Isolated DC/DC converter with wide input voltage or output voltage range is featured in many applications such as renewable energy generation system, electrical vehicle chargers and LED drivers. LLC resonant converter is an attractive topology which has significant advantages, such as soft switching, high efficiency and low EMI issue. Generally, LLC resonant converter adopts pulse frequency modulation (PFM). However, in order to obtain a wide voltage range, the switching frequency of LLC resonant converter needs to be away from the resonant frequency which could generate high circulating energy and reduce the efficiency.
In order to realize wide range voltage regulation in a narrow switching frequency range, many improved control schemes have been proposed. Such as the phase-shift modulation (PSM) and pulse width modulation (PWM) scheme. But the peak current of the secondary rectifier is increased. The hybrid control combining PFM and PSM or PWM is also an effective method. This method takes into account the capacity of voltage regulation and working efficiency, but the control strategy becomes more complex. Besides, the reconstruction of circuit can also help to extend the voltage gain range. Mode switching of full bridge and half bridge can obtain a double gain range. However, it is difficult to switch smoothly between the two modes. Auxiliary switches or components are also commonly adopted to extend the voltage range with narrow switching frequency. Unfortunately, the complex of model and control are increased. Although the improvement of modulation or topology can increase the range of voltage regulation to a certain extent, the control and operation modes of the converter become more complex. To deal with this, the two-stage cascade topology consisting of a non-isolated DC/DC converter and an unregulated LLC resonant converter working as DC transformer (DCX) can be used. In this kind of topology, the voltage regulation is only realized by the traditional non-isolated DC/DC converter, and the voltage regulation ability is improved compared with the single stage converter. However, the extra stage introduces more switches and components, which result in low power density and high cost. In order to solve this issue, the integrations of power switches are studied. For example, Boost converter can be integrated with resonant converter by sharing switches. But there are still some shortcomings. First, the integrated topology still needs to add additional Boost inductors, which will reduce the overall power density. Secondly, the analysis of voltage gain and soft switching in the integrated structure will become complex.
In this paper, a coupled inductor integrated series resonant converter (CISRC) with wide voltage range and load independent voltage gain is proposed. Only one magnetic component and two switches are used in proposed CISRC. Compared with the traditional two-stage cascade converter, the number of magnetic components and switches is effectively reduced. Under the proposed modulation, the primary side switch can realize zero voltage switching (ZVS) and the secondary side diode can realize zero current switching (ZCS). In addition, this paper also proposes an adaptive frequency control without current sensor. By observing the average value of the excitation current, the proposed CISRC can adjust the switching frequency adaptively and optimize the current ripple under different power. The working characteristics and control method of the proposed CISRC are described in detail, and a 200 W experimental prototype is built. The experimental results prove the feasibility and effectiveness of the proposed scheme.
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2023, Vol. 38 
