Impact of a Frequency Modulated Pulsewidth Modulation (PWM) Switching Converter on the Input Power System Quality

Abstract

In this paper, we show that a frequency modulated (FM) source of conducted emission has an adverse effect on a dc power system, although it is believed that frequency modulation of the interferer reduces such effects. Spread spectrum modulation is a commonly proposed technique of electromagnetic interference (EMI) reduction. It is often suggested that an FM pulsewidth modulation ramp be used in a voltage mode controlled dc-dc switching converter for reduction of conducted and radiated EMI. However, this paper shows that this technique is not as beneficial as is commonly perceived. The power quality standard MIL-STD 704, which is the origin of the EMI standard MIL-STD 461, puts restriction on several parameters other than the frequency spectrum of the disturbance, which is the only parameter that is specified in the EMI norms and tested in electromagnetic compatibility (EMC) tests. In this paper, we show that different interferers which are compatible to the same EMC limit, may affect the same power system differently. This difference is noticed if the other power quality parameters are considered. We have also mentioned that the actual benefit from spread spectrum modulation-based EMI reduction depends on the savings in printed circuit board (PCB) footprint area that we get by using a smaller filter. Through a numerical study, we have shown that such a method leads to only a negligible savings in PCB area in spite of significant EMI reduction. Moreover, this brings about a degraded performance on the other parameters that are not normally checked in EMI tests.