This article presents a novel reduced order average model for dual-active-bridge converter which can be applied to all modulation methods, such as single-phase-shift modulation, dual-phase-shift modulation, extended-phase-shift modulation and triple-phase-shift modulation. This model considers conduction, inductance, and transformer power losses. Furthermore, the input-output filters are suitable for system performance analysis. Based on the large-signal average model, the small-signal model and the output transfer function are derived. The detailed model for predicting the large-signal transient is established and the small signal analysis in frequency domain is carried out. This model can accurately predict the steady-state and transient-state response of the system. The experimental results agreed well with the simulation results which proved the accuracy and correctness of the model.

An isolated high step-up converter, which is derived from the reverse structure of a regular LLC resonant converter, is proposed in this paper. The proposed converter consists of an inverter stage, a secondary-side resonant tank and an active voltage-doubler rectifier. An additional input diode is introduced into the regular push-pull inverter so to make the magnetizing inductance free from being always clamped by the input voltage, and make up new voltage gain features. The leakage inductance, which always shows a significant value when reflected to the secondary side in a high step-up transformer, can be utilized to make up the resonant tank. Under a simple Pulse-Frequency-Modulation control with 0.5 duty-ratios for both the primary-side and secondary-side switches, high step-up function can be easily realized by the converter with good load regulation ability. Meanwhile, synchronous rectification (SR) can be naturally achieved by the secondary-side switches. The additional input diode can be replaced by a MOSFET with an analog SR chip in order to further improve efficiency, without changing the original control scheme. The validity of the study is confirmed by a 500W prototype with 36V~42V input and 380V output. The peak efficiency can reach 96.9%.

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.

A transformer winding method that can realize an adjustable turns ratio is proposed. In this article, the LLC resonant converter is taken as an example, where the full-bridge (FB) LLC converter is realized by two split half-bridge (HB) units in parallel. The numbers of the switch and diode are still the same as those of a regular FB LLC converter. By controlling the phase shift angle of the two HB units, the magnetic flux of the central column of the transformer core can be adjusted, so the turns ratio can be changed. In any operating condition, the two HB units always share the same power stress, which is only half of the total. A 500 W prototype with an input voltage range of 160∼480 V and an output voltage of 120 V is built, which shows a frequency range of 67–95 kHz covering a wide load range from 5% to 100% rated power.

Dual-Active-Bridge Series-Resonant-Converter (DAB-SRC) is widely applied due to its simple circuit structure. A lot of literature has proposed variety kinds of control strategies to ensure the soft switching realization. However, problems such as complex control algorithms, intricate parameter design processes, and a high number of feedback control variables still persist. This paper introduced a variable frequency & single phase shifted (VF&SPS) control strategy based on the time-domain analysis, which employing only two control variables (frequency and phase shifted angle) and only two frequently-used feedback variables (output voltage and current). The soft switching region and the gain model are presented by visualization method so the converter's operating region is clear. By establishing a linear function between normalized frequency fn and the quality factor Q (fn=k*Q+b), the operating track of the converter is restricted and the soft-switching is also ensured even with a constant switching dead-band time. The output is finally regulated by the phase shifted angle ȷ which is generated by a proportional integral compensator. The correctness of the theoretical analysis is verified by a 1.2kW bidirectional prototype with a maximum efficiency of 97.9%.

Resonant converter is a good solution as the isolated high step-up converter due to its soft-switching features and less component count. However, since the resonant tank is placed on the primary side (low-voltage side) in most traditional resonant converters, large input current still leads to a big proportion of power loss. Besides, synchronous rectification (SR) is also a big issue. The diode conducting time in a resonant converter is hard to be predicted so a current transformer or an analog SR chip is usually employed to detect the current zero-crossing point when the higher efficiency is required. In this article, an isolated high step-up converter with active secondary-side quasi-resonant tank is proposed. Since the whole resonant tank is placed on the high-voltage side, the current stress of the resonant capacitors and their power loss can be greatly alleviated. Besides, the analytical solution of the diode conducting time is derived and two SR control schemes are proposed, without the need to detect the current zero-crossing point. All the switches and diodes achieve the soft-switching so this leads to a good efficiency performance. The validity is confirmed by the experimental results of a 500 W prototype with 36–48 V input and 380 V output.