Source of electrical noise

Source of electrical noise

Noise is generated when there is a rapid jump in the voltage or current waveform. In a switching power supply, many waveforms are periodic, that is, pulses containing high frequency edges repeat at a predictable pulse repetition rate (PRF). For a rectangular pulse train, the reciprocal of the period determines the fundamental frequency of the waveform. The Fourier transform of a rectangular wave contains a large number of harmonics at the fundamental frequency. The reciprocal of the rise time or fall time of the edge of the pulse twice gives the frequency of the frequency component caused by these edges, typically in the megahertz range, and its harmonic frequency is much higher.

In PWM switching power supply, the pulse width of rectangular wave will change continuously with the operation of the device. The result is an energy distribution close to that typical of white noise, with the base spectrum showing peaks whose amplitudes fluctuate as frequency increases. Components of the spectrum extend well beyond 100MHz. If not filtered and screened, it can interfere with consumers' other electronic devices. Quasi-resonant and resonant transform switching power supplies have a very attractive spectral shape of radiation. This is because the current and voltage changes in the switching process are forced by the resonant unit at a lower frequency, so only the low-frequency spectrum components can be displayed (below 30MHz). The low change rate of current and voltage in the transformation process is the reason for the above phenomenon. Higher frequency components of the spectrum are almost nonexistent. Quasi-resonant and soft-switched converters are "silent" and easy to filter

Conducted noise, that is, noise current, flows out of the product housing through the power line and any input or output line and is displayed in two forms: common-mode mode and differential mode mode. Common-mode noise is transmitted through the input power line rather than through the ground wire and can be measured through the input power line. Differential mode noise can only be measured from the ground wire to an input power line. The noise current actually flows out of the ground conductor. The noise of each mode is related to the filter topology, and each power supply requires two types of input filters. These filters contain inductors and capacitors known as "X" and "Y" components, respectively. The X element filters out common-mode noise in the power line, while the Y element filters out differential mode noise between the power line and the ground line.

The inspection organization should check both radiation and conduction noise. The radiated noise is detected by rotating a calibrated antenna and receiver at a certain distance (1m) from the product and drawing the spectrum within the Gih region. Radiated noise interferes with other devices, while conducted noise uses power lines and I/O lines to transmit noise, so it also needs to be detected. The detection of conducting noise is to coupling frequency-modulated current to the input power line, and the detection spectrum is capped at 1GHz.