of electrical equipment, in addition to causing additional loss, but also makes the motor produce mechanical vibration, noise and over voltage transformer, severe local overheating, the capacitors, cables and other equipment overheating, insulation aging and shorten the life span of that damage;
3) it will cause the local parallel resonance and series resonance in the public power grid, so that the harmonics can be magnified, and the harm of the foregoing is greatly increased, and even serious accidents are caused.
4) will cause the relay protection and the automatic device misoperation, and make the electrical measurement instrument inaccurate.
5) it will cause interference to the nearby communication system. The lighter will produce noise, reduce the communication quality, and the heavy will lead to information loss, so that the communication system can not work normally.
3. Sudden wave (or surge Power Surges)
The effective value of the output voltage in a moment (between milliseconds) is higher than 110% of the rated value, and the duration of the voltage is one or several cycles. It is the main culprit to destroy the precision electronic equipment. In addition to the lightning strike, it is mainly due to the high voltage caused by the sudden unloading of the power grid when the large electrical equipment connected to the power grid is shut down. The harm of electric surge: since the development of computer technology, multi-layer and super scale collector chip has been concentrated in circuit, and tends to be more integrated, and the gap between components is smaller. A few years ago, a square centimeter of computer chips had 2000 transistors and now more than 10000000 Pentium machines. Thus the probability of a computer to be damaged by a surge is increased. Because of the design and structure of the computer, it should work in a specific voltage range.
When the surge beyond the acceptable level of computer, the computer will appear garbled data, the chip is damaged, part of premature aging, these symptoms include: beyond expectation data errors, data receiving / delivery failure, loss of documents, disorders, often in need of repair, unexplained failure and hardware problems. Lightning surge is far beyond the level that computers and other electrical equipment can bear. In most cases, computers and other electrical equipment will be destroyed instantly or the data will always be lost. Even a small surge induction motor 20 hp start or shutdown will generate 3000-5000 volts, and make it share the same distribution box computer will damage or interference in every surge in the number of this surge is very frequent.
4. Sharp wave (or high pressure tip pulse Spikes)
. The peak value is 6000V, the duration from 1/10000 to 1/2 period (10ms) voltage. This is mainly due to lightning strikes, electric arc entry, static discharge or switch operation of large electrical equipment. Harm：
In the steel mill, the rolling mill or the use of a large number of thyristor equipment, electric spark equipment, electric locomotive and other places, this kind of peak interference is especially harmful. Its amplitude is large up to hundreds of volts to 1000 volts, and the pulse width is generally of the order of S. Lightning also often invades with peak pulse. When the peak pulse amplitude is very large, it will destroy the input filter, rectifier and even main vibration tube of the switching power supply of the industrial control machine. In addition, its spectrum is very wide, it will also move into the computer to cause interference.