Duty Cycle Exchanging Control for Input-Series-Output-Series Connected Two PS-FB DC-DC Converters

Input-series-output-series (ISOS) connected dc-dc converters enable low voltage rating switches to be used in high voltage input and high voltage output applications needing electrical isolation. In order for ISOS connected two phase-shifted-full-bridge (PS-FB) dc-dc converters to achieve input voltage sharing (IVS) and output voltage sharing (OVS), duty cycle exchanging control is implemented by exchanging their duty cycles. Based on small-signal analysis, the stability mechanism of ISOS connected full-bridge converters with independent OVS control is revealed by using the Routh-Hurwitz criterion. Loop gain design is also accomplished by employing double-closed-loop control based on the duty cycle exchanging control. With the proposed scheme, although sensing individual input voltages is avoided, excellent IVS and OVS can be achieved not only during steady state but also during transients facing turns-ratio mismatches. The performance of the proposed control strategy is verified by experimental results from 1.6 k-W ISOS connected two full-bridge dc-dc converters.