Simulating the EMI characteristics of flyback DC/DC converters

In this article, the influence of diode reverse recovery on EMI performance of a low power flyback converter is investigated. Two different diode models (SPICE and SABER
^®
) are analyzed and the conducted emissions from a simulated converter with a thorough characterization of the parasitic elements are compared to measurements. It is concluded that the emission levels to a large extent is determined by the mutual couplings within the input filter. The proposed simulation model predicts EMI levels up to 20MHz with good accuracy. Furthermore, it is concluded that the diode reverse recovery is reflected into the conducted emissions. For the investigated case, a recovery peak of I
_RR
=2.8A gives an increase in emission levels with 3dB around 10MHz compared to a diode with I
_BR
=1.0A. Finally, it was found that Schottky diodes may decrease the EMI performance of the converter; the large inherent capacitance can cause large oscillations in the circuit that are reflected into the conducted emissions.