Batarya şarj uygulamalarında kullanılan LLC rezonans çeviricilerde optimum verim eldesi için yeni bir yöntem
A novel method on obtaining optimal operation efficiency of LLC resonant converters in battery charging applications
Yazar: ESER ÇALIŞKAN
Danışman: DOÇ. DR. ÖZGÜR ÜSTÜN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Elektrik Mühendisliği Ana Bilim Dalı / Elektrik Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Akümülatör şarjı = Battery charge ; Optimum verim = Optimum efficiency ; Rezonans çevirici teknoloji = Resonant converter technology

Özet:
Dünya genelindeki nüfus artışı ve globalleşme, mobilite kavramını tetiklemiştir. Mobilite ile yeni teknolojilerin hayatımıza girmesi kaçınılmaz olmuştur. Yeni teknolojilerin hayatımıza girmesi her geçen gün artan enerji talebini beraberinde getirmektedir. Günümüzde ulaşımda enerji talebinin büyük bir kısmı petrol ve petrol türevleri olan yakıtlar tarafından karşılanmakta olup gelecekte alternatif enerjilerin kullanıma alınmasını zorunlu kılmaktadır. 
Hayatın birçok alanında mobil olma ihtiyacının yanı sıra bunun bir sonucu olarak ortaya çıkan enerji gereksiniminin de mobiliteye hizmet edecek şekilde taşınabilir ve paylaşımlı olması kaçınılmazdır. Mobiliteye en çok hizmet eden cihazların başında elektrikli araçlar gelmekte olup her geçen gün yeni bir model piyasaya sürülmektedir. Elektrikli araçlar ve neredeyse tüm mobil cihazlarda enerji ihtiyacı büyük çoğunlukla dahili bataryalar ile sağlanmakta olup şarj ve deşarj işlemleri ile enerji paylaşımı sağlanabilmektedir. Batarya şarj ve deşarj döngüsünde enerji kayıplarının en az seviyeye indirilebilmesi için kullanılan güç çeviricisi tüm çalışma bölgesinde en yüksek verim ile çalıştırılmalıdır. Güç elektroniği çeviricisinin mümkün olan en yüksek verim ile çalıştırılabilmesi amacıyla farklı kontrol yöntemleri ve devre topolojileri geliştirilmektedir.
Bu doktora tez çalışmasında, yeni tip GaN anahtarlama elemanları kullanılan bir LLC rezonans çeviriciye yönelik yeni bir verim optimizasyonu yöntemi üzerinde durulmuştur. Hafif elektrikli araçlar için tüm batarya şarj sürecinde en yüksek verim ile güç akışı kontrolünün en iyileştirilmesi amacıyla yeni bir verim optimizasyonu algoritması geliştirilmiştir. Klasik kontrol yöntemi olan frekans modülasyonu (FM), ölü zaman kontrolüne dayanan S-PWM ve kesintili çalışma modları LLC rezonans çeviricinin verim değerinin tüm batarya şarj sürecinde mümkün olan en yüksek seviyede kalması amacıyla birlikte kullanılmıştır. 
İlk olarak potansiyel batarya şarj topolojileri incelenmiş olup ardından bir rezonans çevirici kullanılarak klasik bir batarya şarj sürecine ait grafik verilerek şarj bölgeleri ve temel verim problemi ele alınmıştır. Düşük ve yüksek yük durumları arasındaki farklar ve rezonans çeviricinin çalışma karakteristiği birlikte değerlendirilerek özellikle düşük yük durumlarında çevirici veriminin düşmesine ait detaylar aktarılmıştır. 
Problemin tanımının ardından GaN tipi anahtarlar kullanılan bir LLC rezonans çevirici ile alakalı teorik altyapıya değinilmiş olup yapılan detay tasarımlar, hesaplamalar, elektronik kartlara ait şema ve baskı devre çizimleri, VHDL blokları ve tasarımları, kart testleri ve doğrulaması verilmiştir. 
LLC rezonans çevirici tasarımlarını takiben üç farklı anahtarlama ve kontrol yöntemine ilişkin modelleme ve benzetim çalışmalarına yer verilmiştir. Benzetim çalışmalarında temel çalışma prensipleri ve modeller, batarya şarj işlemi ve temel dalga şekilleri verilmiştir. 
Benzetim çalışmalarının ardından yapılan tasarım detaylarına göre üretilen ve entegre edilen deney düzeneği üzerinde üç farklı anahtarlama yöntemine ait testler gerçekleştirilmiştir. Deneysel testlerin sonuçlarına göre iteratif olarak önerilen verim takibi algoritması iyileştirilmiştir. 
Sonuç olarak önerilen algoritmanın batarya şarj sürecine uygulanması ve oluşturduğu etki tartışılmıştır. Önerilen verim takibi algoritması ile batarya şarj sürecinde kullanılan LLC rezonans çeviricinin toplam verim değerinde özellikle düşük yük durumlarında %25'e varan artış gözlenmiştir. 
Tez çalışmasında, yeni bir verim takip algoritması ortaya koyularak GaN temelli bir LLC rezonans çevirici üzerinde hafif elektrikli araçlara ait bir batarya şarj uygulamasında testleri ve doğrulaması yapılmıştır. Sonuçlar değerlendirilmiş olup gelecek çalışmalar için bir yol haritası çıkarılmıştır.

Abstract:
Population growth and globalization triggers the mobility concept. With the mobility, it has become inevitable for new technologies to enter our lives. New technologies bring with them increasing energy demand. Today, a large part of the energy demand in transportation is met by fuels with petroleum derivatives, and it necessitates the use of alternative energies in the future.
The energy requirement arises as a result of the mobility requirements in many areas of life. Energy should also be portable and shared in a way that serves mobility. Electric vehicles are one of the most supporting devices of mobility and a new model is being launched every day. Energy demand in electric vehicles and mobile devices is mostly achieved by internal batteries and energy transfer between mobile devices, i.e., vehicle-to-vehicle and device-to-device, can be achieved through power converters for charging and discharging directions of the batteries. On the other hand, fast charging is essential to make electric vehicles more desirable. Nowadays, long charging times seem to be the number one reason for the fading tendency to use more electric vehicles. Solutions for shorter charging times lie in more powerful charging supplies and high efﬁciency power electronic circuits. In order to minimize energy losses in the battery charging and discharge cycle, power converters shall be operated with the highest possible efficiency in the entire charging cycle. Researchers are pursuing the design of smaller and lighter charging circuits with exceptionally high efﬁciencies. Therefore, fast charging of batteries, especially Li-Ion batteries, is an eventual target for today's power electronic circuit designers. New methods or circuit topologies are proposed continuously for improving fast charging process. In that effort, the LLC resonant converters have a special place due to their high power densities, higher efﬁciencies, small size and lighter structures. Because of their unique features, they are capable of transferring DC power with the highest possible efﬁciency in both directions, i.e., grid-to-battery and battery-to-grid.
One of the most critical issues in the battery charging and discharging cycle is increasing the average efficiency of the total cycle. Since energy transfer between the vehicles and the grid is done via LLC resonant converters in each of the charging and discharging directions, the efficiency of the LLC resonant converter shall be kept at the highest level in the entire cycle. Power converters usually operate at high efficiency under high rated power and inversely the LLC resonant converters are at the lowest levels in terms of efficiency in light-load situations. Because of the high variability at the output and input voltages and currents of the LLC resonant converters, the average efficiency of the LLC converter is minimal, especially at low loads. There are basically two zones in battery charging cycle. One of them is constant current or fast charging zone and the other one is constant voltage zone which can be also called as slow charging zone. During constant current charging, battery voltage increases almost linearly at the beginning, and efficiency increases due to closing to the nominal (rated) operation point of the converter. The output voltage of the converter is controlled to keep the charging current under the rising voltage. However, the state of charge (SoC) of the battery reaches 80% approximately and then the charging procedure is changed and shifted to a constant voltage region, wherein the charging current diminishes due to the total transferred charge to battery. For providing a decaying current, the output voltage of the LLC converter is reduced as well. To decrease output voltage, switching frequency of LLC converter is increased, when charging process nears to its end, wherein the efficiency of LLC converter also reduces substantially. The efficiency of the LLC converter is below 75% at a quarter of the total charging time. When the total charging process is considered, the efficiency variation implies a substantial amount of energy loss. On the other side, when the frequency is increased, the operation point of the converter moves to the off-resonance region. However, the essential requirement  to provide higher efficiencies at light loading conditions which cover an important portion of the battery charging process is the aim of this study.
In this study, a novel smart efficiency tracking (SET) algorithm for GaN-based LLC resonant converters for quick battery charging of light electric vehicles is proposed. Conventional frequency modulation (FM) method, single pulse width modulation method (S-PWM) and discontinuous operation mode method (DOM) are used to track the converter efficiency which varies for different loading conditions during the battery charging process. 
In the first part, the most suitable isolated and non-isolated power electronics topologies for the battery charging process are examined,  advantages and disadvantages of the possible topologies are explained in detail, and the superiority of the LLC resonant converters over other topologies is emphasized. The details of the battery charging cycle for electric vehicles are given by considering the LLC resonant converter as a battery charger. General problems and limits in the battery charging process are discussed. Low load operation and low converter efficiency in this situation are defined. In addition, a comprehensive literature research on LLC resonant converters and batery charging process is conducted.
After the definition of battery charging cycle and efficiency variation problem,  the theoretical background of GaN-based LLC resonant converters is presented. LLC resonant converter topology is given and the working principle is defined in detail.  
Then, to carry out the experimental studies about the proposed algorithm, a high efficiency and high power density LLC resonant converter is designed and manufactured by using GaN power transistors. Design details, equations, calculations and the characteristics of the LLC converter resonant tank circuit are given. Furthermore transformer design parameters, core selection and the winding design are presented. Besides the power electronics circuits, a precision FPGA-based control and monitoring circuit is designed in the scope of the study. In addition to electronic hardware design, VHDL top-modules and sub-modules design details, input / output ports, register maps and the simulation results of VHDL blocks are included in this study. The experimental setup is built by integrating the power electronics and the control boards, FPGA codes, power sources, electrical loads and the measurement devices, such as multimeters and an oscilloscope. 
After the design and manufacturing of the GaN-based LLC resonant converter experimental setup for battery charging applications, the principles of three different switching control strategies, the modeling study and the simulation results are given. Frequency modulation (FM), single pulse width modulation (S-PWM) and discrete operation modes (DOM) are modelled in simulation environment, by using PSIM 64-Bit V9.0. Three control principles are simulated under various input voltage, switching frequency, duty cycle and battery charging current conditions and the simulation results are presented. 
After simulation phase, experimental work is also carried out similarly. First, the three switching modes are tested under various input voltage, switching frequency, charging current and duty cycles. The efficiency values resulting from the three control strategies are calculated and presented according to measurement results in the experimental setup. The efficiency values are used for the development of the proposed smart efficiency tracking algorithm by comparing the experimental results with the battery charging process perspective. By analyzing the results given in experimental results with different switching modes, S-PWM and DOM modes are evaluated as advantageous for charging currents below 3 A, wherein the average efficiency of the LLC resonant converter is lower than 70%. By using S-PWM and DOM switching strategies, the efficiency can be kept over 80%. 
For the constant current charging mode, the LLC resonant converter is run at the conventional resonance operation which enables the maximum charging power. To keep the charging current at 20 A, the switching frequency is reduced from 250 kHz down to 218 kHz. Then, for shifting to constant voltage charging, the switching frequency is increased to obtain the lower charging currents and the frequency modulation voltage control ends with a charging current of 4 A, and the efficiency drops to 85%. While the current is between 4 A and 2 A, the S-PWM operation mode is applied between 100% to 75% S-PWM duty by setting the switching frequency to a fixed frequency of 218 kHz. The S-PWM operation is useful only for a narrow operation region. If the duty ratio is less than 75% in a half period, a substantial efficiency loss occurs due to the resonance operation between the magnetizing inductance and resonant tank capacitor. If a lower charging current is needed, the switching frequency is kept at 218 kHz and the DOM control is started. For charging currents under 2 A, the full DOM is employed. In DOM operation, the emerging inrush current at each switching start causes extra loss in efficiency. In the planned future studies, the DOM operation will be analyzed further and an adaptive frequency modulation will be used to reduce the inrush current for optimal efficiency operation. On the otherside, when the S-PWM is combined with DOM, the battery is fully charged without decreasing the charging efficiency to a level lower than 80%. The achieved improvement of efficiency by using this approach a 20% increase in efficiency is obtained especially for low charging currents. 
Finally, an algorithm is developed for maximum available efficiency to shift among the operation modes. In the algorithm flowchart, first it is checked whether constant voltage charging (slow charging) or constant current charging (fast charging) is necessary, then according to the battery charging current level, S-PWM or DOM is selected for optimum charging efficiency. 
In the study, a new method which is dedicated to obtain the highest available efficiency is implemented for a GaN switch based LLC converter. The novel approach tracks the converter efficiency for varying loading conditions during fast battery charging. The aim of the studied algorithm is to monitor the efficiency and to keep or alter the operation mode of the converter according to the monitored efficiency and loading condition. As a result of pursuing a wide operation range of efficiency, a multi-mode switching and control technique is applied for shifting among the mentioned modes and explained by simulation and experimental results. The proposed smart efficiency tracking algorithm can be defined as a "monitor-and-change" scheme. Thus, during the whole charging process the efficiency of the converter can be kept at the highest value. A maximum 25% increase in efficiency can be provided, which is quite high for efficiency-critical applications such as battery charging.