14 Battery Charging - Wiley

14

Battery Charging

“Power can be, and at no distant date will be, transmitted without wires, for allcommercial uses, such as the lighting of homes and the driving of aeroplanes.I have discovered the essential principles, and it only remains to develop themcommercially. When this is done, you will be able to go anywhere in the world— to the mountain top overlooking your farm, to the arctic, or to the desert —and set up a little equipment that will give you heat to cook with, and light to readby. This equipment will be carried in a satchel not as big as the ordinary suit case.In years to come wireless lights will be as common on the farms as ordinary elec-tric lights are nowadays in our cities.”

Nikola Tesla (1856–1943)

In this chapter, we investigate the charging of electric vehicles from the electric grid.Battery charging connects the vehicle to the electric grid, and many factors must be con-sidered, such as available voltages and wiring, standardization, safety, communication,ergonomics, and more. Various charging architectures and charging standards are used.Conductive and wireless standards are discussed. The boost power-factor correctionpower stage is investigated in detail.

14.1 Basic Requirements for Charging System

Many important issues must be considered when selecting the charging system. Theprincipal issues are safety, reliability, user-friendliness, power levels and charging times,communications, and standardization. These issues are briefly discussed as follows.

Safety: This is the most serious consideration for any automotive manufacturer introdu-cing an electric vehicle (EV) to the consumer marketplace. The battery charger systemmust minimize the risk of electrical shock, fire, and injury to the end user for a widerange of operating and fault conditions. The system must provide various levels ofinsulation and safety checks in order to ensure a safe system. There are a numberof electrical safety standards which are used around the world. The principal standardsare from Underwriters Laboratories (UL) in the United States and VDE in Germany.

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Electric Powertrain: Energy Systems, Power Electronics and Drives for Hybrid, Electric and Fuel Cell Vehicles,First Edition. John G. Hayes and G. Abas Goodarzi.© 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd.Companion website: www.wiley.com/go/hayes/electricpowertrain

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It is necessary to obtain safety approvals from these types of agencies in order to sellproducts in many countries.

Reliability: The automotive environment is very harsh. The same performance isexpected whether a car is driven in the dry heat of the Arizona desert, the freezingcold of Minnesota, or the humid conditions of Florida. The car is exposed to signif-icant shock and vibration in addition to corrosive solvents, salt, water, and mud. Thecharger for the EV must have a long service life with daily operation. The electricalconnector must be designed to withstand over 10,000 insertions and withdrawalsin these harsh conditions and still remain safe for the consumer for the many faultscenarios. For example, the plug and cable should be able to withstand the vehicleweight in case the vehicle accidentally drives over the assembly.

User-friendliness: A consumer product such as a car requires that significant attention bepaid to customer expectations and ergonomics. The present method of fueling a vehiclewith an internal combustion engine is simple and straightforward. EV charging mustalso be simple and should pose minimal challenge to EV users or to young childrenwhom a parent may send to “plug in the car.” Greater emphasis has to be placed onthe ergonomics of refueling an EV compared to a gasoline-fueled car. The basic reasonfor this is that the EVmay require daily charging, whereas the gasoline-powered car mayonly be fueled once a week or less. Such ergonomic considerations as single-handedoperation and intuitive insertion and withdrawal processes help the user.

Power levels and charging times: The charging time for an EV can range from tens ofminutes, if high-power charging is used, to many hours, if low-power chargers areused. Thus, for EVs to gain widespread acceptance, the charging power levels shouldbe maximized in order to reduce the charging times. However, in practice the powerlevels can be limited by the household electrical wiring, electrical grid impacts, batterychemistries and degradation due to high charging levels, and, of course, size and cost.Given the importance of optimizing society’s use of energy, it is important to chargeefficiently. However, as, discussed in Chapter 3, battery charging efficiency can dropwith higher power levels.

Communication: At a basic level, the plug and cable assembly must not only transmitpower, but must also provide a communications path between the charger and thevehicle in order to ensure a safe and optimized power flow. Communication has takenon a greater role in society in the twenty-first century as smartphones and the Internetare part of the overall communications and control interfaces. Simple messages relat-ing to availability, maximum power output, charging time, and problem or faultreporting are also critical communications.

Standardization: Market acceptance of a new product can be accelerated by creating aproduct standard. A market standard can reduce the cost by ensuring a larger marketwith access to more charging points and ease of communication. An advantage of acommonly agreed standard is that the automotive companies do not have to competeon the charging but can focus on the vehicle.

Compliance:Many regulations already exist to ensure safe and reliable operation of elec-trical equipment. The equipment must also comply with standards to limit electricalnoise emissions, commonly known as electromagnetic interference (EMI), an area ofgovernment regulations in order to protect other electrical devices, for example, car-diac pacemakers, and to limit susceptibility and increase immunity to unwanted emis-sions from other equipment or from events, such as lightning strikes. Chargingsystems must comply with all these regulations.

14.1 Basic Requirements for Charging System 413

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14.2 Charger Architectures

Before the various charging topics are investigated in more depth, it is useful to considerthe basic electrical power conversions required to charge a battery.A basic block diagram of the charging power flow is shown in Figure 14.1. A wonder of

the modern world is the electrical grid. The electrical grid provides alternating current(ac) voltages and currents at an electrical frequency. Voltages and frequencies varyaround the globe and will be discussed in the next section. The ac supply frequencyis low and is typically 50 Hz or 60 Hz. A battery requires direct current (dc) electricity,and so the first stage of the power conversion is to rectify and filter the ac waveformfrom the grid to dc. However, this first-stage conversion to dc cannot be supplied directlyto the battery, as a transformer is typically required to order to provide electrical safety tothe user. Thus, the dc is chopped to create a stream of very high-frequency ac (hfac)waveforms. Frequencies of tens or hundreds of kilohertz are common to minimize trans-former size. A transformer is used to provide a physical barrier in the electrical pathbetween the ac grid and the battery and to minimize failure modes which could resultin life-threatening mishaps. The high-frequency ac is rectified and filtered to createdc to supply the battery.There are a number of options for charging the vehicle, and there are a variety of char-

ging technologies available for EVs. Choices and decisions must be made by the variousmanufacturers, infrastructure providers, and the consumers with respect to the followingareas: conductive or wireless/inductive charging, high-power or rapid charging, on-board versus off-board chargers, and integral charging.Conductive charging is the common approach to charging a vehicle. Conductive char-

ging simply means that the vehicle is electrically connected to the off-board poweringsystem by a conductive plug and socket assembly, similar to the operation of commonhousehold electrical appliances. Generally, vehicles feature a low-to-medium power on-board charger, with ac being supplied from the electrical grid to the vehicle. Figure 14.1 ismodified to that shown in Figure 14.2 to include all the charging components on thevehicle.While low-to-medium power chargers are expected to be the commonly used charging

approach, an enabler for EVs is the capability to rapidly charge using a high-powercharger. High-power chargers can be very large physically and are designed as stationaryoff-board devices to be operated in a similar manner to a gasoline pump at a filling sta-tion. In this case, the charger is off-board, and dc is supplied on-board. The conductive dcblock diagram is shown in Figure 14.3.

Rectifier

and

Filter

Inverter

ac

50/60 Hz

dc

Transformer

Rectifier

And

Filter

Batteryhfac dchfac

Figure 14.1 Basic power block diagram for battery charging.

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Wireless or inductive charging does not connect the vehicle to the electrical grid by aconductive coupling using copper wires. Instead, the magic of transformer coupling isused to couple power from the grid to the vehicle without conductive contacts. Suchan approach can result in safety enhancements and consumer ease of use, but also comeswith significant engineering challenges. For wireless charging, the partition between theon-board and off-board components of the charging system is within the transformeritself, as shown in Figure 14.4.

Transformer

Rectifier

and

Filter

Inverter

Rectifier

And

Filter

Battery

On-boardConductive ac

Off-board

dc hfac dchfac

ac

50/60 Hz

Figure 14.2 Conductive ac charging power block diagram.

Rectifier

and

Filter

Inverter

Rectifier

And

Filter

Battery

On-boardConductive dc

Off-board

dc hfac dchfac

ac

50/60 Hz

Transformer

Figure 14.3 Conductive dc charging power block diagram.

Transformer

Rectifier

and

Filter

Inverter

Rectifier

And

Filter

Battery

Off-board On-board

Wireless/Inductive

dc hfac dchfac

ac

50/60 Hz

Figure 14.4 Wireless/inductive ac charging power block diagram.

14.2 Charger Architectures 415

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Integral charging was originally employed for the prototype predecessor of the GMEV1, known as the GM Impact, and the technology has since been developed and usedby such companies as AC Propulsion and Renault. As shown in Figure 14.5, the technol-ogy reconfigures the traction power electronics and machine to re-employ the compo-nents to also perform the power conversions required for charging. Such an approachreduces the overall parts count on the vehicle while requiring additional measures onelectrical safety and isolation.A significant advantage of the integral charging approach is that the power flow can be

made bidirectional. Thus, the vehicle can supply power back to the local grid if required –such operation is known as vehicle to grid (V2G).Microgrid operation is also possible. Amicrogrid is a term for a local power grid which

can be connected to the main grid, or can be disconnected from the main grid for inde-pendent operation if required.

14.3 Grid Voltages, Frequencies, and Wiring

Electrical equipment, especially mobile devices, are designed to operate globally. Thereare many configurations of wiring and voltage, power, and frequency around the world.This section briefly considers the principal configurations.First, the world is divided in terms of the electrical frequency of operation. Most coun-

tries, with a handful of exceptions, operate at either 50 Hz or 60 Hz. Japan is the notableexception as it has two separate grids, one with 50 Hz to the east and one with 60 Hz tothe west. The simplest broad classifications would be that the world operates at 50 Hzexcept for North and Central America, many countries in South America, South Korea,Saudi Arabia, and a handful of other countries.Similarly, the world generally operates with a standard voltage of 230 V (in the nominal

range of 220–240 V), except for North and Central America, where 120 V is standard,and South Korea and Japan with 100 V.Note that there can be significant variations in the voltage during regular operation.

Equipment working off the standard 230 V would likely be designed to operate off a volt-age range of 180 V to 270 V. Equipment can also be designed for universal operation, inwhich case the input stage is designed to accept a voltage range of 80 V to 270 V. Notethat the voltages just discussed are all root-mean-squared (rms) values.

FilterTraction

InverterFilter Battery

On-boardIntegral ac

Off-board

acMotor

windingsdc

ac

50/60 Hz

Figure 14.5 Integral ac charging power block diagram.


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The basic residential wiring system provides a phase voltage of 230 V, 50 Hz betweenthe line and neutral, as shown in Figure 14.6(a). The neutral is typically grounded byphysically connecting the neutral to a copper grounding rod which is driven into theground at a location close to the residence. The three wires of line, neutral, and groundare hard-wired into the charging assembly.The single-phase connection is typically provided by a three-phase transformer. Com-

mercial premises often have a three-phase connection in order to power electric motors,fans, compressors, and so on. The typical three-phase configuration is shown inFigure 14.6(b). Three phase can be an option for many charger power levels, in whichcase the five wires of the three lines, neutral, and ground are supplied to the chargingequipment. Simply multiply the phase voltage by 3 in order to get the appropriate linevoltage. Thus, 400 V is the line voltage when the phase-to-neutral voltage is 230 V.It is common in the 60Hz regions for 100/120 V to be available. Again, basic household

wiring provides a single-phase connection, as shown by Figure 14.7(a). As the 120 V sys-tem outputs a relatively low power, it is usual to have a higher voltage available. It is com-mon to have the grounded midpoint single-phase wiring system of Figure 14.7(b), fromwhich two 120 V outputs are available. Note that these outputs are 180 out of phase witheach other, which means that the sum of the two outputs is 240 V, the high-voltageoutput.

Neutral

Ground (earth)

Phase Voltage = 230 V

Line

Neutral

Ground

(earth)

Line Voltage = 400 V

Line

Line

Line

Ground (earth)


(b)(a)

Figure 14.6 Basic 230 V wiring system.

Neutral


Line

Neutral

(a)


Line Voltage = 240 V


Line

Line

(b)

Ground (earth)

Ground

(earth)

Figure 14.7 Basic 100/120 V wiring system.

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The 120 V output, shown in Figure 14.7(a), may itself be the output of a three-phasetransformer with a nominal 208 V line voltage, as shown in Figure 14.8.The 240 V winding of Figure 14.7(b) can be the output of a single-phase winding of a

three-phase star or delta transformer.The voltages mentioned so far in this section are the typical voltages considered for

household and commercial premises. Higher voltages are likely to be used for high-power charging. These again are three phase, and the wiring configurations can be instar or delta and may feature auxiliary windings.

14.4 Charger Functions

EV chargers are similar in operation but have some key differences compared to batterychargers used in other applications, such as mobile phones and laptop computers. Thebasic functions of a low-power battery charger are shown in Figure 14.9. First, we willreview the functions of the more basic charger and then turn to the functions of the

Neutral

Ground (earth)

Phase Voltage = 120 VLine Voltage = 208 V

Line

Line

Line

Ground (earth)

Figure 14.8 Three-phase 208 V system.

vac

L

N

Full-bridge

Ac-dc

Rectifier Flyback dc-dc

Battery

Pack

Dc

Capacitor

iac Vdc

idciac

Figure 14.9 Low-power charger.


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automotive charger. The related voltage and current waveforms are shown in Figure 14.10.The basic charger power blocks are as follows.

Ac-dc rectifier: The function of the ac-dc rectifier is to rectify, or make positive, theinput ac voltage vac and current iac when they are in the negative half cycle. Thus,the output of the diode rectifier is always positive, as can be seen in the waveformsof the rectifier output voltage │vac│ and current │ iac│.

Dc capacitor: The dc capacitor is charged to the peak ac voltage when the rectified volt-age │vac│ exceeds the capacitor voltage. This only happens during a portion of thecycle, and there is a surge of current from the ac input through the diodes and intothe capacitor. Thus, the current waveform has a sharp pulsed waveform.

Dc-dc converter: The dc-dc converter converts the high voltage on the dc capacitor to asafe lower voltage for input to the laptop or mobile phone for use in charging. Thesimplest and most cost-effective dc-dc is the switch-mode flyback converter, whichswitches at a high frequency and has the transformer isolation that is essential forsafety.

At this point it is useful to identify some very commonly used power terms.

14.4.1 Real Power, Apparent Power, and Power Factor

The apparent power S is the product of the rms voltage V and the rms current I. Thisproduct is also known as the volt-ampere product. In equation form:

S =VI 14 1

The apparent power has the units of volt-amperes (symbol VA).The real power P is the true power delivered to an electrical circuit. The real power is

the power measured by a wattmeter and has the units of watts (symbol W).In a power circuit, the apparent power can vary significantly from the real power due to

the distortion introduced by the power-stage components, such as the diode-capacitiverectifier above, or by the load itself.The power factor PF is the ratio of the real power to the apparent power:

PF =PVI

=PS

14 2

vac

2ππ ωt

iac

Vdc

2ππ ωt

vaciac

(a) (b)

Figure 14.10 (a) Input and (b) rectifier-capacitor waveforms.

14.4 Charger Functions 419

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The power factor is a dimensionless quantity and has no units. A low power factor isundesirable for a number of reasons.First, a poor power factor results in an increased current for a given voltage in order to

supply the required power. From a charging perspective, a low power factor means thatmaximum power cannot be sourced from the supply even though the maximum currentis being supplied.Second, the diode-capacitor front-end, described earlier, results in high-frequency har-

monics of the fundamental 50 or 60 Hz waveform. These harmonics create increasedpower losses through the distribution system, increasing conductor and transformertemperatures and reducing the efficiency and system reliability.Third, the current distortion results in voltage distortion of the supply voltage, which

affects the supply and the other loads fed from the supply.Fourth, commercial and industrial customers of the power utilities can be financially

penalized for demanding currents with a low power factor.Global standards have been developed to govern power quality and harmonic distor-

tion. IEC 61000 contains commonly referenced standards addressing harmonics, EMI,and other grid-related issues.Thus, simple diode–rectifier–capacitor front-ends are only permitted in low-power

applications. At levels above hundreds of watts, the simple capacitive filter is bufferedwith another switch-mode power converter, known as the boost converter. The boostconverter serves to maintain the input current waveform identical with the input voltagewaveform and so eliminates any harmonic distortion and improves the power factor tounity. The power-factor-corrected boost converter is shown in Figure 14.11, and thewaveforms are presented in Figure 14.12. A basic requirement for the boost converteris that the output dc voltage must be greater than the peak of the input ac voltage. Notethat the power converter has been changed from the simple flyback to the full-bridge (seeChapter 12) for higher power.A more detailed overview of the EV battery charging system is shown in Figure 14.13.

This charging system is representative of the on-board conductive systems. The circuithas a number of different functions as follows:

• RCCB: The residual current circuit breaker (RCCB) detects an imbalance in the lineand neutral currents, usually between about 5 to 20 mA, and triggers a circuit breakerto take the charger off-line to prevent fatalities. This circuitry is also known as aground-fault circuit interrupter (GFCI).

• EMI filter: Switching power electronics can generate significant radiated and con-ducted noise, known as electromagnetic interference. A high-current filter with com-mon-mode and differential-mode stages is required to meet legal emissionstandards in the United States (FCC) and the EU (VDE). FCC Part 15b is commonlyreferenced in the United States, while the VDE B standard is commonly referenced inEurope.

• Rectifier: A simple diode bridge rectifies the 50/60 Hz ac waveform.

• Boost PFC: A boost converter, typically switching at tens or hundreds of kHz, chopsup the low-frequency rectified power and boosts it to a voltage level of about 400 Vdc, avalue higher than the peak ac value.

• Dc link: An electrolytic capacitor is usually used for bulk storage to filter the 50/60 Hzcomponent.


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v ac

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• Dc-hfac chopper: A full-bridge or H-bridge converter is used to chop the nominal 400V dc link voltage into a high-frequency pulse stream going from -400 V to +400 V atthe switching frequency.

• Transformer: the high-frequency pulse stream is galvanically isolated for safety by thetransformer. The pulse stream must be high frequency in order to minimize the sizeand weight of the transformer.

• Rectifier-filter: The output of the transformer secondaries are rectified and filtered tocreate dc current to charge the battery.

14.5 Charging Standards and Technologies

A number of charging standards have emerged or are emerging globally. A global stand-ard, IEC 62196, has been developed by the International Electrotechnical Commission(IEC) and acts as an umbrella standard for a number of the charging standards. Theglobal standard covers the basics of power and communication interfaces, while the var-ious charging standards describe the mechanical and electrical specifications of the par-ticular plug and socket assemblies.The main charging standards are

1) SAE J1772 for use in North America and for ac and dc charging.2) VDE-AR-E 2623-2-2 for use in Europe and for single-phase and three-phase ac

charging.3) JEVS G105-1993, known as CHAdeMo and developed in Japan, for use globally for

high-power dc charging.

Tesla vehicles can be charged from a dedicated 240 V Tesla wall charger or by using astandard plug connected to a standard 240 V socket. The ac charger is on-board. TheTesla vehicles can interface to SAE and VDE outlets by using an adapter.

14.5.1 SAE J1772

This standard has been developed by the Society of Automotive Engineers (SAE) for usein North America [1,2]. The standard covers a number of different power levels. Level 1charging is for low-power convenience charging using a standard 120 V outlet and

π/2 ωt

iac

Vdc

iRB=

D2, D3 D1, D4–π/2

vacvRB=

π/2 ωt

iac

–π/2

vac

(a) (b)

D1, D4

Figure 14.12 Power-factor-corrected waveforms: (a) input and (b) rectifier.


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Res

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Curr

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Cir

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supplying up to 1.44 kW or 1.96 kW maximum, depending on the outlet, whereas theLevel 2 standard installed home charger would feature up to 19.2 kW, if available. Abovethese power levels, the standard includes options for high-power off-board dc charging.The various power levels for the SAE standard are presented in Table 14.1.The SAE standard also enables use of a combined socket featuring ac and dc which is

known as the SAE J1772 Combo or CCS Combo.The SAE J1772 Level 2 plug features three power contacts – line, neutral, and ground –

and two signal contacts. A sample socket is shown in Figure 14.14(a).

Table 14.1 SAE J1772 levels

Voltage Max. Continuous Current

Ac Level 1 120 V (input ac) 12/16 A (input ac)

Ac Level 2 240 V (input ac) <80 A (input ac)

Dc Level 1 50–500V (output dc) 80 A (output dc)

Dc Level 2 50–500V (output dc) 200 A(output dc)

(a) (b)

(c)

Figure 14.14 (a) SAE J1772 Level 2 socket, (b) VDE-AR-E 2623-2-2 plug, and (c) VDE-AR-E 2623-2-2 plusSAE Combo socket.


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14.5.2 VDE-AR-E 2623-2-2

The VDE-AR-E 2623-2-2 standard has been developed for use in Europe. It facilitates theuse of three phase, in addition to single phase, as three phase is widely available in parts ofEurope. The standard includes five power wires – three lines, a neutral, and a ground –and two signal wires, as shown in Figure 14.14(b), and is often known as the Mennekesconnector. This standard can bematched with the SAE high-power dc charging standardsocket, as shown in Figure 14.14(c), to create a version of the CCS Combo.

14.5.3 CHAdeMo

Several thousand high-power chargers have been installed globally using the CHAdeMOstandard. The basic high-power charger is 40 kW. The 40 kW charger, plug, and socketare all shown in Figure 14.15.

14.5.4 Tesla

The Tesla residential charger is designed to operate using commonly provided outlets.The vehicle comes with an adapter set which allows the charger to interface to the avail-able power or off-board charging outlet as shown in Figure 14.16(a). The charger cableplug to the vehicle is shown in Figure 14.16(b).

14.5.5 Wireless Charging

14.5.5.1 InductiveWireless charging or inductive charging is a method of transferring electrical power fromthe source to the load magnetically rather than by direct ohmic contact. The technologyoffers the advantages of galvanic isolation, safety, connector robustness, and durability in

(a) (b) (c)

Figure 14.15 CHAdeMO: (a) off-board dc charger, (b) plug, and (c) on-board socket.

14.5 Charging Standards and Technologies 425

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power delivery applications where harsh or hazardous environmental conditions mayexist. Examples of these applications are mining and sub-sea power delivery and EV bat-tery charging. The GeneralMotors EV1 featuredmany new technologies including a rad-ically new design for inductively coupled battery charging.The basic principle underlying inductive coupling is that the two halves of the induc-

tive coupling interface are the primary and secondary of a separable two-part trans-former. When the charge coupler (i.e., the primary) is juxtaposed with the vehicleinlet (i.e., the secondary), power can be transferred magnetically with complete elec-trical isolation, as with a standard transformer. The coupler and vehicle inlet featuredin the EV1 are shown in Figure 14.17(a). The coupler is attached via the cable to the off-vehicle charging module. When the coupler is inserted into the vehicle inlet, powerfrom the coupler is transformer-coupled to the secondary, rectified, and fed to the bat-tery by the battery cable. Note that the coupler contains a ferrite block or “puck” at the

(a) (b)

Figure 14.16 Tesla plug adapters and charging plug.

Coupler

TranformerInlet

Cores

puck

Primary

Secondary

(a) (b)

Figure 14.17 EV1 coupler and vehicle inlet.


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center of the primary winding to complete the magnetic path when the coupler isinserted into the vehicle inlet. The disassembled transformer components are shownin Figure 14.17(b).An off-vehicle high-frequency power converter feeds the cable, coupler, vehicle-

charging inlet, and battery load. The EV user physically inserts the coupler into the vehi-cle inlet where the high-frequency power is transformer-coupled, rectified, and fed to thebattery. The technology was researched and productized at levels ranging from a fewkilowatts to tens of kilowatts, with a high-power demonstration at 120 kW [3–6].A recommended practice for inductive charging of EVs, SAE J1773, was published bythe SAE. The specifications, as outlined in SAE J1773, for the coupler and vehicle inletcharacteristics were to be considered when selecting a driving topology. Among themostcritical parameters are the frequency range, the low magnetizing inductance, the highleakage inductance, and the significant discrete parallel capacitance. The off-vehicleEV1 charge module features a frequency-controlled series-resonant converter. Drivingthe SAE J1773 vehicle interface with the series-resonant converter results in a four-element topology with many desirable features. This resonant topology is discussed indetail in Chapter 12, Section 12.4.

14.5.5.2 WirelessThe inductive charging system developed by GM is no longer used as the market shiftedto conductive standards. However, new wireless charging standards are being developedfor EVs as wireless charging is once again being viewed positively for developing the EVmarket. An interesting application of wireless charging is driverless vehicles, as chargingcan be facilitated without human actions. Recent interest in wireless charging has been inloosely coupled transformer systems. The principles are similar to the inductive couplingjust described, with the difference that the transformer primary and secondary assem-blies are spaced many centimeters apart and have relatively greater leakage inductances.Worldwide standards are being developed. SAE J2954 is the SAE standard. The tech-

nology is also dependant on the types of resonant circuits discussed in Chapter 12 forapplication to inductive coupling. Additional safety issues must be addressed, such asthe effects of radiation on humans and animals and the presence of metal objects inthe magnetic fields.

14.6 The Boost Converter for Power Factor Correction

The front-end of the charger is a power-factor-correction stage utilizing a boost con-verter. An example of an automotive charger is shown in Figure 14.18. The charger fea-tures an interleaved boost and so has two boost inductors, shown with an L. The input(IP) power first flows through the EMI filter (EMI) and is then rectified (R) and boosted(Q+D, L). The charger requires a significant electrolytic bulk capacitor stage (C) in orderto filter the 50/60 Hz ripple. The full-bridge converter stage features the power transfor-mers (Xo), the full-bridge switches and diodes (Qo+Do), the output rectifier (Ro), theoutput inductor (Lo), and output filtering (EMIo). The full-bridge converter is discussedin depth in Chapter 12. The focus of this section is on the boost PFC, as shown inFigure 14.19(a).

14.6 The Boost Converter for Power Factor Correction 427

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14.6.1 The Boost PFC Power Stage

The input current is controlled to be in phase with the supply voltage. This is achieved byusing two control loops: an inner current loop and an outer voltage loop. The convertertypically achieves very high power factors with values greater than 0.99 being reasonableat full load [7–9].The ac voltages and currents are rectified by the input bridge. Let the ac voltage be

vac θ = 2Vph cosθ 14 3

Using the boost PFC, the ac current is controlled to be in phase with the voltage:

iac θ = 2Iph cosθ 14 4

The voltage vRB and current iRB from the rectifier bridge are

vRB θ = 2Vph cosθ 14 5

iRB θ = 2Iph cosθ 14 6

The boost converter controls the inductor current iL to be in phase with the rectifiedinput voltage vRB, as shown in Figure 14.19(b) The inductor current carries the PWMripple current as shown in Figure 14.19(c).The capacitor at the output of the rectifier CR is there to filter the PWM ripple current

of the inductor.The size of the dc link capacitor in a PFC boost converter is based on three factors:

(1) the desired hold-up time of the capacitors, (2) the current rating and aging of theelectrolytic capacitors, and (3) the low-frequency voltage ripple. All three factors must

L

L

C

EMI

Q

+

D

R

IP

OP

EMIo

Xo

Xo

Lo

Ro

Qo

+

Do

Figure 14.18 Automotive EV charger.


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be considered when designing the power and control stages. The dc link capacitor is usu-ally a bank of bulky electrolytic capacitors, which are sized to minimize the low-frequency voltage ripple due to the high second harmonic of the dc link current.The EMI stage provides common-mode and differential-mode EMI filtering in order

to meet the applicable EMC standards [10–13]. It is common for EMI stages to comprise10% to 20 % of the volume of the charger or of the generic power converter.The power pulled from the ac source pac has a pulsing sine2 characteristic given by

pac θ = vac θ × iac θ = 2Vph cosθ × 2Iph cosθ = 2VphIphcos2θ 14 7

π/2 ωt

Vdc

D2, D3 D1, D4–π/2

iL

(b)

(c)

π/2–π/2

(d)

Pac, Idc

iDpac

vac

L

N

iac

Idc

Vdc

iD

iQ

iL

vac

iac

+

–

+

–

Cdc

CR

iRB =

iaciRB =

Q

DD1

D2

D3

D4

L

(a)

EMI

Ground (earth)

vRB =

vacvRB =

D1, D4

π/2 ωt

ωt

D2, D3 D1, D4–π/2 D1, D4

Figure 14.19 Boost PFC and waveforms.


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with an average power Pac given by

Pac =VphIph 14 8

The characteristic of the power is as shown in Figure 14.19(d). The input power pulsesfrom zero to twice the average power with a sine-squared (sine2) characteristic.If we consider the low-frequency characteristic of the diode current, then the diode

current also has a low-frequency sine2 characteristic at twice the line frequency as shownin Figure 14.19(d). Neglecting PWM, the low-frequency dc link power is given by

pD θ =Vdc × iD θ 14 9

If we ignore the rectifier and boost power loss, the instantaneous dc link power pDequals the input power pac:

pD θ = pac θ 14 10

and so

iD θ =pac θ

Vdc=2VphIphVdc

cos2θ 14 11

Thus, the boost diode has a similar low-frequency sine2 current characteristic, withpower pulses from zero to twice the average power as shown in Figure 14.19(d). Theaverage of the diode current equals the dc link current Idc and is given by

Idc =PacVdc

=VphIphVdc

14 12

This low-frequency current at twice the line frequency is typically filtered by the dc linkcapacitor Cdc. It is also an option in battery chargers to have the sine2 current flow intothe battery, significantly reducing the requirement for the dc link capacitor [14]. How-ever, sine2 charging, as it is known, increases the ripple current flowing into the battery.

14.6.2 Sizing the Boost Inductor

The boost inductor is sized in order to limit the ripple current and reduce harmonics andEMI. From the analysis of the CCM boost in Chapter 11 Section 11.4.1, the duty cycle ofthe boost switch is given by

d θ = 1−vRB θ

Vdc= 1−

2Vph cosθVdc

14 13

The peak-to-peak ripple current is

ΔIL p−p θ =2Vph cosθ

fs Ld θ =

2Vph

fsLcosθ −

2Vphcos2θVdc

14 14

and so the ripple current magnitude clearly varies with duty cycle.

14.6.2.1 Example: Sizing the InductorA 3.3 kW PFC boost is designed for a nominal input voltage of 230 V, 50/60 Hz with aninput voltage ranging from a low line of 180 V to a high line of 265 V. Determine the valueof the boost inductor and the peak inductor current if the switching frequency is 100 kHz


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and the peak-to-peak ripple ratio is 20% of the peak low-frequency current at the peak ofthe nominal input voltage. The dc link voltage is 380 V. Ignore the power loss.

Solution:The rms input current is

Iph =PVph

=3300230

A= 14 35A

The peak of the input current is

Iph peak = 2Iph = 2× 14 35A= 20 29A

The peak-to-peak ripple current at the peak of the input voltage is

ΔIL p−p = riIph peak = 0 2 × 20 29A= 4 06A

The duty cycle at the peak of the input voltage is

d 0 = 1−2Vph cos0

Vdc= 1−

2Vph

Vdc= 1−

2 × 230380

= 0 1440

The required inductance is determined by rearranging Equation (14.14):

L=2Vph cos0

f ΔIL p−p 0d 0 =

2 × 230

100 × 103 × 4 06× 0 144H=115μH

The peak current in the inductor is the sum of the peak of the line current and half ofthe peak-to-peak ripple:

IL peak = Iph peak +ΔIL p−p

214 15

In this case:

IL peak = Iph peak +ΔIL p−p

2= 20 29A+

4 062

A= 22 32A

Once the peak and rms currents are known, the inductor can be sized as covered inChapter 16, Section 16.3.7, using the area-product method. The rms input currentcan be used as a reasonable approximation for the rms inductor current.

14.6.3 Average Currents in the Rectifier

The input rectifier diodes alternately conduct. DiodesD1 andD4 conduct during the pos-itive half cycle, while diodes D2 and D3 conduct during the negative half cycle.The average current in the rectifier diodes IR(dc) for a half cycle is

IR dc = ID1 dc = ID4 dc =12π

π2

−π2

iRB θ dθ =12π

π2

−π2

2Iphcosθdθ =Iph2π

π2

−π2

cosθdθ =2π

Iph

14 16


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The rms current in the rectifier diodes IR(rms) for a half cycle is

IR rms = ID1 rms = ID4 rms =12π

π2

−π2

iRB θ 2dθ =12π

π2

−π2

2Iph cosθ2dθ =

1

2Iph

14 17

The values of the dc and rms currents in diodesD2 andD3 are the same as in diodesD1

and D4.

14.6.3.1 Example: Input Rectifier Power LossDetermine the power loss in the input rectifier stage of the earlier example if each rec-tifier has the following parameters: Vf0 =0.8 V and rf = 10 mΩ.

Solution:The average current in the rectifier diodes for a half cycle is

IR dc =2π

Iph =2π

× 14 35A= 6 46A

The rms current in the rectifier diodes for a half cycle is

IR rms =1

2Iph =

1

2× 14 35A= 10 15A

Per the section on semiconductor losses in Chapter 11, Section 11.5, the conductionloss per rectifier diode is

PR cond =Vf 0IR dc + rf IR rms2 14 18

In this case:

PR cond =Vf 0IR dc + rf IR rms2 = 0 8 × 6 46W+0 01 × 10 152W=6 2W

The loss in the rectifier bridge PRB is four times the loss in a single rectifier diode:

PRB = 4× PR cond 14 19

For this example:

PRB = 4× PR cond = 4 × 6 2W= 24 8W

14.6.4 Switch and Diode Average Currents

The duty cycle of the boost switch is given by Equation (14.13).The low-frequency time-averaged current in the switch can be approximated by

iQ θ = d θ × iac θ 14 20

Substituting in Equation (14.4) and Equation (14.13) yields:

iQ θ = 1−2Vph cosθ

Vdc× 2Iph cosθ 14 21


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or

iQ θ = 2Iph cosθ−2VphIphcos2θ

Vdc14 22

The average current in the switch is given by

IQ dc =1π

π2

−π2

iQ θ dθ 14 23

Substituting Equation (14.22) into Equation (14.23) yields

IQ dc =Iphπ

π2

−π2

2cosθ−2Vphcos2θ

Vdcdθ 14 24

which simplifies to

IQ dc = Iph2 2π

−Vph

Vdc14 25

since

π2

−π2

cosθdθ = 2 and

π2

−π2

cos2θdθ =π2

14 26

Similarly, the low-frequency time-averaged current in the boost diode is given by

iD θ = 1−d θ × iac θ 14 27

which expands to

iD θ =2Vph cosθ

Vdc× 2Iph cosθ 14 28

The diode conducts the current for half the period, and the average current is

ID dc =1π

π2

−π2

iD θ dθ =1π

π2

−π2

2VphIphcos2θVdc

dθ =2VphIphπVdc

π2

−π2

cos2θdθ 14 29

which simplifies to

ID dc =VphIphVdc

14 30


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14.6.5 Switch, Diode, and Capacitor RMS Currents

The rms current in the boost switch can be approximated by

IQ rms =1π

π2

−π2

d θ iac θ 2dθ 14 31

Substituting in Equation (14.4) and Equation (14.13) gives

IQ rms =1π

π2

−π2

1−2Vph cosθ

Vdc× 2Iph cosθ

2dθ 14 32

which simplifies to

IQ rms = Iph1π

π2

−π2

2cos2θ−2 2Vph cosθ cos2θ

Vdcdθ 14 33

Since

π2

−π2

cosθ cos2θdθ =43

14 34

the rms current in the switch is given by

IQ rms = Iph 1−8 2Vph

3πVdc14 35

Similarly the rms current in the boost diode is given by

ID rms = Iph8 2Vph

3πVdc14 36

The rms current in the high-voltage dc link capacitor ICdc(rms) is simply given by

ICdc rms = ID rms2− ID dc

2

= Iph8 2Vph

3πVdc−Vph

2

Vdc2

14 37

14.6.6 Power Semiconductors for Charging

While the silicon IGBT and the silicon diode are dominant for low-to-medium switchingfrequencies, the silicon MOSFET is typically preferred at higher switching frequencies


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for low- and medium-range voltages (<600 V). The conduction loss in the MOSFET canbe simply modeled by the drain-source on-resistance, typically designated RDS(on). Themaximum RDS(on) is usually specified for the component. A characteristic of the powerMOSFET is that the on-resistance typically doubles between 25 C and 120 C.The MOSFET conduction loss is given by

PQ cond =RDS on IQ rms2 14 38

AsRDS(on) increaseswith an index somewhere between the square and cube of the devicevoltage rating, the siliconMOSFET cannot compete with the silicon IGBT for higher vol-tages. However, the MOSFET is a competitive device for a high-frequency PFC boost.A significant source of power loss for the switching pole is the reverse recovery loss of

the boost diode. This loss can be eliminated by employing the more expensive wide-band-gap silicon-carbide (SiC) Schottky diode, which has no reverse recovery loss.The average current in the diode and switch when switched is

IQ sw,avg = ID sw,avg =2 2π

Iph 14 39

Similar to the IGBT in Chapter 11, Section 11.5.1.3, the MOSFET switching powerlosses remain

PQ sw = fs Eon + EoffVdc

Vtest14 40

It is assumed for simplicity that the switching loss for the SiC diode is zero.In the final part of this section, the losses are estimated for the semiconductor switches

in the boost PFC.

14.6.6.1 Example: Silicon MOSFET and SiC Diode Power LossesThe PFC boost converter of the examples used in this chapter features a representativesilicon MOSFET and a SiC diode, nominally the part MKE 11R600DCGFC from man-ufacturer IXYS [15].The nominal maximum RDS(on) of the device is specified as 0.165 Ω at 25 C, but

increases to about 0.375 Ω at 125 C as shown in Figure 14.20(a).The diode conduction drops are shown in Figure 14.20(b) and can be modeled by Vf0 =

0.8 V and rf = 8.8 mΩ at 125 C.The MOSFET switching losses are shown in Figure 14.21. The diode switching losses

are ignored.Determine the MOSFET and diode power losses, assuming junction temperatures

of 125 C.

Solution:The switch rms current is

IQ rms = Iph 1−8 2Vph

3πVdc= 14 35 × 1−

8 2 × 2303π× 380

A= 7 504A

The MOSFET conduction loss is

PQ cond =RDS on IQ rms2 = 0 375 × 7 5042W=21 1W


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0.0

0.1

0.2

0.3

0.4

0.5

(a) (b)

–60 –20 20 60 100 140 180

RD

S(on

) (Ω

)

Tj (°C)

0

5

10

15

20

25

0 1 2 3 4

I f (A

)

Vf (V)

25°C150°C

125°C

On resistanceRDS(on)

Figure 14.20 Representative 600 V, 15 A MOSFET and SiC diode conduction characteristics.

0.00

0.05

0.10

0.15

0.20

0.25

0 5 10 15 20 25

E (

mJ)

ID (A)

Eon

Eoff

Figure 14.21 Representative 600 V, 15 AMOSFET (and SiC diode) turn-on andturn-off switching losses at 125 C and atest voltage of 380 Vdc.


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The diode average and rms currents are

ID dc =VphIphVdc

=230 × 14 35

380A= 8 69A

ID rms = Iph8 2Vph

3πVdc= 14 35×

8 2 × 2303π× 380

A= 12 23A

The diode conduction loss is

PD cond =Vf 0ID dc + rf ID rms2 = 0 8 × 8 69W+0 0088 × 12 232W=8 3W

For a sinusoidal excitation, the average switching current in the switch and diode is

IQ sw,avg = ID sw,avg =2 2π

Iph =2 2π

14 35A= 12 92A

The turn-on and turn-off power loss in the switch is

PQ sw = fs Eon + EoffVdc

Vtest= 100 × 103 0 1 × 10−3 + 0 013 × 10−3 380

380W=11 3W

where Eon = 0.1 mJ and Eoff = 0.013 mJ at 12.92 A from Figure 14.21.The power losses in the switch and diode are

PQ =PQ cond +PQ sw = 21 1W+11 3W=32 4W

and

PD =PD cond = 8 3W

14.6.6.2 Example: PFC Stage LossesIf the equivalent series resistance of the inductor is Rcu = 50 mΩ, and the combined aux-iliary power and stray power loss Paux is 15 W, determine the overall PFC converterpower loss and efficiency.

Solution:The inductor loss is

PL =RcuI2ph = 0 05 × 14 352W=10 3W

The auxiliary and stray loss is

Paux = 15W

The total loss is the sum of the losses in the inductor, the auxiliary circuits, the bridgerectifier, and the boost switch and diode:

Ploss = PL + Paux + PRB +PQ + PD = 10 3W+15W+24 8W+32 4W+8 3W=90 8W

The approximate efficiency is

η=P

P +Ploss× 100 =

33003300 + 90 8

× 100 = 97 3


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References

1 SAE Electric Vehicle and Plug in Hybrid Electric Vehicle Conductive Charge Coupler, SAEJ-1772, Society of Automotive Engineers.

2 SAE Power Quality Requirements for Plug-In Electric Vehicle Chargers, SAE J-2894,Society of Automotive Engineers.

3 J. G. Hayes, Resonant Power Conversion Topologies for Inductive Charging of ElectricVehicle Batteries, PhD Thesis, University College Cork, 1998.

4 SAE Electric Vehicle Inductive Coupling Recommended Practice, SAE J-1773, Society ofAutomotive Engineers, Draft, Feb. 1, 1995.

5 R. Severns, E. Yeow, G. Woody, J. Hall, and J. Hayes, “An ultra-compact transformer for a100 W to 120 kW inductive coupler for electric vehicle battery charging,” IEEE AppliedPower Electronics Conference, pp. 32–38, 1996.

6 J. G. Hayes, N. O’Donovan, and M. G. Egan, “Inductance characterization ofhigh-leakage transformers,” IEEE Applied Power Electronics Conference, pp. 1150–1156, 2003.

7 N. Mohan, Power Electronics A First Course, Chapter 6, John Wiley & Sons, 2012.8 S. Abdel-Rahman, F. Stuckler, and K. Siu, PFC Boost Converter Design Guide 1200W Design Example, Infineon Application Note, 2016.

9 Texas Instruments, UCC2817, UCC2818, UCC3817 and UCC3818 BiCMOS PowerFactor Preregulator, Unitrode Products from Texas Instruments, revised 2015.

10 M. Nave, Power Line Filter Design for Switched-Mode Power Supplies, Van NostrandReinhold, 1991.

11 H. W. Ott, Electromagnetic Compatibility Engineering, John Wiley & Sons, 2009.12 P. Bardos, “Predicting the EMC performance of high-frequency inverters,” IEEE Applied

Power Electronics Conference, pp. 213–219, 2001.13 M. Kacki, M. Rylko, J. G. Hayes, and C. R. Sullivan, “Magnetic material selection for EMI

filters,” IEEE Energy Conversion Congress and Exposition, 2017.14 M. G. Egan, D. O’Sullivan, J. G. Hayes, M. Willers, and C. P. Henze, “Power-factor-

corrected single-stage inductive charger for electric-vehicle batteries,” IEEE Transactionson Industrial Electronics, 54 (2), pp. 1217–1226, April 2007.

15 Website of IXYS Corp., www.ixys.com.

Further Reading

1 N. Mohan, T. M. Undeland, andW. P. Robbins, Power Electronics Converters, Applicationsand Design, 3rd edition, John Wiley & Sons, 2003.

2 R. W. Erickson, Fundamentals of Power Electronics, Chapter 17, Kluwer AcademicPublishers, 2000.

3 M. Yilmaz and P. T. Krein, “Review of battery charger topologies, charging power levels,and infrastructure for plug-in electric and hybrid vehicles,” IEEE Transactions on PowerElectronics, 28 (5), pp. 2151–2169, May 2013.

4 R. Ryan, J. G. Hayes, R. Morrison, and D. Hogan, “Digital control of an interleaved BCMboost PFC converter with fast transient response at low input voltage,” IEEE EnergyConversion Congress and Exposition, 2017.


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Problems

14.1 Determine the following for the converter example used in this chapter when theline voltage is (i) 180 V and (ii) 265 V, at an output power of 3.3 kW: input current,rectifier bridge and inductor power losses, switch and diode average and rms cur-rents and losses, overall converter loss, and efficiency.

[Ans. (i) Iph = 18.33 A, PRB = 33.1W, PL = 16.8W, IQ(rms) = 12.04 A, ID(dc) = 8.68 A,ID(rms) = 13.82 A, PQ = 69.9 W (Eon ≈ 0.025 mJ, Eoff ≈ 0.13 mJ), PD = 8.6 W, Ploss =143.4 W, Eff = 95.8%; (ii) Iph = 12.45 A, PRB = 21.0W, PL = 7.8 W, IQ(rms) = 5.02 A,ID(dc) = 8.68 A, ID(rms) = 11.39 A, PQ = 18 W, PD = 8.1 W, Ploss = 69.9 W, Eff= 97.9%]

14.2 A 2 kW PFC boost is designed for a nominal input voltage of 230 V with an inputvoltage ranging from a low line of 180 V to a high line of 265 V. The dc link voltageis 380 V. Assume an equivalent series resistance of the inductor of 50 mΩ andauxiliary and a stray power loss of 15 W. Use the semiconductor characteristicsof Section 14.6.6.1.i) Determine the value of boost inductor if the switching frequency is 60 kHz and

the peak-to-peak ripple ratio is 15% of the peak low-frequency current at thepeak of the nominal input voltage.

ii) Determine the following at the nominal voltage: input current, rectifier bridgeand inductor power losses, switch and diode average and rms currents, andswitch and diode losses, overall converter loss, and efficiency.

0.00

0.05

0.10

0.15

0.20

0.25

0 10 20 30 40 50

E (

mJ)

ID (A)

Eon

Eoff

Figure 14.22 Representative 600 V, 30 AMOSFET (and SiC diode) Turn-on andturn-off energy curves at 380 Vdc.

Problems 439

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[Ans. 422 μH, Iph = 8.7 A, PRB = 14W, PL = 3.8W, IQ(rms) = 4.55 A, ID(rms) = 5.26 A,ID(rms) = 7.41 A, PQ = 11.6W, PD = 4.7W, Paux = 15W, Ploss = 49.1W, Eff = 97.6%]

14.3 A 6.6 kW PFC boost is designed for a nominal input voltage of 220 V, 50/60 Hz.The switching frequency is 40 kHz and the peak-to-peak ripple ratio is 10% of thepeak low-frequency current at the peak of the nominal input voltage. The dc linkvoltage is 380 V.i) Determine the values of the boost inductor and the peak inductor current.ii) Determine the power losses in the boost switch if the RDS(on) of theMOSFET is

0.19 Ω, and the turn-on and turn-off energy losses are given by the curves inFigure 14.22. Note that the curves are for a dc link voltage of 380 V.

[Ans. 332 μH, IL(peak)= 44.5 A, PQ = 57 W]

Assignments

What are the voltages, currents, frequency, and wiring configurations in your region?Which charging standards are used in your region?


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Documents

14 Battery Charging - Wiley