Research On Spread Spectrum Pulse Width Modulation Method Of Digital Class-D Audio Power Amplifiers

Filterless digital class-D audio power amplifiers have received high attention from researchers because of high power efficiency,easy to interface with digital audio sources and integrate system.Since these amplifiers do not utilize LC low-pass filters,its output signal will generate high energy high-frequency components at the pulse repetition frequency(PRF)and its multiples,which will produce serious electromagnetic interference(EMI).Therefore,researchers have focused on how to effectively reduce EMI in the filterless digital class-D audio power amplifiers.In this paper,the working principle and research status of spread spectrum pulse width modulation(PWM)are analyzed,and then a new all-digital spread spectrum PWM method with distortion correction is proposed.The method firstly constructs a spread spectrum PWM scheme with dual randomization of pulse position and PRF based on the decomposability of the asymmetric double-edge uniform-sampling PWM(UPWM).This spread spectrum PWM scheme decomposes the asymmetric double-edge UPWM generator into a leading-edge UPWM generator and a trailing-edge UPWM generator.Then random numbers generated by the pseudo-random number generator are used to randomize the levels of the leading-edge UPWM generator and the trailing-edge UPWM generator,so as to randomize the pulse position and PRF of the UPWM signal,and then the high frequency peak energy of the ternary UPWM signal is extended to its nearby frequency domain to reduce the EMI of the amplifier.However,the use of UPWM technique and spread spectrum technique can cause distortion to the input signal.To solve this problem,an interpolation block and a decimation block are added to the front and back of the digital Σ-Δ modulator respectively,and reconstructing the state-space expression of the digital Σ-Δ modulator to take advantage of the high open-loop gain characteristics of the digital Σ-Δ modulator,and constructing a digital closed-loop block including the digital Σ-Δ modulator and the UPWM generator with spread spectrum,so that the input and output of the digital closed-loop block are approximately linear,which effectively reduces the distortion introduced by the UPWM and spread spectrum.Based on field programmable gate array(FPGA),a filterless digital PWM modulator based on the proposed spread spectrum method is designed and implemented in this paper.At the same time,this paper verifies the spread spectrum and distortion correction performance of the proposed method by building a filterless digital PWM modulator test system.Under the same test conditions that the input signals are 24-bit single-frequency sinusoidal digital signals,compared with the seven existing spread spectrum methods,the proposed method has the lowest total harmonic distortion(THD)of 0.00001%,the highest signal to noise and distortion ratio(SNDR)of 100.87 d B and the highest out-of-band peak spectral amplitude reduction of 17.75 d B.By comparing and analyzing the experimental results,it is concluded that the proposed method not only effectively reduces the EMI of the amplifier system,but also basically eliminates the distortion introduced by UPWM and spread spectrum,so that the amplifier system has a better signal fidelity.