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Leonardo Myers
Leonardo Myers

Bidirectional Buck Boost Converter Topologies !!INSTALL!!



Abstract:In this article, the practical comparison of the operational performance of the modular (or multiport) and non-modular bidirectional buck/boost (bi-BB) DC/DC converter is realized. The main contribution of the work is the evaluation of both concepts based on various aspects, considering the qualitative indicators of the systems relevant for microgrids. Here, we discuss efficiency, electrical properties, costs, and component values. At the same time, critical comparisons are provided for converters based on SiC and GaN technology (non-modular high-voltage SiC-based dual-interleaved converter and modular low-voltage GaN-based). The concepts are specific with their operating frequency, whereby for each solution, the switching frequency is different and directly influences relevant components. The efficiency, overall system volume, output voltage ripple, and input current ripple are compared mutually between both concepts with a dependency on power delivery. These factors, together with overall volume and costs, are very important considering modern converters for microgrid systems. The summary of pros and cons is realized for each of the proposed converters, whereby the evaluation criterion is reflected within the electrical properties targeting microgrid application.Keywords: bidirectional converter; high efficiency; GaN; SiC; buck-boost converter; high switching frequency




Bidirectional Buck Boost Converter Topologies


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In the inverting topology, the output voltage is of the opposite polarity than the input. This is a switched-mode power supply with a similar circuit topology to the boost converter and the buck converter. The output voltage is adjustable based on the duty cycle of the switching transistor. One possible drawback of this converter is that the switch does not have a terminal at ground; this complicates the driving circuitry. However, this drawback is of no consequence if the power supply is isolated from the load circuit (if, for example, the supply is a battery) because the supply and diode polarity can simply be reversed. When they can be reversed, the switch can be on either the ground side or the supply side.


When a buck (step-down) converter is combined with a boost (step-up) converter, the output voltage is typically of the same polarity of the input, and can be lower or higher than the input. Such a non-inverting buck-boost converter may use a single inductor which is used for both the buck inductor mode and the boost inductor mode, using switches instead of diodes,[2][3] sometimes called a "four-switch buck-boost converter",[4] it may use multiple inductors but only a single switch as in the SEPIC and Ćuk topologies.


Like the buck and boost converters, the operation of the buck-boost is best understood in terms of the inductor's "reluctance" to allow rapid change in current. From the initial state in which nothing is charged and the switch is open, the current through the inductor is zero. When the switch is first closed, the blocking diode prevents current from flowing into the right hand side of the circuit, so it must all flow through the inductor. However, since the inductor doesn't allow rapid current change, it will initially keep the current low by dropping most of the voltage provided by the source.


The four-switch converter combines the buck and boost converters. It can operate in either the buck or the boost mode. In either mode, only one switch controls the duty cycle, another is for commutation and must be operated inversely to the former one, and the remaining two switches are in a fixed position. A two-switch buck-boost converter can be built with two diodes, but upgrading the diodes to FET switches doesn't cost much extra while efficiency improves due to the lower voltage drop.


With the massive use of efficient energy accumulators (batteries and super capacitors), the trend is towards a better management of the electric current. A bidirectional DC/DC converter can keep the battery healthy and extend its life.


The entire article has been dedicated to covering the current state of the art in bidirectional DC-DC converter topologies and their smart control algorithms identifying the research gaps and concluding with the motivation for taking up the work. It covers the literature survey of bidirectional buck-boost DC-DC converters, and control schemes are carried out on two aspects, one is on topology perspective and another one is on control schemes. Different topologies with and without transformers of bidirectional DC-DC converters are discussed. Non-isolated converters establish the DC path between the input and output sides while transformer-based converters cancel the DC path in between the input and output sides since it introduces AC line between two DC lines just like in a flyback converter. The transformer-less converter is preferred when there is no much protection needed for load from high voltage levels, also these converters are used in high-power applications. The bidirectional DC-DC converter can switch the power between two DC sources and the load. To do so, it has to use proper control schemes and control algorithms. It can store the excess energy in batteries or in supercapacitors. In contrast, isolated topologies contain transformers in their circuits. Due to this, it offers advantages like safeguarding sensitive loads from high power which is at the input side. In addition to it, multiple input and output ports can be established. With the isolation in DC-DC converters, input and output sections are separated from the electrical standpoint of view. With isolation, both input and output sections will not be having a common ground point. The DC path is removed with isolation due to the usage of the transformer in DC-DC converters. In contrast to its features, it is capable to be used in low-power applications since the transformer is switching at high frequency, the size of the coil reduces and hence it can handle a limited rate of current. The bidirectional DC-DC converters are categorized based on isolation properties so-called isolated bidirectional converters. Features and applications of each topology are presented. Comparative analysis w.r.t research gaps between all the topologies are presented. Also, the scope of control schemes with artificial intelligence is discussed. Pros and cons of each control scheme, i.e. research gaps in control schemes and the impact of control schemes for bidirectional DC-DC converters, are also presented.


In the technology of power electronics, bidirectional power converters are identified as significant subsystems in the design of systems where power flow is required to flow in both forward and reverse directions. The control technology for power converters has been modernized over the past few decades. The advancements in the semiconductor industry have helped in the easier implementation of the control strategies. The power converter can find its own importance in the system of any applications only when the power converter can offer tight regulation of load and line of the power source. On top of that efficiency is also prime factor in deciding the best power converter. For that, entire controller plays a vital role to obtain tight regulation and efficiency. The next point of interest in choosing the best power converter is that reliability of power converter which can be enhanced with functions of monitoring the system added to it. In addition to it, optimization of controllability and minimization of component count and flexibility of settings in the control circuit [1,2,3,4,5,6,7,8]. The general block diagram of bidirectional converter is as shown in Fig. 1. Based on type of current mode control or voltage mode control, the control system regulates the voltage or current in the system. The DC-DC converter is a power switching system in power electronics. It accepts DC signal of certain voltage and converts it into another DC signal with certain voltage. The voltage levels will change in input and output sides. But on either side of input and output power levels remains same, i.e. power is not amplified. It is widely used in battery charging and discharging applications with constant voltage and constant current so that battery like is increased.


The topologies of BBBC are divided into two main types such as isolated bidirectional DC-DC converter and non-isolated bidirectional DC-DC converters. The non-isolated topologies convert one level of DC-voltage to another level of DC-voltage, and they do not contain transformer which offers galvanic isolation in the system of circuits. Therefore, these topologies lack the advantages like high step-up voltage gain ratio and isolation between source and load. Nevertheless their weights are reduced since no transformer is used and system is going to be compact without transformer. When the transformers are used with converter, it generates reactive power in supply lines so more compensation is required. Since transformer is used with high frequency, size of the coil reduces and hence the size of the transformer reduces; therefore, it cannot handle high current. Also the transformer usage with the converter can cause core loss and skin effect with the conductor. With all these impacts non-isolated converter finds the applications in high-power applications (Fig. 6).


This converter is the first basic converter in the family of BBBC. In particular, its transformer-less bidirectional DC-DC converter as shown in Fig. 7a. It replaces two different basic converters for bidirectional power flow and hence reduce the component count. It pushes the energy from VH to VL in step-up time while it pushes the power from VL to VH in step-down time. The mode of transition is an automatic process by the controller. It can find the applications starts from renewable energy systems to automotive systems.


The voltage boosting ability of the converter is increased by the cell made up of switched capacitor. Figure 7f shows the switched capacitor-based BBBC. The bidirectional switched cells are made with the unidirectional switched cells [22]. In this topology, there is no magnetic utilization and high weight of the converter since there is no inductor used. By connecting strings made up of cells in parallel results in availing a continuous input current and those can be operating in all additive storing cells.


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