Research On EMI Reduction Of Converters Applying Switching Frequency Jittering

Electromagnetic interference(EMI) and electromagnetic compatibility(EMC) issues of power converters are the problem that must be confronted with and solved. Switching frequency jittering is a preferred method applied to mitigate the EMI of the power converter from the interference source in essence by redistributing the total energy into a number of sidebands with a lower amplitude and a wider bandwidth but keeping the total energy unchanged.The principle of the frequency modulation which is the fundamental of frequency jittering was introduced in the beginning and was applied to a modulated PWM signal. The spectrum characteristics of a modulated PWM wave were dissertated in detail. Then the parameters of modulation are introduced for the purpose of achieving a better reduction in sidebands amplitudes. Of the modulation parameters, the modulating waveform gives a large attribution to the distribution characteristic of the modulated carrier spectrum. In this paper, the spectrum of a pure sine-wave modulated by sinusoidal, triangular and exponential waveforms is calculated by analytical method with Matlab, and the relative mitigation of the amplitudes of the resultant harmonics compared with the original carrier is obtained, whereas, the essence of the general phenomena is revealed. It shows that a triangular modulation process presents a best reduction of the sidebands amplitudes with a more flat spectrum envelop. The rules revealed could be applied to predict the spectra of PWM signal modulated by any other periodic modulating wave. Besides, the limitation to deviation of the switching frequency is discussed from the view of output ripples in open loop and oscillation of close loop to obtain an optimal modulating waveform.To realize the performance of frequency jittering, an exterior analog circuit combined with conventional PWM chip and a digital circuit with DSP are selected respectively. Both circuits realized the function of frequency jittering by three basic modulating waveform only with a difference of two exponential waves. The analog circuit has the ability to adjust the amplitude, frequency and duty cycle of the wave consecutively. The detailed principles and parameters of each analog circuit for modulating wave are discussed and the control flow is also introduced. The spectra of the PWM signal produced by DSP with frequency jittering are presented to verify the calculation results. Through a suitable selection for the parameters of the modulation, the measured harmonics amplitudes of the input current as well as the voltage across the transistor of a prototype of a 30W flyback converter with triangular modulation reduced by10dB contrast with the fixed frequency control mode.