# Design and Control of a Battery Charger/Discharger Based on the Flyback Topology
###### Abstract  
Devices connected to microgrids require safe conditions during their connection, disconnection and operation. The required safety is achieved through the design and control of the converters that interface elements with the microgrid. Therefore, the design of both power and control stages of a battery charger/discharger based on a flyback is proposed in this paper. First, the structure of a battery charger/discharger is proposed, including the battery, the flyback, the DC bus, and the control scheme. Then, three models to represent the battery charger/discharger are developed in this work; a switched model, an averaged model, and a steady-state model, which are used to obtain the static and dynamic behavior of the system, and also to obtain the design equations. Based on those models, a sliding-mode controller is designed, which includes the adaptive calculation of one parameter. Subsequently, a procedure to select the flyback HFT, the output capacitor, and the \(K_{\mathrm{v}}\) parameter based on operation requirements of the battery charger/discharger is presented in detail. Five tests developed in PSIM demonstrate the global stability of the system, the correct design of the circuit and controller parameters, the satisfactory regulation of the bus voltage, and the correct operation of the system for charge, discharge and stand-by conditions. Furthermore, a contrast with a classical PI structure confirms the performance of the proposed sliding-mode controller. adaptive control parameter; battery interface; circuital simulations; co-design; HFT; safety requirements +
Footnote †: journal: Applied  
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balance and return the DC bus voltage to its nominal value. Finally, when generation and load are equal, the ESD does not supply or absorb power (stand-by). Moreover, it is worth noting that the DC bus voltage regulation is important in a microgrid to provide safe operating conditions for the loads since most of the RESs work as current sources assuming a regulated DC bus voltage. An ESD charging/discharging system is typically formed by a storage device, a bidirectional power converter, and a control system. The DC bus voltage (\(v_{dc}\)) is usually greater than the battery voltage (\(v_{b}\)); hence, bidirectional step-up converters like Boost [2; 3; 4] or Buck-Boost [5; 6] are commonly used. Considering that these converters have limited voltage gains, a typical value of \(v_{b}\) is 12 V [7; 8; 9], and the values of \(v_{dc}\) in microgrid applications vary in wide range, but are significantly greater than \(v_{b}\) (e.g., 48 V [10; 11; 12], 200 V [13]), or higher [14]; then, multiple batteries are connected in series and parallel to reach the required voltage and storage capacity levels, respectively. Those batteries have different electrical characteristics due to manufacturing imperfections, uniform aging, and continuous charging/discharging cycles; these differences produce excessive heat and accelerated degradation of the batteries [15]. That is why there is a necessity for battery balancing systems [15; 16] or charging/discharging systems implemented with high-gain power converters to connect each battery, or arrays of parallel batteries, directly to the DC bus. Such a charging/discharging system would avoid the necessity of a balancing system providing some advantages like a high ESDs modularity, and the connection of batteries with several technologies and electrical characteristics. Moreover, this charging/discharging system would permit a simple connection and disconnection of batteries to adjust the storage capacity and the use of second-life batteries coming from applications like electric vehicles. A bidirectional flyback converter is a feasible option to implement such an ESD charging/discharging system since it provides galvanic isolation between the ESD and the DC bus, it has a simple structure, and it can provide high voltage gains.