A Capacitor-Cross-Connected Boost Converter With Duty Cycle < 0.5 Control for Extended Conversion-Ratio and Soft Start-Up

The series-capacitor boost converter (SCBC) requires a duty cycle (
$D) \ge0.5$
control, otherwise has issues of power switch overstress and inductor currents imbalance. Then, we cannot use the SCBC in wide conversion ratio (
CR
) applications that require
CR
< 4. Meanwhile, the
$D \ge0.5$
control does not allow
$D$
to increase smoothly from 0 during start-up, causing severe inrush inductor currents. As a solution, we propose two control schemes on the capacitor-cross-connected (CCC) boost converter, which is the symmetric version of the SCBC. With the proposed hard-charging control, the CCC converter allows
$D < 0.5$
without overstress or imbalance issues, thus extending the
CR
range and facilitating a soft start-up. In addition, we propose a soft-charging control that enhances the efficiency for 3<
CR
< 4. We study the output impedance and
CR
expressions under these two types of controls. Finally, we propose a compact centrosymmetric floorplan that reduces the switching nodes’ ringing, verifying the proposed schemes with a 2.8-to-8.4-V input / 24-V output prototype converter. It exhibits a soft start-up with the proposed control. Under an 8-V input voltage and 1.5-A output current, the peak efficiency of the proposed hard-charging and soft-charging control is 94.7% and 96.3%, respectively.