Common-Mode Noise Reduction and Capacitor Voltage Auto-Balance Using Bridged Midpoints and Coupled Inductor in a 3-L Buck-Boost Converter

The symmetry principle has been widely used to mitigate the power stage common-mode (CM) emissions. Based on a symmetrical three-level (3-L) buck-boost converter in [1], this letter finds a midpoints interconnection that can reduce the CM noise further and realized the dc-link voltage auto-balance at the same time. By bridging the input and output midpoints, the noise is trapped inside the power converter and a 30 dB CM noise reduction is achievable on the line impedance stabilization network (LISN) side. Also, a combination of bridged midpoints and coupled buck-boost inductor grants the converter with a desirable dc-link capacitor voltage auto-balance feature. Furthermore, uniquely the voltage ratio between the split dc-link capacitors is tightly related to the turn ratio of the coupled inductor, meaning the auto-balance nature is valid also for asymmetrical dc-link voltage cases. Detailed modeling and analysis are provided in this letter to explain the observations. All the analyses are verified on a custom built SiC-based 30 kHz 50 kW prototype. This letter is accompanied by two videos to visualize the discussed voltage balancing feature.