Valley Current Control for the Flying Capacitor Voltage Balancing in the Three-Level Boost Converter with Variable Switching Frequency

Valley current control (V-CMC) in the three-level flying capacitor (FC) boost converter in case of triangular conduction mode (TCM) operation under variable switching frequency $\left(f_{\mathrm{SW}}\right)$ and $T_{\mathrm{ON}}$ control is discussed in this paper. Main differences of this operating mode with the results previously presented in the literature for the constant switching frequency operation in terms of static instability of the inductor current and FC voltage are discussed. It is shown that the issue of subhamronic oscillations that exists in case of constant $f_{\mathrm{SW}}$ operation and demands inclusion of the compensation ramp, does not exist in case of variable switching frequency operation. Detailed theoretical analysis is conducted on the subject of the FC voltage imbalance and it is concluded that under equal magnetizing times in both halfs of the period, and balanced corresponding dead-times, FC voltage is perfectly balanced. On the other side, in case of no consumption at the output, it is discovered that, even under theoretically ideal conditions, charge balance of the flying capacitor for boosting factor higher than 2 can be achieved for any value of $V_{C \mathrm{fly}}$ voltage. This leads to the static imbalance of the flying capacitor voltage for $V_{\mathrm{IN}}\lt V_{\mathrm{OUT}} / 2$ and no-load conditions. In order to solve the problem of the previously described FC voltage imbalance, a very simple correction in the control loop is proposed that does not require any additional hardware and it is based on a very simple manipulation of the data that is already available in the FPGA. All the theoretical analysis presented is verified by the comprehensive prototype measurements under all theoretically analyzed conditions.