Three-frequency pseudo-random varying spread spectrum method for filterless digital class-D audio amplifiers
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ORIGINAL ARTICLE
Three‑frequency pseudo‑random varying spread spectrum method for filterless digital class‑D audio amplifiers Zeqi Yu1 · Ge Bai1 · Ke Zhang1 · Jin Xu1 Received: 11 January 2020 / Revised: 27 May 2020 / Accepted: 31 May 2020 © The Korean Institute of Power Electronics 2020
Abstract This paper presents a novel spread spectrum (SS) method with a pseudo-natural sampling technique for filterless digital class-D audio amplifiers. This method is proposed to reduce the electromagnetic interference (EMI) emissions of amplifiers by lowering the output out-of-band spectral amplitudes around the pulse width modulator carrier frequency and its multiples. The main idea of this method is that the sampling frequency of the modulating signal and the carrier frequency are varied synchronously and randomly among three different values. Since the carrier frequency is variable, the polynomial interpolation Newton–Raphson (PI-NR) algorithm is modified to correct the harmonic distortion caused by uniform-sampling pulse width modulation (UPWM). Based on a field programmable gate array (FPGA), an experimental system is established to analyze the performances of the proposed method and other six reported SS methods. Experimental results show that the proposed method features the lowest total harmonic distortion (THD) of 0.009% with a 6.6 kHz, 0 dBFS (full scale) input, a relatively high signal-to-noise ratio (SNR) of 52.68 dB and a relatively high output out-of-band peak spectral amplitude reduction of 17.75 dB. Keywords Class-D audio amplifier · Spread spectrum (SS) · Uniform-sampling pulse width modulation (UPWM) · Pseudonatural sampling · Finite state machine (FSM)
1 Introduction Nowadays, the miniaturization of audiovisual portable devices makes it necessary for devices to have a high power efficiency to assure a long battery lifetime. Compared with class-A, class-B and class-AB amplifiers, digital classD audio amplifiers have a higher power efficiency (typically ≥ 85%) and a lower power dissipation, which means they have become popular for most portable devices, such as hearing aids, smartphones, notebook computers, wireless speakers, etc. [1–4]. Generally, a digital class-D audio amplifier consists of three basic blocks: a uniform-sampling pulse width modulation (UPWM) modulator, a power stage and an LC low-pass filter [5]. The UPWM modulator converts the audio input signal into a UPWM signal. Then, the UPWM signal is amplified by the power stage. Finally, the LC low-pass filter filters the high-frequency (beyond the * Zeqi Yu [email protected] 1
School of Computer and Communication Engineering, Zhengzhou University of Light Industry, Zhengzhou, China
audio range) components of the amplified UPWM signal to drive a loudspeaker. Since the LC low-pass filter, which occupies about 70% of the amplifier printed circuit board (PCB) area, is area inefficiency, for further miniaturization, the filterless digital class-D audio amplifier was invented [6, 7]. The filterless configuration means that the power stage directly
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