Analysis and solution to voltage unbalance of the flying capacitor in ZVS three-level DC/DC converter

In order to lower the voltage stress on power devices, the three-level topologies are widely used in high voltage converters. In the topologies a flying capacitor is added to achieve true ZVS under phase shift control. But the ZVS three-level converter will run abnormally at light load if the flying capacitor is added. The main issue is the charge unbalance across the flying capacitor. Due to the unbalance, the voltage stress on the switches of lagging leg becomes high so much as 500 V under 800 V bus condition. This issue will also appear under open voltage loop condition (this condition may occur in dynamic load). Aiming at this issue, the reason of charge unbalance of the flying capacitor is firstly analyzed under the light load and open voltage loop in this paper. An experiment is implemented for testifying the analysis. A novel method is presented for this issue. This method not only effectively balances the charge of the capacitor but also is lossless.