The pulse-density modulation technique (PDM) has been used in resonant inverters because it maintains soft-switching for any output power level. Its major drawback is that subharmonics of the resonant frequency are generated. In this paper, the frequency spectra of PDM waveforms, as a function of the duty cycle and the load time constant, are investigated. The current is analyzed as an amplitude modulated waveform, and results for regular and nonregular PDM are obtained. It is shown that the nonregular PDM, with the highest number of partial sequences and the best symmetry, produces the lowest subharmonics.
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