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Conducted EMI Analysis and Filter Design for MHz Active Clamp Flyback Front-end Converter

Fig. 1. (a) Flyback transformer with shielding.
High frequency is the major catalyst for size reduction in power conversion technology. It is essential to understand the EMI characteristic of the high frequency converter since the EMI filter typically occupies one third of the total system volume. The modeling and analysis of conducted EMI noise of a flyback converter has been studied for decades. The CM/DM noise transformation has been described and some analysis was reported in the past, however, the fundamental mechanism by which the unbalanced current is induced has not been revealed. This paper presents an insight view of the CM/DM noise transformation and a better understanding of the flyback converter EMI noise.

A shielding technique is used as an alternative to block the CM noise path from either side of the transformer. The traditional flyback transformer is in hand-made and is complicated to insert a shielding. With a much higher switching frequency, a PCB winding based transformer is feasible and it is easier to integrate the shielding layer as well as precisely control the parasitics, as shown in Figure 1. Conventional theory only expects shielding to have an impact on CM noise reduction, however, it also helps reduce the total DM noise by reducing the part of mix-mode noise.

With a better understanding of the EMI noise mechanism and impact of shielding on total noise, the EMI filter can be designed effectively. High frequency operation and shielding have significant impact on EMI filter design. The corner frequency of the CM and DM filter shifts to higher frequency, which requires a smaller CM and DM choke. A one-stage filter can be used to achieve the required attenuation over the whole conducted EMI noise testing frequency range (150kHz ~ 30MHz). Figure 2 shows the CM/DM noise spectrum with peak mode measurement under a 110VAC input full load output condition, which is the worst case for the prototype design. The red curves are the results with shielding but without filters. The blue curves are the final results with the EMI filter. It clearly shows that the blue curves are already lower than the EN55022B standard.

Fig. 1. (b) Small-signal model of the converter.
Fig. 2. Figure 2 EMI noise measurement of MHz flyback converter prototype.