Design And Research Of A Low EMI BUCK DC-DC Controller With Synchronous Rectification

DC-DC switching power supply is widely used in life and industry because of its high transmission efficiency.But at the same time,the switching power supply will produce electromagnetic noise,which will have a great impact on some noise sensitive loads.In order to reduce EMI,we choose to suppress the interference from the source and adopt the method of frequency hopping to reduce EMI.In this paper,a current mode buck converter with a spur-free constant-cycle frequency-hopping controller is studied.Firstly,the loop of buck converter is analyzed,and the power stage loop of buck is modeled,and the zeros and poles of the loop are determined.Then the appropriate compensation circuit is selected to ensure the loop stability.Because the traditional switching power supply has large spurs at the switching frequency.In order to reduce this kind of spurious,we can use frequency hopping technology.This is because frequency jitter can disperse energy,so as to achieve the goal of reducing EMI.The original fixed clock frequency is adjusted to the randomly switched clock frequency controlled by random number generator,and finally the clock jitter is realized.In this scheme,frequency hopping is combined with current loop control to achieve spurious free operation.At the same time,the output noise is low and there is no subharmonic caused by frequency hopping.Even if there are only two frequencies,the phase of them will be cut off to eliminate the spurious completely.It can be used as a low noise converter to supply power to noise sensitive loads without post linear adjustment or additional passive filters.In addition,spurious free operation helps to integrate it into a mixed signal system on chip without interfering with sensitive circuits sharing the same substrate or power rail.The small signal modeling of peak current control mode adopted in this paper is more accurate than the traditional modeling.At the same time,the Fourier series theory is used to analyze how to select the two kinds of frequency in dithering frequency,so as not to cause subharmonics.Through the simulation analysis of the overall circuit,the voltage ripple increases by about 6m V,but the output EMI decreases by about 15 d B.At the same time,the peak efficiency of the converter reaches 92%.