EMS Analysis on Digital Pulse Width Modulators

In this paper, a digital pulse width modulator (DPWM) is analysed from the point of view of the susceptibility to radio frequency interferences (RFI). The chosen DPWM architecture is based on the hybrid delay-line/counter scheme commonly used in the digital control of switched mode power supplies, since it increases the DPWM performance in terms of maximum switching frequency, duty-cycle precision and area consumption. This paper focuses on the DPWM performance degradation in presence of electromagnetic interferences (EMI) and, for this purpose, a hybrid PWM with time resolution down to 350 ps has been designed and manufactured in a standard 0.35 micrometer CMOS technology. Measurements and analytical analyses are presented for both EMI effects on the internal generated clock signal and on the PWM duty-cycle precision. The immunity degradation mechanisms are investigated, in particular by means of the proposed mathematical approach, demonstrating that the PWM susceptibility is strictly related to its functional design.