Week 4 Session 5

2.1. Conventional Boost DC–DC Converter (BC)

• A conventional step-up or pulse-width modulation (PWM) boost converter is depicted below, which consists of a DC input voltage source Vs., energy storage element (i.e., inductor and capacitor), controlled switch (MOSFET, IGBT, etc.) Q, diode D, filter capacitor C, and load (electric motor)
• In a boost DC–DC converter, the output voltage is always more significant than the input voltage, hence the name “Boost”

Figure Conventional Boost DC–DC Converter.

• The conventional boost converter has several merits, including simpler circuitry, lower cost due to fewer component counts, filtering and reducing electromagnetic interference efficiently, and high efficiency
• Despite many merits, this converter cannot achieve high voltage gain; extra protection requires to protect the circuit from short-circuit, and the power-switching devices require a parallel arrangement for handling high power, and the system is quite large in volume and heavily weighted because the large capacitor that is used to filter out the output voltage ripples
• To maintain a constant output voltage despite changes in input supply, designing a high-performance control system for DC–DC converters is very difficult due to the nonlinearity such as bifurcation, multiple equilibrium points, periodic behavior, and chaos of the converter
• Controllers can be of two types: voltage mode controllers, and current mode controllers. Current mode controllers are widely utilized for DC–DC converters due to their several benefits
• In , several state-of-the-art control system design methods, such as sliding mode control (SMC) , model predictive control (MPC) , intelligent fuzzy logic/control system , fractional-order proportional-integral-derivative (FOPID) control systems were proposed for conventional DC–DC boost converter topologies to mitigate these problems
• The SMC method has invariance to internal parameter variations, insensitiveness to external disturbances, fast transient response, and can improve the robustness quickly against nonlinear uncertainties; the MPC can easily consider the state variables/input constraints in the design procedure and control the conventional DC–DC converter
• The fuzzy logic-type PID control methods are generally utilized due to their effective, simple, practical, and easily tuned capabilities
• In a fractional-order PID (FOPID) control method was proposed and verified via experimental studies, which showed that the control system can provide a faster recovery time and less overshoot for a DC–DC boost converter.

2.2. Interleaved Four-Phase Boost DC–DC Converter (IBC)

• The interleaved boost converter comprises a parallel connection of boost or step-up converters
• The current is divided due to a parallel connection. So, current stresses are decreased as the power losses are minimized
• The interleaved four-phase boost DC–DC converter (IBC) comprises four similar inductors (L1, L2, L3, L4) in four step-up levels to reduce the conductor weight and input current ripples, four parallel power-switching devices for successive phase shifting, diodes, and a filtering capacitor to eliminate the output voltage ripples
• All these inductors contain individual magnetic cores for better energy storing and release. As a result, the IBC topology can increase the voltage level by more than four times

Figure. Four-phase Interleaved DC–DC Boost converter.

• Moreover, the four-phase interleaved boost DC–DC converter has been chosen for EVs for its several compatible reasons, such as a reduction in inductor size and output capacitor size, a more significant reduction in input current and output voltage ripples, and higher overall system efficiency
• Nevertheless, the IBC is sensitive to any changes in the duty cycle, high cost, and impact on the magnetic core due to any changes in load
• The interleaved multi-phase boost converter has an enormous controlling process, but to make the system stable for significant disturbancessome advanced control methods such as model predictive control, fuzzy controller, sliding mode control (SMC) and PI hybrid controller , high order sliding mode control (HOSMC) , active disturbance rejection control (ADRC) were proposed, designed, analyzed and applied to the multi-phase IBC. Moreover, an advanced hybrid Super-Twisting (ST) ADRC dual-loop controller was proposed and developed in , where they discussed that the ST-ADRC controller provides stronger robustness against the input voltage and load disturbances, better voltage tracking performance, and a more significant reduction in both the recovery time and voltage fluctuations compared to the conventional control systems and can improve the control performance of the converter tremendously.

3.3. Boost DC–DC Converter with Resonant Circuit (BCRC)

• Conventional boost DC–DC converters are typically operated with hard switching, which increases the switching loss of converters in BEV and PHEV powertrains
• The boost DC–DC converter topologies uses a soft-switching configuration to suppress this loss
• In the soft switching technique, during the switching transition (i.e., turn ON or turn OFF), voltage or current across the switch becomes zero
• As a result, the product of the voltage and current is zero; hence power losses are zero
• Thus, the converter can achieve high switching frequency by reducing switching losses
• Due to the switching loss reduction, the heatsink size becomes lessened, which decreases the converter volume
• The soft-switching configuration for the boost DC–DC converter with the resonant circuit, which consists of two switching devices, the main switch Q1 and the auxiliary switch Q2

Figure DC–DC Boost Converter with auxiliary Resonant Circuit.

• Moreover, any abnormality in load power will not affect the converter as the converter has high safety regulations. However, BCRC is incompatible with high-power EV powertrains and does not support bidirectionality
• Soft-switching techniques are considered the best-suited way to enhance the efficiency and reliability by reducing switching losses of electric vehicles’ DC–DC converters

2.4. Full Bridge Boost DC–DC Converter (FBC)

• The circuit diagram of an isolated full-bridge bidirectional DC–DC converter is depicted below
• This converter works in buck-and-boost mode, thus it is a bidirectional converter
• In the forward direction, it works as a buck converter; in the backward direction, it operates as a boost converter
• Due to this bidirectionality, an isolated full-bridge bidirectional DC–DC converter can charge the batteries and provide the voltage to the load
• The bidirectional full-bridge DC–DC converter has three working stages: DC–AC conversion as an inverter; step-up/step-down AC voltage with a high-frequency transformer (HFT); and AC–DC conversion as a rectifier

Figure Isolated Full-bridge Boost DC–DC Converter.

2.5. Isolated ZVS DC–DC Converter (ZVSC)

• For isolation, cold starting, and soft switching, an isolated ZVS DC–DC converter is needed an isolated zero-voltage switching DC–DC converter (ZVSC), where a dual half-bridge topology is placed on both sides of the transformer, and for soft-switching, each power switching device has a parallel capacitor
• ZVSC has a simple control technique, higher efficiency, soft-switching without extra circuity, high-power density, less component count, compact packaging, and lightweight, and no real device rating consequences compared to the traditional full-bridge DC–DC converter

Figure Isolated Zero Voltage Switching (ZVS) DC–DC Converter.

2.6. Isolated Multiport DC–DC Converter (MPC)

• The isolated multiport DC–DC converter is used when more than one input source is needed with galvanic isolation between the source and load
• The multiport converter is classified into three main categories: single-input multi-output (SIMO) converter, multi-input-single-output (MISO) converter, and the multi-input-multi-output (MIMO) converter
• Among them, the MIMO multiport DC–DC converter is used in battery and plug-in hybrid vehicles
• It has coupled multiple input sources (generally supercapacitor and battery) and uses them as a single source with the advantages of various sources
• The circuit diagram of the MIMO-MPC boost DC–DC converter which consists of a parallel connection of two boost DC–DC converters with bidirectional power flow, and it is advantageous for BEVs powertrains because it allows the converter to recharge the input sources during regenerative braking
• Because of this, the effectiveness and functionality of the MPC increase, which makes it a high-power density converter
• Moreover, all the input ports, as well as the output port, remain isolated from each other due to the interior multi-winding transformer

Figure Isolated Multiport DC–DC Converter.

2.7. Multidevice Interleaved DC–DC Bidirectional Converter (MDIBC)

• A transformer is a heavy component that increases the weight of an isolated boost DC–DC converter
• For this reason, a non-isolated multidevice interleaved bidirectional DC–DC converter is used in high-power vehicular applications
• A non-isolated multi-device interleaved bidirectional DC–DC converter (MDIBC) which uses a battery as the primary power source and a supercapacitor as a secondary or auxiliary power source
• MDIBC is a multiphase multiport bidirectional converter consisting of a phase interleaving technique with four high-frequency switching devices per phase
• The number of parallel devices per phase decreases with the increasing number of phases
• Multidevice interleaved boost DC–DC converters merge the power from two or more input sources and are supplied at a constant single output voltage level

Figure Bidirectional Multidevice Interleaved DC–DC Boost Converter.

3.Inverter(DC - AC Converter)

• The inverter is the key component for an electric vehicle because it drives the powertrain of EVs by running the electric motor with three-phase AC from the DC-link
• An inverter can be classified according to input source wise, output phase wise, output voltage wise, number of voltage levels wise, PWM wise, etc
• The voltage level-wise classification of the inverter
• According to the number of voltages level-wise, an inverter can be classified into two main categories:

(a) the two-level inverter

(b) multi-level inverter

3.1. Two-Level Inverter (TLI)

• The two-level inverter which consists of six power-switching devices
• These six power switches are connected into three legs, and each leg contains two switches, which are connected in series
• An antiparallel diode is connected to each switch to allow the current to flow in the opposite direction
• By controlling these six switches in different manners, the inverter can generate eight other states

Figure Topology of a two-level inverter (TLI).

3.2. Multi-Level Inverter (MLI)

• To gain less current and voltage total harmonic distortion (THD), less voltage stress on semiconductor devices, high efficiency, low EMI, and common-mode voltage, many conventional two-level inverters have been replaced in the past decades by their multilevel counterparts
• In addition, due to the modularity and fault tolerance capability, some MLI topologies becomes more beneficial for specific applications
• The basic concept of an MLI where switching devices carry much lower voltages than TLI, and the output filter size is decreased due to the ability to produce various voltage levels with better voltage/current quality at the lower switching frequency
• As a result, higher power levels can be achieved with MLIs without switches derating

Figure One leg of (a) 2-level, (b) 3-level and (c) n-levels inverter

• Although multiple-DC-source multilevel inverters (MDCS-MLIs) require multiple isolated DC supplies , in high-power motor driving applications like electric vehicles, cascaded H-bridge inverter (CHB) MDCS-MLIs are most commonly utilized due to their high modularity and identical voltage rating of the employed switches
• For military combat vehicles and heavy-duty trucks, cascaded H-bridge (CHB) MLI was first recommended in 1998 as a suitable choice due to it can drive high-voltage motors easily with low-voltage switching devices
• Furthermore, there are many different topologies, such as Neutral Point Clamped (NPC) topology, Packed U-Cells (PUC) topology , and Flying Capacitor Inverter (FCI) topology , by which a multi-level inverter can be built but the topology is the cascaded multilevel inverter
• Moreover, a novel H-bridge two-transistor cascaded multi-level voltage source converter is introduced, and this promising multi-level converter could be explored for electrified transportation applications
• Below figure depicts a multi-level inverter, which consists of series-connected H-bridges
• These H-bridges can be controlled independently, and each H-bridges consists of an individual energy storage V_DCML and four power-switching devices
• The MLI can create different outputs like V_DCML, −V_DCML, 0, and open circuits by controlling these power switches in different manners

Figure Topology of a 7-level multilevel inverter (MLI)