Optimizing EMI Input Filters for Switched Mode Power Supplies

Almost every switched-mode power supply (SMPS) needs an EMI (Electro Magnetic Interference) input filter to suppress any disturbances of the SMPS on the power lines. This requirement having an input filter in the design ensures that no negative effect will occur in other parts of the systems connected to the power lines. Therefore, the design and the validation of the input filter is a major task during a typical power supply design. The conducted emission (CE) test according to a specific standard is a suitable and a common validation method to release the design at the end of the development cycle. Nowadays, this conducted emission test will be performed as pre-compliance test during the development phase in the lab as well. In this case, the designer will obtain an early feedback whether the design has to be optimized regarding any disturbances present on the power lines. In most cases, the designer has to adjust the input filter to obtain a more effective suppression of disturbances generated by the SMPS. However, the designer needs to know details about the noise spectrum to optimize the input filter as effective as possible. In addition to magnitude and frequency information of the noise source, it would be very helpful to know whether the noise is generated by a common mode source or by a differential mode source. During the standard conducted emission test, common and differential mode noise is a combination in the measurement results and thus not possible to gain deeper insights. This document will describe a method to separate common-mode and differential-mode separation using two oscilloscope channels. This separation approach works without any additional hardware component like a noise separator. The designer will be able to distinguish between common-mode (CM) and differential-mode (DM) noise. This additional information about the dominant mode provides the capability to optimize input filters very efficiently.