A Control Strategy for the Bidirectional LLC-L Converter to Extend the Gain Range Based on the Time Domain Analytical Model

A control strategy to shrink the switching frequency range and widen the gain range for the bidirectional
inductor
-
inductor
-
capacitor
(
LLC
) -
inductor
(
L
) converter is proposed. The phase-shifted angle (
ϕ
) between the rectifier stage and the inverter stage is introduced so as to increase the control freedom degree besides the switching frequency (
f_s
). The analytical expression of the voltage gain with respect to
f_s
and
ϕ
is solved by the precise time-domain analysis. Based on this, a function between
f_s
and
ϕ
is proposed so as to realize a steep slope for the
M_g
–
f_s
curve. A design procedure is also presented so as to ensure a wide soft-switching range. The proposed control strategy employs the same 0.5-duty cycles for both the primary and the secondary side switches. By this means, the rectifier stage always operates with a natural synchronous rectification without the need of detecting the current zero-crossing point. The correctness of the theoretical analysis is verified by a 1-kW experimental prototype, where the switching frequency only changes 22.6% within a load variation from 20% to 100%. Besides, the superiority of the proposed control strategy is also verified by the experimental comparison with the traditional pulse frequency modulation control strategy.