MINISTRY OF EDUCATION AND TRAINING HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY AND EDUCATION FACULTY FOR HIGH QUALITY TRAINING GRADUATION THESIS AUTOMOTIVE ENGINEERING RESEARCH AND DE
INTRODUCTION
Motivation
A robust transportation system is essential for a thriving national economy, as it facilitates the efficient movement of goods and people, both in public and personal contexts This efficiency plays a crucial role in empowering the economy, while economic growth, in turn, leads to increased traffic and demand for transportation services.
In 2022, the Vietnam Association of Motorcycle Manufacturers (VAMM) reported that motorcycle sales in Vietnam exceeded 3 million units, averaging over 8,000 vehicles sold daily This highlights the continued popularity of motorbikes as the most efficient mode of transportation in Vietnam, given the country's economic, cultural, and infrastructural characteristics Additionally, there is a growing global trend towards the production of environmentally friendly vehicles, particularly electric vehicles, which are gaining traction in Vietnam.
Combining humanistic, economic, social and market backgrounds, we have every reason to believe that electric motorbikes will soon become a dynamic future for Vietnamese people
However, the issue that many consumers are concerned about is: Are electric scooters safe to use and charge? How long does an electric motorcycle take to fully charge?
To accelerate the adoption of electric motorcycles as a viable alternative to traditional bikes, it is essential to enhance their convenience and speed, which are key advantages of conventional motorcycles In response to this evolving trend, we have chosen to focus our graduation project on researching and designing charging systems for electric motorcycles that utilize pulse transformers, leveraging our knowledge of automotive technology and electronics.
Research status
The article "Investigating the Impact of Continuous and Intermittent Current Operating Modes of Transformers on High-Frequency Switching Power Supplies" by Cao Xuan Tuyen and Nguyen Anh Tuan's team was published in the journal Science and Technology The authors conducted calculations, tests, and simulations to analyze transformer operations for high-frequency switching power supplies in both continuous and intermittent current modes Utilizing a Flyback converter for transformer control, they applied formulas to determine necessary parameters at three distinct output power levels while maintaining the same input power With an input voltage range of Vin = 9-15 VDC, the team examined outputs of V_out = +5 VDC at I_out = 1.1 A, V_out = +15 VDC at I_out = 0.6 A, and V_out = -15 VDC at I_out = 0.6 A The power supply simulation was carried out effectively.
The author's team utilized PSIM software to analyze two operational modes of a transformer, finding that both modes exhibit stability Notably, the intermittent current mode offers superior voltage quality compared to the continuous current mode However, the circuit in intermittent current mode is more compact but necessitates a greater number of wire turns, while the continuous current mode features a larger circuit size with fewer turns.
The research project titled “120-V, 200-W, 90% Efficiency, Interleaved Flyback for Battery Charging Applications Reference Design” was carried out by Texas Instrument Corporation It focuses on monitoring a charger designed for a battery pack comprising five Lithium-ion batteries The power circuit operates with an input voltage of 100-120 VAC at a frequency of 50-60 Hz, delivering an output voltage of 21 VDC and a current of 9.5 A This charger utilizes a Flyback pulse source combined with a pulse transformer, achieving an output power of up to 200W.
The research conducted by Sheng-Yu Tseng and Jun-Hao Fan focuses on the "Buck-Boost/Flyback Hybrid Converter for Solar Power System Applications." This study explores the integration of Buck-Boost and Flyback pulse sources in the lithium-ion battery charging and discharging process Utilizing solar energy, the system provides input power to the Buck-Boost charging circuit with a voltage range of 17.5-20.6 VDC and an output current of 8-12 A to charge lithium-ion batteries during the day At night, these batteries discharge through the Flyback power circuit, generating an output voltage of 10 VDC.
𝐼 𝑜𝑢𝑡 = 2 (A) to supply LEDs at night.
Objectives of the research
Research objective: Design charging circuit for electric vehicle using pulse transformer
- Learn the theory and operation of Flyback pulse sources, electronic components, and simulation software
- Calculation and design of charging circuit for electric motorcycle
- Experiment, evaluate the stability of the circuit.
Research methods
Method of document review: Scientific articles from the most reputable sources
(ScienceDirect, Springer, IEEE, Elsevier) were downloaded, read, and analyzed by the research authors to find out the problems of scientists learning ahead has not solved
Simulation and control: use Proteus simulation software to simulate electrical circuits using STM32CubeIDE software to program microcontrollers
Experiment and evaluate results: conduct circuit tests with different conditions, based on calculated parameters and experimental results to evaluate the effectiveness and safety of the circuit.
Object and scope of the research
- Pulse Transformer and Pulse Source Flyback
- MOSFET 11N90 and MOSFET Driver IR2103
This article explores the structure and fundamental operating principles of charging circuits for motorcycles, emphasizing practical applications in design while intentionally avoiding complex algorithms and advanced electronics concepts.
Expected result
Design the charging system that can use to charge the 72-battery-connected Lithium-ion The system can adjust the duty cycle base on the feedback signal
LITERATURE REVIEW
Pulse power supply
2.1.1 Elements used in pulse power supply a The Law of Inductance
The instantaneous voltage drop across an inductor is directly proportional to the rate of change of the current passing through the inductor The mathematical relationship is given by: v = L × (𝑑𝑖
With: L is inductance of the inductor in Henries (H)
𝑑𝑡) instantaneous rate of current change in amperes per second (A/s) Important characteristics of the inductor according to the law of inductance:
The voltage across the inductor is valid if and only if the current varies with time
If a constant DC current is applied to the inductor, there will be no voltage across it (except in the case of a small voltage drop across the copper coil)
In an inductor, voltage can change instantaneously, while amperage remains constant unless the voltage reaches an infinite value A larger voltage drop across the inductor results in a quicker change in current.
Below is an illustration of the voltage characteristics of the inductor:
Figure 2 1 Inductor Voltage/Current Relationship [4]
According to the phenomenon of self-inductance, the current through the inductor does not change instantaneously but increases and decreases with time
There are two modes of inductor current operation:
In continuous operating mode (CO), the current in the inductor, denoted as \$I_L\$, rises from a minimum value \$I\$ to a maximum value \$I_{max}\$ and subsequently falls back to \$I\$ This process of continuous increase and decrease occurs without allowing the current to drop to zero.
In Discontinuous Operation mode (DO), the inductor current 𝐼 𝐿 rises from 0 to its maximum value 𝐼 𝑚𝑎𝑥 when the switch is closed, and subsequently drops back to 0 when the switch is opened This principle is essential in the functioning of transformers within pulse power supply systems.
The primary component of this project is the pulse transformer, which differs from a standard transformer as its coil functions as an inductor Typically, each pulse transformer consists of two or more pairs of magnetic coils A schematic diagram of the transformer is provided below.
The input and output voltages of a transformer are influenced by the number of turns in the winding; a coil with more turns produces a higher voltage but lower current, while a coil with fewer turns results in lower voltage and higher current The polarity of the transformer is indicated by a dot, and reversing this dot changes the polarity and output current Additionally, Pulse Width Modulation (PWM) plays a significant role in controlling the output.
Pulse width modulation (PWM) is a technique used to regulate output voltage by varying the width of a square pulse sequence within a cycle \( T_P \) This output voltage is managed by toggling a switch in the converter on and off The output DC voltage \( V_{Out} \) can be calculated as illustrated in the accompanying figure.
Figure 2 3 Basic Principle of PWM [4]
The DC output voltage is determined by multiplying the peak pulse amplitude, denoted as \$V_{PK}\$, by the duty cycle, which is defined as the ratio of the switch ON time \$T_{ON}\$ to the total period \$T_{P}\$.
The working principle of Flyback circuit:
The pulse transformer circuit uses a Flyback circuit that acts as an inductor or a device that accumulate magnetic energy
Figure 2 4 Basic Flyback Converter Schematic [5]
When switch 𝑄 1 is closed, it directs current to the primary coil of the pulse transformer, allowing it to store energy The transformer 𝑇 1 behaves like a pure inductor, causing the primary current to increase linearly until it reaches a peak value, 𝐼 𝑃𝐸𝐴𝐾 Meanwhile, the diode 𝐷 1 in the secondary coil remains non-conductive, resulting in no current flow in the secondary coil During this period, the output load current is solely provided by the output filter capacitor 𝐶 1.
While 𝑄 1 is open, all winding voltages reverse under flyback action, bringing the output diodes into conduction and the primary stored energy 1
The output current from 2𝐿𝐼 𝑃 2 is essential for supplying load current and recharging the output capacitors after they have lost charge when 𝑄 1 was closed This current not only meets the load requirements but also replenishes capacitor 𝐶 1, ensuring it is ready to supply the load again during the next closed 𝑄 1 phase.
Figure 2 5 Current and Voltage in primary & secondary coil [5]
When implementing a Flyback circuit, it's crucial to note that voltage exists across the pulse transformer between the primary and secondary windings, even when they are not conducting simultaneously When the switch 𝑄1 is OFF, the input voltage 𝑉𝐷𝐶 combines with the reverse voltage from the secondary coil Consequently, 𝑄1 must be capable of withstanding the combined magnitude of these voltages The voltage 𝑉𝑅𝑂 can be calculated using a specific formula.
And the maximum voltage on Q1 is defined as:
Flyback converters function in two modes: continuous conduction mode (CCM), where the secondary current remains above zero, and discontinuous conduction mode (DCM), where the secondary current drops to zero during each cycle A flyback converter is classified as discontinuous if the secondary current reaches zero before the subsequent "ON" period of switch 𝑄1 begins.
In DCM, the current in the secondary coil reaches 0 A (point I) before the primary current initiates the next conduction cycle (point F), a phase known as the dead time \( T_{dt} \) The average output current at the secondary is determined by the average of the GHI triangle, multiplied by the interrupt ratio of the \( Q_1 \) lock, \( T_{off} \).
The DCM enhances the switching performance of the output diode \( D_1 \) in the secondary by ensuring that the current through the diode returns to 0 A before it experiences reverse bias This results in reduced energy storage in the transformer, contributing to a smaller size of the pressurized machine However, it is important to note that the output current in DCM will be lower compared to that in CCM.
9 discontinuous current mode is often applied in circuits with high voltage and low current output
The change of current in the DCM is shown as shown below:
Figure 2 8 Waveforms of a discontinuous-mode flyback [5]
The current of the CCM will be shown in the following figure:
Figure 2 9 Waveforms of a continuous-mode flyback [5]
CCM and DCM exhibit similar current waveforms, with the operating mode influenced by magnetized inductance and output load current The key distinction lies in the current storage within the two windings, as there is no dead time \(T_{dt}\) In Continuous Conduction Mode (CCM), the primary coil maintains a consistent current flow.
During the off time of switch \( Q_1 \), the current in the secondary coil exhibits a combination of triangular and stepped waveforms, transitioning from point \( V \) to \( W \) Similarly, as \( Q_1 \) begins to close, the primary coil experiences a buffer current that varies in magnitude from point \( M \) to \( N \).
Lithium-Ion Battery
Lithium-ion batteries generate DC electricity through chemical reactions, with lithium ions moving between the anode and cathode during charging and discharging The cathode is typically composed of metal oxide materials like Cobalt, Nickel, or Manganese, while the anode is made of graphite, both structured as thin layers containing lithium ions During charging, lithium ions migrate from the cathode to the anode, and during discharge, they flow back to the cathode The electrolyte plays a crucial role by providing a conductive medium for Li+ ions between the plates while also serving as a non-conductive solvent.
2.2.1 Structure of Lithium-ion Battery 18650
Figure 2 10 Structure of Lithium-ion battery [8]
The structure of a lithium-ion battery consists of 4 main parts: cathode (positive electrode), anode (negative electrode), separator and electrolyte
A typical lithium-ion battery features a negative electrode made of carbon-based graphite and a positive electrode often composed of a metal oxide The electrolyte consists of a lithium salt dissolved in an organic solvent, while a separator is utilized to prevent contact between the anode and cathode.
The anode (negative electrode) and cathode (positive electrode) are separated to prevent short-circuiting, with a metal component called a current collector serving to isolate them from external electronics The electrochemical roles of the anode and cathode electrodes vary based on the direction of current flow within the cell.
Table 2 1 Main specifications of 18650 Lithium-ion batteries
Rated capacity: 2600 mAh (0,52A discharge; 2,75V) Nominal capacity: 2550 mAh (0,52A discharge; 2,75V)
Ambient temperature Charging: 0 - 45 o C, Discharging: -20 - 60 o C a Cathode
Cobalt-based cathode materials, such as LiCoO2 and LiMn2O4, are favored for their high theoretical specific charge capacity, excellent volumetric capacity, low self-discharge rates, high discharge voltage, and strong cycle performance However, their high cost and poor thermal stability present significant challenges In contrast, manganese-based materials utilize a cubic crystal lattice system that facilitates three-dimensional lithium-ion diffusion, making them a cost-effective alternative Despite their potential, manganese cathodes face limitations, including poor cycling stability, which must be addressed to enhance battery efficiency and longevity.
Cobalt-based cathodes are widely used due to their popularity, but research is ongoing into alternative materials to lower costs and extend the lifespan of cells Additionally, the propensity of these materials to dissolve into the electrolyte during cycling is a significant concern.
Despite the growing use of newer silicon-based materials, graphite and other carbon-based substances remain the primary choice for negative electrode materials in lithium-ion batteries In 2016, 89% of these batteries utilized graphite (43% synthetic and 46% natural), while 7% used amorphous carbon, and 2% each employed lithium titanate and silicon or tin-based materials These materials are favored for their availability, electrical conductivity, and ability to intercalate lithium ions with only a 10% volume expansion Graphite stands out due to its superior performance and low intercalation voltage, making it the dominant material Although alternative materials with higher capacities have been proposed, they often come with higher voltages that reduce energy density, emphasizing the need for anodes to operate at low voltage to maximize energy efficiency.
To prevent battery self-discharge and short circuit problems, a microporous film made of polypropylene (PP), polyethylene (PE), and other plastics is placed between the positive and negative plates This separator allows lithium ions to flow through its dense micropores, facilitating the battery's charging and discharging processes.
Lithium-ion battery electrolytes, primarily composed of elements like ethylene carbonate (EC) and dimethyl carbonate (DMC), are essential for facilitating lithium ion transfer between electrodes To enhance battery cycle life, safety, and lithium-ion transmission, optimizing electrolyte formulations and incorporating effective additives is crucial The selection of the appropriate electrolyte significantly impacts the overall performance of lithium-ion batteries.
2.2.2 Working principle of Lithium-ion battery
In a Lithium-ion battery, the materials in the two electrodes serve as the reactants for the electrochemical reaction, facilitating the movement of lithium ions through an electrolyte solution During this process, lithium ions transition between the electrodes, with current electrode materials enabling their entry and exit from the lattice structure while minimally disturbing the surrounding atoms.
During the charging process, lithium ions are released from the positive electrode and utilize the electrolyte as a transmission medium These ions then pass through the separator and attach to the anode plate.
Figure 2 11 Lithium-ion battery charging process [9]
Figure 2 12 Lithium-ion battery discharging process [9]
During discharge, Lithium ions are released from the cathode plate Lithium ions use the electrolyte as a conducting medium, penetrating through the separator and returning to the cathode plate
The following equations exemplify chemistry
The positive electrode (cathode) half-reaction in the lithium-doped cobalt oxide substrate is:
CoO2 + Li + + e - ↔ LiCoO2 (left to right: discharging, right to left: charging)
The negative electrode (anode) half-reaction for the graphite is:
LiC6 ↔ C6 + Li + + e - (left to right: discharging, right to left: charging)
The full reaction for charging and discharging is:
LiC6 + CoO2 ↔ C6 + LiCoO2 (left to right: discharging, right to left: charging)
The overall reaction has its limits During discharge, C6 1- (Cathode) is oxidized to
During the charging process, Co 4+ in C6 0 is reduced to Co 3+, and this reaction can reverse If a Lithium-Ion battery is over-discharged, Lithium Cobalt Oxide transforms into Lithium Oxide, following a specific reaction pathway.
If the LCO battery is overcharged with a voltage above 5.2 V, it will convert to Cobalt IV Oxide, in one direction of the following reaction:
2.2.3 Charging and discharging characteristics of lithium-ion battery
A Lithium-ion battery cell has a 2-stage charging process:
Stage 1: Constant current (CC) charging
During constant current (CC) charging, the current remains steady, typically set at C/2-C (where C represents the battery's capacity in Ah) While a higher charging current reduces the duration of the stable charging phase, it extends the voltage stabilizer charging phase Generally, the total charging time for both phases does not exceed 3 hours However, increased current can raise the battery's temperature, necessitating careful monitoring during the charging process, as excessive heat may lead to ignition or explosion risks.
The optimal temperature for Li-ion batteries typically should not exceed 45ºC; however, those utilizing Lithium-Ferro-Phosphate (LiFePO4) technology can safely reach temperatures up to 60ºC Fast charging, which involves constant current charging, allows for a higher charging current and consequently reduces charging time, making the increased temperature limit advantageous.
Figure 2 13 Charging characteristic of lithium-ion battery [10]
Stage 2: Constant voltage (CV) charging
During constant voltage (CV) charging, the voltage is maintained at 4.2V per cell As the battery approaches full capacity, its electromotive force rises, leading to a decrease in current The voltage stabilization phase concludes when the current falls below 3% C, indicating that the battery capacity has been restored to approximately 99%.
Li-ion batteries cannot tolerate overcharging, so it is crucial to stop the charging current immediately once the battery reaches full capacity Continuous trickle charging poses safety risks and can lead to the plating of metallic lithium To minimize stress on the battery, it is important to keep the peak cut-off time as short as possible.
Electronic components used in the project
2.3.1 MOSFET a Structure and working principle of MOSFET
MOSFETs, or Metal-Oxide Semiconductor Field-Effect Transistors, are semiconductor devices that operate based on the effects of magnetic fields on metal oxide and semiconductor junctions Known for their rapid switching capabilities, typically in the range of a few MHz, MOSFETs are commonly utilized in pulse source circuits and high voltage control applications.
MOSFETs are primarily categorized into two types: N-MOSFETs and P-MOSFETs, with each type further classified into enhancement and depletion modes A MOSFET consists of three terminals: Gate (G), Drain (D), and Source (S) The G-S terminal is utilized in the control circuit, while the D-S terminal is integrated into the power circuit.
Depletion mode MOSFETs function as "Normally Closed" switches, requiring a Gate-Source voltage (VGS) to turn the device "OFF." In contrast, enhancement mode MOSFETs act as "Normally Open" switches, needing a Gate-Source voltage (VGS) to switch the device "ON."
The following diagram illustrates the symbols and fundamental construction for both MOSFET designs
In this project, the Enhancement-mode N-Channel MOSFETs are used in the control circuit and the feedback circuit b Enhancement-mode N-Channel MOSFETs
The enhancement-mode MOSFET (eMOSFET) operates oppositely to the depletion-mode type, featuring a conducting channel that is either undoped or lightly doped, making it nonconductive Consequently, when the gate bias voltage (VGS) is zero, the device remains in the "OFF" state and does not conduct The circuit symbol for an enhancement MOS transistor is characterized by a broken channel line, indicating a normally open, non-conducting channel.
The n-channel enhancement MOS transistor operates as a transconductance device, allowing drain current to flow only when the gate voltage (VGS) exceeds the threshold voltage (VTh).
Increasing the positive gate voltage reduces channel resistance and enhances the drain current (\$I_D\$) in an n-channel enhancement mode MOSFET Specifically, a gate-source voltage (\$V_{GS}\$) of zero or negative turns the MOSFET "OFF," while a positive \$V_{GS}\$ activates it, functioning similarly to a "normally open" switch.
Figure 2 17 V-I characteristic of enhancement-mode MOSFET [13]
The V-I characteristic of enhancement-mode MOSFET is divided into 3 main regions:
1 Cut-off Region – with VGS < 𝑉 𝑇ℎ the gate-source voltage is much lower than the transistors threshold voltage, so the MOSFET transistor is switched “fully-OFF” thus, ID = 0, with the transistor acting like an open switch regardless of the value of
2 Linear (Ohmic) Region – with VGS > 𝑉 𝑇ℎ and VDS < VGS the transistor is in its constant resistance region behaving as a voltage-controlled resistance whose resistive value is determined by the gate voltage, VGS level
3 Saturation Region – with VGS > 𝑉 𝑇ℎ and VDS > VGS the transistor is in its constant current region and is therefore “fully-ON” The Drain current ID Maximum with the transistor acting as a closed switch [13]
IC (Integrated Circuit) is an integrated circuit consisting of many semiconductors and passive components They are combined to perform a predetermined task
The IR2103 is a high voltage, fast processing MOSFET and IGBT driver IC featuring dependent low and high side output channels It operates with a logic input that adheres to CMOS or LSTTL standards, utilizing a logic voltage level of 3.3V Designed to minimize driver cross-conduction, the output drivers include a high side pulse current buffer stage This driver IC is capable of driving an N-channel power MOSFET or IGBT in a high-side configuration, supporting operation up to 600 volts.
HIN Logic input for high side gate driver output (HO), in phase
𝐋𝐈𝐍̅̅̅̅̅ Logic input for low side gate driver output (LO), in phase
HO High side gate drive output
𝑽 𝑺 High side floating supply return
𝑽 𝑪𝑪 Low side and logic fixed supply
LO Low side gate drive output
- VCC: Low side and logic fixed supply voltage: 10V - 20V
- VS: High side floating supply offset voltage: -5V - 600V
- VB: High side floating supply absolute voltage: VS +10V - VS +20V
- VHO: High side floating output voltage: VS - VB
- VLO: Low side output voltage: 0 - VCC
- VIN: Logic input voltage (HIN & LIN): 0 - VCC
Figure 2 19 IR2103 input and output control pulse [14]
The HO pin outputs a HIGH voltage when both the HIN and LIN pins are HIGH Conversely, if the HIN and LIN pins are LOW or have differing voltages, the HO pin will output a LOW voltage.
At the LO pin, when the voltage at the HIN and LIN pins is LOW, the voltage at the
LO pin is HIGH, when the voltage at the HIN and LIN pins is HIGH or different, the voltage at the LO pin is LOW
In addition, IR2103 has an internal deadtime to prevent cross transmission; this function is applied to control MOSFET or IGBT in Full-Bridge circuits
In this project, the pulse output from the A0 pin of the STM32 microcontroller serves as the input signal for the HIN pin of the IR2103 To protect the microcontroller and the circuit from potential reverse current, an opto-isolator, the HCPL2631, is utilized to transfer the pulse while isolating the microcontroller from any reverse current that may occur.
An optocoupler, also known as an opto-isolator, photo coupler, or optical isolator, is a device that converts electrical signals into light for transmission, providing significant advantages in voltage isolation between input and output circuits The only connection between these circuits is a beam of light, allowing for an impressive isolation resistance of up to thousands of MΩ This makes optocouplers ideal for use in high voltage applications, where the voltage levels of the two circuits can differ significantly.
The optocoupler is made of a light emitter and a light detector
Light emitter: This device receives electrical signals from the input side and converts them into light signals A light emitting diode is commonly used as the light emitter (LED)
In an optocoupler, the light detector plays a crucial role by converting incoming light signals from the light emitter back into their original electrical signals This component can take various forms, including phototransistors, photodarlingtons, or photodiodes.
Infrared (IR) light is generated by an LED when current flows through it, with the supply voltage and current influencing the light's intensity A phototransistor, exposed to this IR light, converts it into current, which in turn regulates the current flow between its emitter and collector The light intensity is directly related to the LED's current supply, while the current quantity is determined by the light's brightness Consequently, the input current on the low voltage side can affect the current flow in the high voltage circuit without any direct electrical connections.
There are four types of optocouplers available, each with a different combination of photo-sensitive components and an infrared LED source These are the four optocouplers:
Figure 2 21 Types of optocoupler [15] d Used optocoupler in the project
STM32CubeIDE
STM32CubeIDE is an all-in-one multi-OS development tool, which is part of the STM32Cube software ecosystem
STM32CubeIDE combines STM32 configuration and project creation features from STM32CubeMX, providing a comprehensive tool that streamlines installation and development time Users can create projects by selecting an empty STM32 MCU or MPU, a preconfigured microcontroller, or a board example, which generates the necessary initialization code Throughout the development process, users can revisit the initialization and configuration of peripherals or middleware, allowing them to regenerate the initialization code without affecting their custom user code.
STM32CubeIDE offers essential build and stack analyzers that deliver valuable insights into project status and memory needs It features both standard and advanced debugging tools, allowing users to view CPU core registers, memory, and peripheral registers Additionally, it includes a live variable watch, Serial Wire Viewer interface, and fault analyzer for enhanced debugging capabilities.
After starting the software, the initial interface of the software is as follows:
Figure 2 35 Initial startup window of STM32CubeIDE
In this startup window, users can select the appropriate shortcut based on their intended use For this discussion, we will focus on the STM32F103C8 microcontroller and begin by choosing the "Start new STM32 project" option.
Figure 2 36 MCU/MPU Selector window
To begin the STM32 board selection process, enter "STM32F103C8T6" in the Commercial Part Number box, then click Next to proceed to the naming step After naming your project, click Finish to complete the selection and initiate the programming process.
Figure 2 37 STM32CubeIDE naming window
The pins are selected as follows:
- PD0 and PD1: External crystal/ceramic resonator
- PA13 and PA14: Used when loading the program without pressing the reset button to run the program
We choose PWM Generation for pin PA0:
Figure 2 39 PA0 Mode and Configuration
To achieve the desired PWM frequency of 100KHz, select the appropriate parameters for the Prescaler (PSC) and Counter Period (ARR), which represents the maximum value of the counter The frequency can be calculated using the specified formula.
(𝑃𝑆𝐶+1)×(𝐴𝑅𝑅+1) (2.2) With 𝐹 𝑐𝑙𝑐𝑘 is timer clock, we choose the value 72 𝑀𝐻𝑧
Then, we choose PSC = 0 and ARR = 719
Figure 2 41 PWM generation mode at PA0
STM32F411 supports 2 PWM modes as follows:
- Mode 1: If using the count-up mode, the output will be at logic 1 when CNT < CCR and vice versa, at 0 if CNT > CCR
- Mode 2: If counting up mode is used, the output will be at logic 0 when CNT CCR
After selecting the mode for the control pins, we configure the processing time and frequency of the microcontroller
Finish setting up the pins and related parameters, click Project > Generate Code, or use Alt + K shortcut to open the programming workspace, built into STM32CubeIDE
After clicking on Generate Code, a window with the name main.c will pop up, we will enter our code in this window
Figure 2 43 Coding window of STM32CubeIDE
The interface of the code window consists of 3 main areas:
- Zone 1: Users will enter the code here
Zone 2 serves as the project window where users can access imported projects In this area, users have the option to select main.c for coding in Zone 1 or choose the ioc file to reset the pins Additionally, users can efficiently manage libraries, subroutines, and files associated with the project.
Zone 3 serves as the display area for the program compilation process, providing insights into the number of errors and warnings found in the Problems tab of the project.
The microcontroller monitors the signal from pin PA4, outputting a PWM pulse at pin PA0 with a 20% duty cycle when the voltage at PA4 exceeds 0 This PWM signal serves as the input for the IR2103, which drives the MOSFET to operate at 100 kHz Conversely, if the voltage at PA4 is 0, the duty cycle is adjusted to 10%.
CALCULATION AND DESIGN
System description
Electric motorcycles utilize a Lithium-ion battery pack that produces 288V DC power, consisting of 72 batteries connected in series To effectively charge this battery pack, it is essential to meet specific voltage and amperage requirements Each battery has a maximum voltage of 4.2V, necessitating a charger voltage greater than 302V, with an amperage of approximately 0.32A The accompanying diagram illustrates a Lithium-ion battery charger circuit that employs a pulse transformer.
Figure 3 1 Lithium-ion battery charger circuit using pulse transformer
The lithium-ion battery charger circuit operates with a 220V - 50Hz AC input, which is rectified to 311V DC This 311V is then processed through a pulse transformer, regulated by a control circuit, to generate a charging current of 320V at 0.32A for the lithium-ion battery pack.
This is a Flyback type isolating load circuit The circuit is isolated by a pulse transformer, so in the event of a power failure, the battery pack will not be affected.
Operating principle of the system
The charging circuit operates on an input power of AC 220V at 50Hz, which is converted into direct current by a full-wave rectifier bridge circuit However, the resulting output DC voltage exhibits an undulating waveform, leading to instability in the system.
Therefore, a capacitor will be connected at the output of the rectifier bridge to smooth this voltage
A snubber circuit is essential for safeguarding electrical circuits against residual voltage spikes and oscillation effects It effectively mitigates voltage spikes that occur when a MOSFET disconnects, preventing potential damage such as burning Additionally, the snubber circuit reduces these voltage spikes while maintaining the frequency of the main circuit.
The circuit used in this study, which is a Flyback type is Resistor Capacitor Diode (RCD) snubber circuit including a resistor, diode, and capacitor connected to the circuit as shown below:
Figure 3 5 Drain pole voltage without snubber circuit [24]
Figure 3 6 Drain pole voltage with snubber circuit [24]
The switching speed of a MOSFET between its on and off states is influenced by the trigger resistors By adjusting the values of the resistors at the control gates, one can effectively control the switching speed, as well as the closing and interrupting times.
𝑅 𝐷𝑅𝑝 and 𝑅 𝐷𝑅𝑛 , respectively Below is the structure of an N-channel MOSFET
In the structure of the MOSFET, there are parasitic capacitances that affect the closing and interrupting process of the MOSFET Therefore, we must consider these capacitances
Figure 3 7 Parasitic capacitances in MOSFET [25]
The MOSFET activation process consists of three main phases:
Figure 3 8 VGS as a function of gate charge [25]
The process from 𝑇 0 to 𝑇 1 : at time 𝑇 0 , pole 𝐺 begins to be powered and the voltage
The voltage \( V_{GS} \) begins at zero, during which the majority of the current flows into the \( C_{GS} \) capacitor, charging it A smaller portion of the current also charges the \( C_{GD} \) capacitor, but since \( C_{GD} \) has a lower capacitance than \( C_{GS} \), this phase primarily represents the charging of the \( C_{GS} \) capacitor This initial stage is referred to as the \( ON \) delay, as both the current and voltage from the source remain constant, keeping the MOSFET in the off state.
The process from 𝑇 1 to 𝑇 2 : this is the MOSFET stage that is almost fully conducted
At this point, the 𝑉 𝐺𝑆 voltage increases very slowly or not even at all, and the 𝑉 𝐺𝐷 voltage increases rapidly
𝑇 2 to 𝑇 3 : MOSFET completes the excitation cycle at this stage Capacitors 𝐶𝐺𝑆 and 𝐶𝐺𝐷 are loaded and 𝑉 𝐺𝑆 increases to the final point
The MOSFET interrupting process is the reverse of the triggering process a ON resistance
The current through terminal G and the voltage \( V_{GS} \) lack a defined equation, making accurate resistance calculation impossible However, the IR Rectifier manufacturer has developed a straightforward yet highly effective method to determine these values.
The average excitation current, denoted as \( I_{avg} \), is defined as the current during the excitation process, while \( t_{sw} \) represents the switching time, which is the duration from the start of excitation until the MOSFET is fully closed This time interval corresponds to the period between \( T_1 \) and \( T_3 \) as analyzed previously.
With 𝑉 𝐺𝑆 is the average voltage for the period 𝑇 2 to 𝑇 3 which was provided by the manufacturer in the datasheet
The switching time, denoted as \$t_{sw}\$, is influenced by the excitation current; a smaller \$t_{sw}\$ results in faster switching times and reduced component losses.
𝑡 𝑠𝑤 is usually chosen according to design criteria and suitable for carrier frequency The optimal 𝑡 𝑠𝑤 switching time when triggered by the driver IC and is usually chosen is:
𝑅 𝑇𝑜𝑡𝑎𝑙 = 𝑅 𝐺(𝑂𝑁) + 𝑅 𝐷𝑅𝑝 (3.5) Thus, the value of ON resistance has been defined
Driver ICs feature distinct trigger and interrupt pins, with the trigger resistor typically chosen to have a lower value than the interrupt resistor This design allows for quicker interrupt triggering, effectively minimizing Dead Time.
𝑑𝑡 (3.6) But we need 𝑉 𝐺𝐸 < 𝑉 𝑇ℎ of the MOSFET, and then we have:
When designing an excitation circuit, it is crucial to carefully calculate the values of the on and off resistors, as these choices significantly impact circuit performance and help prevent damage to other components.
When using a MOSFET to control a circuit, we will usually have two basic ways as shown below:
High-side excitation circuits for MOSFETs connect the MOSFET to a high-voltage source, with the load linked to the ground, distinguishing it from low-side excitation configurations.
Figure 3 11 N-MOSFET driver circuit high (left) and low side (right)
In contrast, with the low-side excitation circuit, the MOSFET will be connected between the load and ground
In the high-side driver circuit, for closing the MOSFET, 𝑉 𝐺𝑆 must be higher than
𝑉 𝐺𝐸 (with a 20N60 MOSFET, 𝑉 𝐺𝐸 = 5V) When the MOSFET is closed, 𝑉 𝐷𝑆 = 0, or the whole voltage 𝑉 𝐷𝐷 will fall on the load, which means 𝑉 𝑠 ≈ 𝑉 𝐷𝐷 = 310V In addition,
𝑉 𝐺𝑆 = 𝑉 𝐺 − 𝑉 𝑆 , and 𝑉 𝐺 has a value of 7.5V < 310V of 𝑉 𝑆 , so the MOSFET will not open
In a low-side trigger circuit, when the MOSFET is off, the drain-source voltage (\$V_{DS}\$) is approximately 0V, and with pole S grounded, the drain voltage (\$V_D\$) is also around 0V Consequently, the entire supply voltage (\$V_{DD} = 310 V\$) is applied across the load without affecting the source voltage (\$V_S\$) Thus, as long as the gate-source voltage (\$V_{GS}\$) exceeds the gate-emitter voltage (\$V_{GE}\$), the MOSFET can be effectively controlled, even when the supply voltage is significantly higher.
Controlling the high-side excitation circuit is significantly more complex than managing the low-side triggering circuit Given that the pulse transformer requires an input source of 310V, a gate voltage (𝑉𝐺) exceeding 310V is necessary to trigger the MOSFET, which can be achieved through an isolation voltage source or a Bootstrap circuit Consequently, our team has opted to control the low-side MOSFET to streamline the control process.
Calculation and design circuit elements
In the MOSFET trigger circuit, the trigger resistor typically requires a minimal power rating of 0.25W However, to enhance circuit safety, a 2W resistor is recommended for selection.
High side floating supply absolute voltage 𝑉 𝐵 𝑉 𝑆 + 10 - 𝑉 𝑆 + 20
High side floating supply offset voltage 𝑉 𝑆 - - 600
High side floating output voltage 𝑉 𝐻𝑂 𝑉 𝑆 - 𝑉 𝐵
Low side and logic fixed supply voltage 𝑉 𝐶𝐶 10 - 20
Low side output voltage 𝑉 𝐿𝑂 0 - VCC
Logic input voltage (HIN & LIN̅̅̅̅̅) 𝑉 𝐼𝑁 0 - VCC
Gate - Source voltage enough to open the gate 𝑉 𝐺𝑆(min) 5 - -
Drain – Source diode forward voltage 𝑉 𝑆𝐷 - - 1.5
Peak diode recovery voltage slope 𝑑𝑉
We choose the gate resistor 𝑅 𝐺 = 1 𝑘Ω
In Vietnam, residential electricity is single-phase alternating current with a value in the range of 85-220V with a frequency of 50𝐻𝑧 Therefore, we will have 𝑉 𝐴𝐶𝑚𝑖𝑛 = 85V and 𝑉 𝐴𝐶𝑚𝑎𝑥 = 220V
We will use the rectified DC current by the diode bridge, so VDC will be calculated according to the following formula:
The output required to charge the Lithium-ion battery is 320V - 0.3A, so the output power will be:
𝑃 𝑂 = 𝑉 𝑂 × 𝐼 𝑂 = 320 × 0.3 = 96 (𝑊) Assuming the efficiency of the circuit is 𝜂 = 70%, we will determine the input power:
Besides, the voltage of the alternating current will also be flattened by the effect of the capacitor, with two factors 𝐶 𝐷𝐶 and 𝐷 𝐶ℎ Where:
𝐶 𝐷𝐶 : The value of the input capacitor 𝐶 𝑖𝑛 per Wattage of input power With the AC voltage range defined above, 𝐶 𝐷𝐶 = 2 – 3 𝜇𝐹 We will choose 𝐶 𝐷𝐶 = 3 𝜇𝐹
𝐷 𝑐ℎ : charge ratio of input capacitor 𝐶 𝑖𝑛 This ratio is referred to as in the following figure:
Figure 3 12 Waveform graph of DC voltage after input filter capacitor [27]
Table 3 3 Input factors and output goals
Because the output requires high voltage and low current, we will choose the discontinuous current mode for the calculation of this pulse transformer
First, we will choose the voltage ratio as well as the turn ratio of the pulse transformer
The 20N60 MOSFET is rated for a Power-Source voltage of 600V, making it suitable for various applications To ensure reliable operation and to handle voltage spikes or noise during use, it is essential to select the maximum stress \$V_{ms}\$ on the transistor while in the "off" state, excluding any leakage inductance spikes.
Where 𝑉 𝐹 is the voltage drop of the output rectifier diode In the circuit using general-purpose rectifier diode type 1N4007 with 𝑉 𝐹 = 1.1V
With 𝑉 𝑚𝑠 as 440 V Then even with a 25% or 110V leakage spike, this leaves a 50V margin to the maximum voltage rating We have:
Thus, 𝑉 𝑅𝑂 = 126𝑉 and we can choose the maximum “on” time by the formula:
Next, we will calculate the primary inductance 𝐿 𝑝 :
=(120× 4.17×10 −6 ) 2 2.5×10×10 −6 ×160 = 62.6 (𝜇𝐻) Besides that, we can determine the value of peak current 𝐼 𝑝 :
62.6×10 −6 = 7.99 (𝐴) The primary RMS current is calculated by:
10 = 1.66 (𝐴) And the secondary RMS current is:
Next, determine the number of turns of the primary winding of the pulse transformer to prevent core saturation, 𝑁 𝑝 will be calculated by the formula:
𝑉 𝐷𝐶𝑚𝑎𝑥 : maximum input DC voltage through the primary wire (V)
𝑇 𝑂𝑁𝑚𝑎𝑥 : MOSFET maximum “on” time (𝑠) 𝐴𝑒: cross-sectional area of the pulse transformer core (𝑚 2 ) 𝑑𝐵: saturation flux density (T)
For 𝑁 𝑝𝑚𝑖𝑛 = 13.51 turns, choose larger than this to ensure the pulse transformer is working properly and leave the rest to wind the secondary winding We choose
Then, the number of turns of the secondary winding is:
The ferrite core of the pulse transformer must have a clearance to prevent premature core saturation The clearance length is calculated according to the following formula:
𝐴 𝐿 : is the inductance of the core According to the manufacturer's specifications, for EE42 type ferrite cores, the 𝐴 𝐿 value will be equal to 1029 𝑛𝐻/𝑁 2
𝐿 𝑝 : is the inductance of the primary coil (𝑛𝐻)
𝑁 𝑝 : is the number of turns of the primary coil (turns)
𝐴 𝑒 : cross-sectional area of ferrite core (𝑚 2 ) Obtaining the core clearance G length is:
According to the manufacturer's specifications, the G clearance parameter of the pulse transformer EE42 is 0.25 mm > 0.0755 mm So, this pulse transformer meets the requirements of the circuit
To determine the output filter capacitor \( C_1 \) for the circuit, we consider the maximum output current of 0.5A and a permissible voltage drop of 0.05V across the capacitor The capacitance can be calculated using the appropriate formula, ensuring optimal performance in the circuit.
0.05 = 38.3 (𝜇𝐹) Select output filter capacitor 𝐶 1 is 100 𝜇𝐹 - 450V capacitor
There is a table of statistics as follows:
Primary winding turn Np 15 turns
Secondary winding turn Ns 40 turns
The 𝑉 𝑅𝑂 voltage in the primary coil is caused by the input voltage 𝑉 𝐷𝐶 and reverse voltage on the secondary coil And this voltage is calculated by the formula:
The leakage inductance on the primary coil is: 𝐿 𝐿𝑒𝑎𝑘 = 0.1 × 𝐿 𝑝 = 6.26 (𝜇𝐻) Peak current in primary coil: 𝐼 𝑝 = 7.99 (𝐴)
The clamp voltage (\$V_{Clamp}\$) represents the maximum safe voltage for a component's operation, with lower clamping voltages providing enhanced protection For the 20N60 MOSFET, the \$V_{Clamp}\$ is determined by its VDS, incorporating a safety margin of 90%.
𝑉 𝐶𝑙𝑎𝑚𝑝 = 0.9 × 𝑉 𝐷𝑆 = 0.9 × 600 = 540 (𝑉) Choosing the maximum switching frequency: 𝑓 𝑠𝑤𝑚𝑎𝑥 = 100000 (𝐻𝑧)
In the Snubber circuit, the capacitor is calculated according to the following formula:
68 × 10000 × 100000 = 7.94 (𝑛𝐹) Then, we choose 𝐶 𝑆𝑛𝑢𝑏𝑏𝑒𝑟 = 22 𝑛𝐹 and the rated voltage 𝑉 𝑅 = 630 𝑉
And the diode needed for the spike-suppressing circuit will be a fast recovery diode Therefore, diode CBB22 will be used
To enhance the efficiency of the charging circuit, the feedback signal plays a crucial role, enabling the microcontroller to modify the pulse width By adjusting the duty cycle, the microcontroller ensures that the output voltage and current are optimized for the specific operating conditions of the charging circuit.
The feedback circuit operates by initially applying a 5V voltage to pin 4 of the optocoupler, which is connected in series with pin PA4 of the microcontroller Once the output voltage is sufficient to activate the Zener diode, it triggers the MOSFET, allowing the feedback signal to turn on the optocoupler Consequently, the 5V on pin 4 flows through pin 3 to ground, resulting in a voltage drop on pin PA4 from 5V to 0V This voltage drop serves as the feedback signal for the microcontroller to decrease the duty cycle.
The 1N4740A Zener diode combined with a voltage divider bridge is used to facilitate the feedback of the circuit, returning the signal to the microcontroller
With a breakdown voltage of 10V, we adjust the input voltage to the diode with a voltage divider circuit
To achieve the breakdown voltage of the 1N4740A Zener diode, the output voltage (\$V_{out}\$) must be at least 10V, significantly lower than the maximum expected output voltage of the charging circuit, which is \$V_{DC(max)} = V_{in} = 311V\$ Therefore, resistors \$R_1\$ and \$R_2\$ are selected to reduce the output voltage of the voltage divider to meet these requirements.
To ensure proper operation of the MOSFET, the calculated voltage must exceed the Zener diode's breakdown voltage, accounting for voltage drops in the circuit This ensures that the feedback signal can effectively activate the MOSFET, which requires a voltage range of approximately 5V to 12V.
Upon reaching the breakdown voltage of the Zener diode, the signal triggers the MOSFET in the feedback circuit, enabling the feedback current to flow through pins 1 and 2 of the optocoupler to ground This activation puts the optocoupler in a conduction state The 5V voltage applied to pin 4 of the optocoupler flows through pin 3 to ground, causing a voltage drop at pin PA4 of the microcontroller When the voltage at pin PA4 reaches 0V, the duty cycle is promptly reduced to the initially programmed level.