Design and implementation of phase-shifted full-bridge converter with a current doubler for HV to LV battery charger in electric vehicles

Abstract

Internal combustion vehicles have been widely used for many years. With the increase in population, the usage rate of internal combustion vehicles also increases. This situation causes significant increases in greenhouse gas emissions. Therefore, increasing emission leads to the growth of the global warming problem worldwide. Many protocols such as the Paris Agreement, have been signed between countries in order to solve this issue. Moreover, efforts are made to increase environmental awareness and reduce people's carbon footprint. Electric vehicles (EVs) have recently become widespread within this scope. It is aimed at making the use of electric vehicles more prevalent. With the extensive use of EVs, different needs have begun to emerge in many different areas. The most prominent of these needs are related to battery systems, power electronic converters, and traction systems. There are two types of batteries in electric vehicles: high-voltage and low-voltage batteries. High voltage batteries have many different types when classified according to their chemical content. The high-voltage battery is used for the traction of the vehicle, and the power is transferred to the electric motor with the inverter, which is one of the power electronic converters in EVs. Another power electronics unit, which is the onboard charger, is required to charge this high-voltage battery. Even though electric vehicles have high voltage battery, low voltage battery is still needed. The low-voltage battery plays a role in feeding the in-vehicle loads. There is another DC-DC converter between this low-voltage battery and the high-voltage battery. A DC-DC converter is a converter that charges the low-voltage battery and supplies all accessories in the EV, such as air conditioner, headlights, sound systems, lighting, electronic control units, wiper, and window system. This converter is used in electric vehicles instead of the belt-driven generator in internal combustion vehicles. Today, due to the increase in vehicle electrification, accessories, and features, the amount of power demanded from DC-DC converters is also increasing. In order to meet this increase, this DC-DC converter must be able to provide very high currents. High current also brings high transmission losses, so it is essential to design properly with appropriate topology selection. In addition, it is an important criterion that DC-DC converters must be efficient due to continuous operation when the vehicle is in use. This means that a DC-DC converter affects the range or vehicle. In addition to efficiency, the converter must also be suitable for placement in the vehicle in terms of volume, owing to the limited space in the vehicle. Therefore, the DC-DC converter is expected to have a high power density (kW/L). As mentioned, since the DC-DC converter converts power between high-voltage and low-voltage batteries, another important point is that these converters are isolated to ensure safe operation. These are the basic features expected and required from a DC-DC converter to be used for electric vehicles. This thesis studies the design and implementation of a high power density and efficient isolated DC-DC converter for electric vehicles. First of all, a short history of EVs from past to present is shared. Brief information about battery electric vehicles, plug-in hybrid electric vehicles, hybrid electric vehicles, and fuel-cell electric vehicles is given. Afterward, high-voltage and low-voltage batteries were mentioned and their properties are shared by classifying them according to their chemical construction. In addition to information about the types and general features of today's EVs, the future of electric vehicles is given. Isolated and unidirectional converters are investigated and evaluated in terms of power rating, efficiency, cost, volume, power density, input and output voltage, advantages and disadvantages. As a result, a phase-shifted full-bridge converter (PSFB) with a current doubler rectifier (CDR) is selected as a suitable converter. The main reasons for this selection are that PSFB with CDR have a wide voltage gain range operation ability, low output current ripple, and ZVS operation capability with constant switching frequency. Moreover, reduced conduction loss, better thermal performance, simpler transformer structure, and lower ripple load current can be achieved thanks to CDR. Operating principles of PSFB with CDR converter and related waveforms of each operating mode are given in detail. All analytical calculations about switches, transformer, critical timings, inductors, capacitors and ZVS condition are given. Information on selected components is shared according to calculations. Moreover, design calculations to manufacture the transformer which is a planar transformer and inductors which are output and shim inductor are given. Another important step, which is the decision on a suitable control method. Voltage mode, average current mode and peak current mode control (PCMC) methods are explained inclusively. PCMC method is selected as a suitable control method. The operating principle and implementation of the selected microcontroller of PCMC are explained. Small-signal model and transfer function of PSFB with CDR converter are obtained. Controller parameters of the PCMC method are determined with the modeled transfer function in MATLAB/Simulink. Open loop and close loop behavior are compared. Afterwards, the converter is simulated with the real model of selected components, which are provided by the manufacturer, in LTspice. All results and waveforms are obtained for the operation at nominal operating conditions with different load currents. In addition, the clamping diodes effect is evaluated and results are shared. PSFB with a CDR converter is designed and implemented. General information about the design and parts of the implemented design are shared. Laboratory tests are completed under the nominal operating conditions with different load currents as the simulation. Detailed experimental results and waveforms are shared and compared with simulation results. As a result of the studies carried out within the scope of this thesis, the efficiency is obtained according to the simulation result at the nominal operating point for 250 A load current as 96.54%. The efficiency obtained from laboratory tests for the same operating point is 93.43%. The reason for this difference in efficiency is conduction losses, which become dominant in the printed circuit board at high currents, and thermal effects, which do not affect the simulation. According to the results of laboratory tests, the highest efficiency is 95.19% at the nominal operating point when the load current is 150 A. Efficiency decreases for load currents which are higher than 150A because of the more dominant conduction losses. It can be observed that the analytical calculations, simulation, and test results are compatible with each other. Another outcome of the thesis is that the average power density of the different manufacturers' designs is 1.2 KW/L, while the power density of the realized design is 1.87 kW/L. Finally, improving efficiency and conduction losses at higher currents are shared as future work. Furthermore, in order to adapt to technological developments, the converter can be revised to operate bidirectionally and at input voltages up to 800 V.