Table of Contents

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Application Report

Power Topology Considerations for Electric Vehicle
Charging Stations

Harish Ramakrishnan, Jayanth Rangaraju
ABSTRACTAs the number of electric vehicles (EVs) increase, there is a growing need to create more energy-efficientcharging infrastructure systems around the world that can charge vehicles faster than ever before. New EVs
have higher ranges and larger battery capacities than their predecessors, necessitating the development of fast
DC charging solutions to support quick charging requirements. In this Application Report we look into topology
consideration for designing power modules that acts as a building block for design of these fast DC Charging
Station.

Table of Contents
1 Introduction.............................................................................................................................................................................2
1.1 AC Charging Station.......................................................................................................................................................... 2
1.2 DC Charging Station.......................................................................................................................................................... 3
2 Power Module in DC Charging Station................................................................................................................................. 3
2.1 Power Stage in a Fast DC Charger....................................................................................................................................4
2.2 AC/DC and DC/DC Power Stages..................................................................................................................................... 5
3 Power Topologies in AC/DC (PFC)........................................................................................................................................ 6
3.1 Single-phase Totem Pole PFC........................................................................................................................................... 6
3.2 Single-phase Neutral Point Clamped PFC.........................................................................................................................7
3.3 Three-phase Two-level PFC.............................................................................................................................................. 8
3.4 Three-phase Vienna PFC...................................................................................................................................................8
3.5 Three-phase ANPC/NPC Three-level PFC...................................................................................................................... 10
3.6 Three-phase TNPC Three-level PFC............................................................................................................................... 11
3.7 Summary of AC/DC Topologies....................................................................................................................................... 13
4 Power Topologies in DC/DC.................................................................................................................................................14
4.1 Conventional Phase-shifted Full Bridge (PSFB).............................................................................................................. 14
4.2 Dual Active Bridge (DAB).................................................................................................................................................15
4.3 LLC Resonant Converter................................................................................................................................................. 16
4.4 DAB in CLLLC Mode........................................................................................................................................................17
4.5 Summary of DC/DC Topologies....................................................................................................................................... 18
5 Upcoming Trends in DC Fast Charging Infrastructure......................................................................................................19
5.1 Portable Chargers............................................................................................................................................................ 19
5.2 Vehicle-to-Grid Technology (V2G)....................................................................................................................................20
6 Conclusion............................................................................................................................................................................ 20
7 References............................................................................................................................................................................ 21

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1 Introduction
A charging station is part of the grid infrastructure installed along a street, parking lot or in a home garage; its
primary purpose is to supply the power to the PHEV for charging the battery. There are mainly two types ofcharging systems, as shown in Table 1-1: AC and DC charging systems.
An AC charger powers the EV battery through the vehicle’s on-board charger, while a DC charger directly
charges the vehicle’s battery.
Table 1-1 details the charging stations classified based on power levels.
Table 1-1. Charging Station Classification
EVSE TypePower Supply

Charger Power

Charging Time* (approximate)for a 24-kWh Battery

AC charging station: L1
residential

120/230 VAC and 12 A to 16 A
(Single Phase)

Approximately 1.44 kW toapproximately 1.92 kW

Approximately 17 Hours

AC charging station: L2
commercial

208–240 VAC and 15 A toapproximately 80 A (Single/split
phase)

Approximately 3.1 kW toapproximately 19.2 kW

Approximately 8 Hours

DC charging station: L3 fast
chargers

300 to 600 VDC and (Max 400 A)(Poly phase)

From 120 kW up to 240 kW

Approximately 30 Minutes

1.1 AC Charging Station
A Level 1 EVSE uses commonly-available 120 VAC/230 VAC power sources, draws current in the order of a 12 A
to 16 A range and can take anywhere from 12 to 17 hours to fully charge a 24-kWh battery. L1 chargers can go
up to a maximum power of 2 kW and is used in residential applications. A Level 2 EVSE (typically used incommercial spaces such as malls, offices, and so forth) uses poly-phase 240 VAC sources to power a morerobust vehicle charger and draws anywhere between 15 A and 80 A to completely charge a 24-kWh battery in
about eight hours (power level up to 20 kW). Figure 1-1 shows a typical block diagram of an AC charging station.
AC Charging System Power Flow
Electric Vehicle
HVDC

Pilot Wire
Grid

EVSE

(OBC)
AC/DC Converter

BMS

AC

Battery Pack

Figure 1-1. AC Charging Station

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Introduction

1.2 DC Charging Station
The DC charging station is a Level 3 charger which can cater for very high power level in the range of 120 to 240
kW. The L3 chargers typically charge batteries to 80% State of Charge (SOC) in under 30 minutes. To achieve
such high power levels modular converters which can be stacked are used. Stacking of converters inside thevehicle makes the vehicle bulky. Hence, these stacked converters are placed outside the vehicle and constitutethe EV charging station. The EV charging station is directly interfaced with the battery of the vehicle bypassing
the onboard charger. Figure 1-2 shows a typical block diagram of a DC charging station.
DC Charging System Power Flow
Electric Vehicle
HVDC

Pilot Wire
Grid

EVSE +EVSE+
AC/DC
Converter
EVSE
+
AC/DC
Converter
AC/DC Converter(OBC)
AC/DC Converter

xN Stack

BypassBMS

AC

Battery Pack

Figure 1-2. DC Charging Station
For more information on on-board chargers and off-board chargers, see the Taking charge of electric vehicles –both in the vehicle and on the grid white paper .

2 Power Module in DC Charging Station
The power module in a DC charging station consists of AC/DC power stage and DC/DC power stage integrated
into the charging station. Each converter is associated with its power stage comprises of power switches andgate driver, current and voltage sensing, and controller. Each of EV charging station power module subsystems
are described in detail in the upcoming sections.

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2.1 Power Stage in a Fast DC Charger
Figure 2-1 shows the system level block diagram of a EV charging station power module captured from TI's EV
charging station power module, web page. On the input side it has three-phase AC mains which are connectedto the AC/DC power stage. This block converts the incoming AC voltage into a fixed DC voltage of around 800 V.
This voltage serves as input to the DC/DC power stage which processes power and interfaces directly with thebattery on the electric vehicle.
Phase 1
Phase 2Phase 3
GRID

Phase 1
Phase 2Phase 3Phase 1
Phase 2Phase 3Current
SensorPower FET

Current
SensorCurrent
SensorPower
FET/ IGBT

Comp

Gate
Driver

Comp

Comp

Gate
DriverAMP

AMP

AC/DC Power
Stage

AMP
ISO
AMP

MCU

AC/DC Input
Current andVoltage Sense

AC/DC Power
Digital Processing

ADC

ISO
AMP

DC/DC Input
Current andVoltage Sense

ISO
AMP

DC/DC Power
Digital Processing

DC/DC Output
Current andVoltage Sense

ADC

RTC
AC/DC Clocking

Reference

AMP

MCU

RTC
DC/DC Clocking

Reference

AC/DC
Analog Front-End

VehicleOutput

Relay
Driver
Relay
Control
Power
Stage

Comp

DC/DC Power
Stage

ISO
AMP

AC/DC Output
Current andVoltage Sense

Relay

Current
SensorDC/DC
Analog Front-End

CAN
Ethernet
Fan Motor
DriverPower Fan
Stage

eFuse
ISO DCRailPower Board
Input Power
Protection

Data
Isolation

Temperature
SensorPower Board Self
Diagnostics/Monitoring

Quad SVS/
Sequencer

DC/DC
VTT

Power Board
Signal Isolation

Low Noise
Power

DC/DC
Power Board Non-iso DC/DC Power Supply

ESD
RS-485
Power Board
Wired Interface

Isolated
DC/DC
Isolated DC/DC
Power Supply

Power Board
Signal I/O
Protection

To:
Central Control Module
HMI Module

Figure 2-1. EERD of EV Charging Station Power Module
The gate drivers driving the power stage MOSFETs also form part of the power stage. In addition to the powerstage, current and voltage sensing blocks are available at the input and output of both stages. This is primarily
used for control, monitoring, and protection purposes. Each power stage has a separate controller which is
responsible for the processing of analog signals and providing fast control action. Apart from this, there aredifferent blocks for temperature sensing, interface for CAN, Ethernet, and RS-485, and isolated and non-isolated
DC/DC converters powering the auxiliary circuits like fans for cooling the heat sinks, isolated amplifiers, and soforth. The focus on the remainder of this application report is on the choice of topology for the AC/DC and
DC/DC power conversion stage.

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Power Module in DC Charging Station

2.2 AC/DC and DC/DC Power Stages
DC charging stations require high-power converters which are capable of charging to 80% SOC in under 30
minutes. These fast charging applications require modular power converters which can be paralleled to cater todifferent power levels, thereby enabling fast charging. The most important parameters are the energy density
and system efficiency. Energy density is the amount of energy that can be transfered for a given volume of
converter. If we can double the power output for the same size, it results in significant cost savings and also
helps in fast charging. This is accomplished by operating the converter at high switching frequencies which
reduces the size of magnetics and thereby helps achieve high power density. A higher system efficiency means
lower losses and a smaller heat sink solution for a given application. It also reduces the thermal stress on
devices and contributes to longer life expectancy of parts. The latest trend in automotive technology is theconcept of Vehicle to Grid (V2G) which allows for the flow of energy from the battery to the grid for stability ofgrid when the vehicle is parked or not in use. This requires both the power stages to be bidirectional for
supporting such applications. The converter must also be capable of providing galvanic isolation between the
input and output stage through a high-frequency transformer with the required voltage conversion ratio as per
the application. The converter must operate at high efficiency through inherent soft switching (like ZVS/ZCS)
over a wide input and output voltage range.
The AC/DC stage (also know as the PFC stage) is the first level of power conversion in an EV charging station. It
converts the incoming AC power from the grid (380–415 VAC) into a stable DC link voltage of around 800 V. As
shown before, the PFC stage is very important to maintain sinusoidal input currents, with typically a THD < 5%,
to provide controlled DC output voltage higher than the amplitude of the line-to-line input voltage, single-stage
power conversion, no galvanic isolation, unidirectional and bidirectional power flow possibly with (limited)capability of reactive power compensation, simple circuit topology, simple modulation and control scheme, andthe possibility to achieve high efficiency and high power density.
The DC/DC stage is the second level of power conversion in an EV charging station. It converts the incoming DC
link voltage of 800 V (in case of three-phase systems) to a lower DC voltage to charge the battery of an electricvehicle. The electric vehicle charging standards are governed by standards such as Combined Charging System(CCS) and CHAdeMO. The DC/DC converter must be capable of delivering rated power to the battery over awide range, for example 50V-500V to accommodate batteries from 48V (e-bikes) all the way up to 400V (PHEV)
with the capability of charging the battery at constant current and at constant voltage mode, depending on theState Of Charge (SOC) of the battery.

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Power Topologies in AC/DC (PFC)

3 Power Topologies in AC/DC (PFC)
The individual topologies are classified into passive, hybrid, and active PFC rectifier systems. With the recent
advancements in semiconductor technology, active PFCs are widely used in EV charging stations. Depending on
the power level, both single-phase as well as three-phase topologies are used in EV charging stations. Singlephase topologies are typically used for power levels less than 3.3 kW and three-phase topologies are used atmuch higher power levels. In this application report single-phase PFC topologies namely totem pole, interleaved
totem pole and Neutral Point Clamped topology are briefly introduced and then we mainly focus on three-phase
PFC topologies; namely the two-level PFC, Vienna PFC, Neutral Point Clamped (NPC) 3 level PFC and T-type
NPC PFC.

3.1 Single-phase Totem Pole PFC
Figure 3-1 shows the totem pole PFC topology which is a conventional boost PFC where one half of the diodebridge is replaced by active switches S1 and S2 in a half bridge configuration, hence the name “totem pole”. The
diode S3 and S4 forms the slow 50- to 60-Hz line frequency leg which can either be slow AC rectifier diodes orcan be replaced by low-RDS(on) synchronous MOSFETs for improved efficiency.
400
VDC

S1
DriverS3

Si
FET

Gate driver

TI-GaN

AC
S2

Si
FET

DriverVoltagesense

Current
sense

TI-GaN

S4
RTN

Isolator

Isolator

ZVS
ZVS
IAC_sense
VAC_sense

Controller

VBUS_sense
LDO/buck

Flyback power
supplyFigure 3-1. Totem Pole PFC
The totem pole PFC has the advantages of improved efficiency. The main current only flows through two
switches at a time. S1 and S2 are driven synchronously with complimentary PWM signals and the S3/S4 on theslow line frequency legs can be diodes or low Rds(on) Si MOSFETs to further reduce the conduction loss.
Secondly, with lower part counts, it offers higher power density and lower BOM cost. Finally, the totem pole PFC
is inherently capable of bidirectional operation, which is ideal in V2G application and onboard bidirectional
battery chargers. The only disadvantage is that with silicon MOSFETs, the totem-pole arrangement allows
operating in only discontinuous-conduction mode (DCM) or critical-conduction mode (CrM) because if
continuous-conduction mode (CCM) is allowed, the reverse recovery of the MOSFET body diodes can cause
excessive losses. The reverse recovery time of the body diode in silicon MOSFETs is much more than that ofstandard fast-recovery diodes. So the reverse recovery losses will be very high and the efficiency will be low.
Although the totem-pole bridgeless-boost PFC is limited in its use with silicon MOSFETs, the advent of siliconcarbide (SiC) and gallium nitride (GaN) switching devices with zero-recovery reverse conduction has made it a6Power Topology Considerations for Electric Vehicle
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preferred choice even in CCM operation. This totem pole PFC can be extended to higher power by interleaving
the power stage as Figure 3-2 shows.
400 VDC
DriverTI-GaN

TI-GaNSi
FET

Gate Driver

DriverAC

Voltagesense

Current
sense

DriverDriverTI-GaN

TI-GaNSi
FET

RTN
Isolator

Isolator

Isolator

Isolator

ZVS
ZVS
IAC_sense

VBUS_sense

Controller

VAC_sense

LDO/Buck

Flyback power
stage

Figure 3-2. Interleaved Totem Pole PFC
The 98.6% Efficiency, 6.6-kW Totem-Pole PFC Reference Design for HEV/EV Onboard Charger and Highefficiency GaN CCM totem pole bridgeless Power Factor Correction (PFC) reference designs show applicationsof interleaved PFCs in electric vehicle charging applications.

3.2 Single-phase Neutral Point Clamped PFC
Figure 3-3 shows a typical neutral point clamped PFC topology in an EV charging station.

400 V

PH
Si FET

Si FET

Gate
Driver
SiC diode
Gate
DriverVoltage
sense

N

N

Current
sense

Voltagesense

Gate
DriverVoltage
sense

400 V

SiC diodeí400 V
Gate
DriverFlyback
Power StageLDO/BuckController

Si FET

Si FETí400 V

Figure 3-3. Neutral Point Clamped PFC
This topology has several advantages over a conventional two-level converter. First multilevel converters not
only can generate the output voltages with very low distortion, but also can reduce the dv/dt stresses with lower
voltage across the power switches; therefore, electromagnetic compatibility (EMC) problems can be reduced.

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Power Topologies in AC/DC (PFC)

Secondly, multilevel converters can draw input current with low distortion and hence they have better total
harmonic distortion performance and require less bulky input inductors. The multilevel feature of this topology
enables more cost-effective MOSFETs to be utilized, further reducing the cost of the design. GaN/SiC can beused in this topology when very high efficiency and power density are needed. Finally active switches allow
bidirectional operation of the converter. One particular disadvantage is the greater number of power
semiconductor switches needed. Although lower voltage rated switches can be utilized in a multilevel converter,
each switch requires an isolated gate drive circuit which can be quite costly and challenging to design.

3.3 Three-phase Two-level PFC
Figure 3-4 shows the typical configuration of a two-level three-phase PFC. The six-switch boost-type rectifierfeatures a very simple circuit topology and easy control. It facilitates bidirectional power flow and can achieve ahigh power factor with reasonable efficiency.
SiC

SiC

SiC

G9

G1

G5

LCL Filter

DC Link

SiC

SiC
G10

Voltagesense

Current
sense

SiC

G6

G2

Current
sense

Voltagesense

ISO5852S
ISO5852S
Gate driver

PWM signals

Vbus sense
IAC_sense
VAC_sense

Controller

LDO/Buck

Flyback
power stage

Figure 3-4. Two-level PFC
Since this topology is a two-level topology, it requires high-voltage blocking switches to block the entire DC link
voltage. For example, in an 800-V DC link voltage application, a 1200-V rated blocking capacity Silicon Carbide(SiC) is required for the power stage. One of the drawbacks of this topology is the bulky filter inductor which it
requires to regulate the input current THD to low values. Hence, the power density is low compared to the othercompetitive three-level topologies which are documented in Section 3.4, Section 3.5, and Section 3.6. Also, thepeak voltage stress on the devices is very high which affects the long-term reliability of the semiconductor andother passive devices used in the power stage. Finally, the electromagnetic interference (EMI) performance ofthe converter is significantly lower when compared to other popular multilevel PFC topologies.

3.4 Three-phase Vienna PFC
The Vienna rectifier power topology, shown in Figure 3-5, is used in high-power, three-phase power factor
correction applications. The Vienna rectifier is popular due to its operation in continuous conduction mode
(CCM), inherent multilevel switching (three level), and reduced voltage stress on the power devices. Bothhysteresis-based and sine triangle based controllers have been used for Vienna rectifiers, hence it is relatively
simpler to control due to the smaller amount of PWMs required for this topology. One of the drawbacks of this
topology is that it only supports unidirectional mode power transfer from the grid to the DC side. Vienna Rectifier
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is characterized by a relatively small overall volume or a high power density as it requires approximately only halfof the inductance for the boost inductors compared to the two level rectifiers discussed in Section 3.3. The
multilevel signature of output voltage also provides a better THD performance. Opposed to the two-level PFC,
the Vienna Rectifier enables high efficiency at elevated switching frequencies with Si MOSFETs or 600-V to 650V IGBTs and SiC Schottky diodes as a result of its three-level characteristic and does not require advanced
1200-V SiC power FETs to lower the switching losses. The Vienna Rectifier-Based Three Phase Power Factor
Correction Reference Design Using C2000 MCU TI reference design contains details on the implementation ofthis converter.
SIC diode

SIC diode

SIC diode

Si FET/IGBTSi FET/IGBTLCL Filter

G4

G3

Si FET/IGBT
DC Link

Si FET/IGBT
G8

G7

G12

G11

Si FET/IGBTSi FET/IGBT
SIC diode

Voltagesense

Current
sense

SIC diode

SIC diode

Current
sense
UCC5320
UCC5320Gate driver
Voltagesense

PWM signals

Current Sense
Voltage Sense

Controller

Vbus sense
LDO/
Buck

Flyback
power stage

Figure 3-5. Vienna Rectifier

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3.5 Three-phase ANPC/NPC Three-level PFC
Figure 3-6 illustrates the basic topology of the Neutral Point Clamped converter. This is similar to the singlephase NPC multilevel topology seen before which has been extended to the three phases where all switches ofthis topology are needed to block only half the bus voltage. Hence the voltage stress on the devices is the lowest
among all the topologies discussed so far. Therefore this topology can be easily scaled across multiple platforms
for implementation with SiC, GaN, and Si MOSFET depending on the power level, cost, and target efficiency.
400 V

GaN/SiC/Si FETGaN/SiC/Si FETGaN/SiC/Si FET+ 400 V
GaN/SiC/Si FETGaN/SiC/Si FET

Flyback power 12 V
supplyLDO/Buck

3.3 V

GaN/SiC/Si FETR

NGate driver

Gate driverY
Gate driver

GaN/SiC/Si FETB

GaN/SiC/Si FETGaN/SiC/Si FETCurrent
sense
R
Gate driver

Gate driver- 400 V

GaN/SiC/Si FETGaN/SiC/Si FET

Gate driver
Gate driverY

B+ 400 V - 400 V

Gate driver
GaN/SiC/Si FETVoltagesense

Controller

Voltagesense

Gate driver

Figure 3-6. Three-phase Three-level NPC
Since only half of the voltage has to be switched and this also cuts the switching losses in the MOSFETs by half,
it is possible to use 600-V components instead of 1200-V types. On top of that, in the 600-V technology much
faster components are available than in 1200 V. This leads to further reduction of the switching losses. The
Neutral Point Clamped topology will have lower ripple in the output current and half of the output voltage
transient. This will reduce the effort for filtering and isolation in the filter inductor. Hence we can achieve highpower density with less inductance needed to regulate the THD of the current waveform. Multilevel converters
not only generate the output voltages with very low disturbance, but also can minimize the dv/dt stresses acrossdevices thereby electromagnetic interference (EMI) issues can be reduced. Moreover, this topology also offers
bidirectional transfer of power and is preferred choice for switching frequencies greater than 50 kHz owing tolesser switching losses and greater efficiency.

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One particular drawback is the high number of power semiconductor switches needed. Despite the fact thatlower voltage rated switches can be utilized in a multilevel converter, each switch needs a linked gate drive
circuit and also the control is quite complex with the increased number of devices. As this topology uses bothactive semiconductor switches and diodes, their unsymmetrical distribution of losses across the power stage andthermal management can be quite challenging. In many situations, to have a more symmetrical loss distribution,
the diodes of the NPC topology are replaced with active switches. This gives rise to an Active Neutral Point
Clamped (ANPC) converter topology as Figure 3-7 shows. Also with reduced blocking voltage across all
switches, Gallium Nitride (GaN) can be used for the high-frequency switches in this topology which improves
efficiency and power density of the converter.
+ 400V

GaN/SiCGaN/SiCGaN/SiC+ 400 V

GaN/SiCGaN/SiCGaN/SiC
Si FET

Si FET

Si FET

Flyback
power
supply12 V

LDO/Buck
3.3 V

R
N

Gate driver
Gate driverY
Gate driver

Si FET

GaN/SiCGaN/SiCB

Si FET
Si FET
Current
sense

GaN/SiCR
Gate driver

GaN/SiCGaN/SiC- 400 V

Gate driver

GaN/SiC
Voltagesense

Gate driverY

B+ 400 V - 400 V

Gate driver

Gate driver

Controller

Voltagesense

Gate driver

Figure 3-7. Three-phase Three-level ANPC

3.6 Three-phase TNPC Three-level PFC
Figure 3-8 depicts the basic topology of the 3L T-Type converters. The conventional two-level Voltage Source
Converter (VSC) topology is extended with an active, bidirectional switch to the DC-link midpoint. For 800-V DC
link voltages, the high-side and the low-side on each phase would usually be implemented with 1200-V IGBTs/
diodes as the full voltage has to be blocked. Differently, the bidirectional switch to the DC-link midpoint has toblock only half of the voltage. It can be implemented with devices having a lower voltage rating like two 600-V
IGBTs including antiparallel diodes. Due to the reduced blocking voltage, the middle switch shows very low
switching losses and acceptable conduction loss. Contrary to the previously-discussed three-level NPC topology,
there is no series connection of devices that has to block the whole DC-link voltage. For the NPC topology,
switching transitions directly from the positive (P) to the negative (N) DC-link voltage level and vice versa areusually omitted as there might occur an uneven share of the voltage to be blocked in the transient case whenboth FETs connected in series turn off at the same time. This undesirable effect cannot occur in the T-type
topology. It is not necessary to implement low-level routines which prevent such transitions or ensure a transient
voltage balancing among series connected IGBTs. Another additional benefit related to using single 1200-V
devices to block the full DC-link voltage is reduced conduction losses. Whenever the output is connected to (P)or (N), the forward voltage drop of only one device occurs, contrary to the NPC topology where always two
devices are connected in series. The conduction losses are considerably reduced making the T-type aninteresting choice even for low switching frequencies.

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Overall, the conduction losses are significantly lower when compared with NPC but the switching losses are highdue to the devices which block the full DC link voltage. Due to limited component count when compared to NPC
topology and better efficiency, power density and bidirectional operation capability when compared to Vienna
Rectifier and two level PFC, the T-type rectifier is best suited for applications upto 50-khz switching frequency
beyond which NPC performs better. One of the drawbacks of this topology is the high peak voltage stress acrossthe high voltage blocking FETs. Finally, similar to the other topologies this also has a good THD performance, so
it does not require bulky inductor at the input. TI's Three-level, three-phase SiC AC-to-DC converter reference
design contains details on the implementation of this converter.
SiC

SiC

SiC

G9

G1

G5

Si FET/IGBTSi FET/IGBTLCL Filter

G3

G4

Si FET/IGBT
DC Link

Si FET/IGBT
G8

G7

G12

G11

Si FET/IGBTSi FET/IGBT
SiC

SiC

G10

SiC

G6

G2
Voltagesense

Current
sense

Current
sense
UCC5320
UCC5320Gate driver
Voltagesense

ISO5852S
ISO5852S
Gate driver

PWM signals

IAC_sense

Controller

VAC_sense

Vbus sense
LDO/
Buck

Flyback
power stage

Figure 3-8. Three-phase Three-level T-type PFC

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Power Topologies in AC/DC (PFC)

3.7 Summary of AC/DC Topologies
Table 3-1 compares and summarizes the previously-discussed PFC topologies.
Table 3-1. Pros and Cons of PFC Topologies
2-Level3-Level NPC

3-Level Vienna

3-Level TNPC

3-Level ANPC

THD of output current HighVery low

Very low

Very low

Very low

Peak voltage stressHighon active and passive
devices

Low

Low

Low /(High Blocking)LowestPower density

Low

HigherHigh

High

HigherBidirectional

Yes

Yes

No

Yes

Yes

Conduction loss

Low

high

high

Mid

high

Switching loss

High

Low

Mid

Mid

Low

Efficiency

Low

Very High (at highfrequency)

High

High

Highest

Cost

Low

High

Mid

Mid

High

Control

Easy

Mid

Mid

Mid

Mild

Input inductor size

Large

Low

Low

Low

Low

Thermal management Easy

Difficult due tounsymmetrical loss
distribution

Mild

Easy

Easy

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Power Topologies in DC/DC

4 Power Topologies in DC/DC
This section explores four different topologies of high-power DC/DC converters; namely the LLC resonant
converter, the Phase-shifted Full Bridge (PSFB), Single-phase Dual-Active Bridge (DAB) and the Dual-Active
Bridge in CLLC mode (DAB – CLLC).

4.1 Conventional Phase-shifted Full Bridge (PSFB)
Figure 4-1 shows the basic topology of the phase shifted full bridge converter. The phase shifted full bridge
belongs to the family of Dual Active Bridge converters where the active switches on the secondary are replacedwith diodes. Due to this, it allows only unidirectional power transfer. The Phase-Shifted Full Bridge DC/DC Power
Converter TI reference design contains details on the implementation of this converter.
SiC

SiC
SiC diode

SiC diode
280 ± 500 V

800 V
SiC

SiC
SiC diode

SiC diode
ISEC

IPRIM

Voltagesense

Current
sense

Gate driver

Current
sense

Current
sense

Voltagesense

Controller

VSEC/VPRIM

Flyback power 12 V
supplyLDO/Buck
3.3 V

Figure 4-1. Phase-shifted Full Bridge
The power transfer between the primary and secondary is controlled by varying the phase between the switch
legs of the primary bridge. As a result it is possible to obtain ZVS turn on of one leg and low voltage turn on of
the other leg minimizing losses. The passive diodes on the secondary can experience hard switching and resultin more conduction losses which can reduce the efficiency of this converter. This converter suffers non ZVS turnon loss at light load condition and non ZVS turn off. Typically burst mode of operation is used to maintain ZVS atlight load condition. This converter is also modular and it can be paralleled to obtain higher power throughput in
EV charging stations. In PSFB, dithering can be easily implemented to reduce the conducted EMI signature. This
topology requires a DC blocking capacitor which is needed to block DC voltage offset saturating the transformer
in voltage mode control. This converter often requires an additional shim inductor which is needed for ZVS
operation and it can make the converter bulky and can hamper power density.

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4.2 Dual Active Bridge (DAB)
Figure 4-2 depicts the basic topology of the Dual Active Bridge converter. It consists of full bridge with active
switches on both the primary and secondary sides connected together by a high-frequency transformer. Becauseof the inherent lagging current in one of the bridges, the current discharges the output capacitance of switches of
one bridge (say secondary side) and some switches of the primary side thereby enabling ZVS turn on. In
addition to this lossless capacitive snubbers can be used across the switches to reduce turn off losses. The main
advantages of this converter are its inherent bidirectional capability which is achieved by controlling the phaseangle between the two bridges and its modularity that allows for it to be scaled to higher power levels.
VPRIM

VSEC
SiC

SiC/GaN

SiC

SiC/GaN

280 ± 500 V
800 V
SiC

SiC
SiC/GaN

SiC/GaN
ISEC

IPRIM

Voltagesense

Current
sense

Current
sense

Gate driver

Gate driver

Current
sense

Voltagesense

Controller

VSEC/VPRIM

Flyback power 12 V
supplyLDO/Buck
3.3 V

Figure 4-2. Dual Active Bridge
The control of the DAB ranges from simple (or single phase shift modulation) to complex (for extended, dual andtriple phase shift modulation). This topology can be used to cover a wide variation of battery voltages with single
phase shift modulation but circulating currents in the transformer increase which drastically reduces efficiency.
But with advanced modulation schemes like triple phase shift, the converter can theoretically achieve ZVS overthe entire operating range. The utilization of the output power to the transformer KVA rating is high for this
topology. The required output capacitor to handle the ripple currents is also low for this converter. This converterwith the relatively fewer number of devices, soft-switching commutations, low cost, and high efficiency is used inapplications where the power density, cost, weight, isolation, and reliability are critical factors. Another limiting
feature is that the converter often requires an additional shim inductor which is needed for ZVS operation and it
can make the converter bulky and can hamper power density. The Bi-directional, dual active bridge reference
design for level 3 electric vehicle charging stations TI reference design contains details on the implementation ofthis converter.

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Power Topologies in DC/DC

4.3 LLC Resonant Converter
The gain of this converter is a function of switching bridge gain, resonant tank gain and transformer turns ratio.
The output voltage regulation is achieved by varying the switching frequency of operation. There are threemodes/regions of operation in the LLC resonant converter namely the operation at resonant frequency, above
resonant frequency and below resonant frequency. During below resonant frequency operation, the resonant halfcycle inductor current reaches the value of magnetizing current within the switching cycle and leads to soft
switching across secondary rectifier diodes but on the other hand leads to more conduction losses due to morecirculating energy. Above resonant frequency operation, leads to increased switching losses and hardcommutation of secondary rectifier diodes, but leads to lower conduction losses due to reduced circulatingenergy. Hence the best performance of these converters is obtained when operated close to resonancefrequency where ZVS turn on and ZCS turn off is possible. This converter provides unidirectional power flow andis generally used in applications less than 5 kW. Figure 4-3 shows the topology of the LLC resonant converter.
VPRIM

VSEC
SiC

SiC
SiC diode

SiC diode
280 ± 500 V

800 V
SiC

SiC
SiC diode

SiC diode
ISEC

IPRIM

Voltagesense

Current
sense

Gate driver

Current
sense

Current
sense

Voltagesense

Controller

VSEC/VPRIM

Flyback power 12 V
supplyLDO/Buck
3.3 V

Figure 4-3. LLC Converter
Paralleling and synchronization of multiple LLC converter modules to increase the power throughput is difficultand often requires external control logic for safe implementation. Low di/dt in high-output-voltage designs(greater than 400 V) makes implementation of synchronous rectification in LLC converter quite tricky. The ripple
current and peak voltage stress across the active and passive devices is significantly high, hence higher output
capacitance is needed for handling high ripple. Also the transformer of LLC converter is also slightly higher insize, hence these passive components significantly reduce the power density of converter. Since this converter
soft switches during turn on and turn off, the EMI performance is better when compared to other hard switched
topologies previously discussed. The Two Phase Interleaved LLC Resonant Converter Reference Design Using
C2000™ MCUs TI reference design contains details on the implementation of this converter.

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Power Topologies in DC/DC

4.4 DAB in CLLLC Mode
The CLLC incorporates all the functionalities of the LLC described before, but a major advantage of this topology
is that with the use of active switches across the secondary, we can obtain bidirectional power transfer as Figure
4-4 shows. The ZVS/ZCS operation of this converter results in increased efficiency. When there is room to vary
the bus voltage by a margin of 10%, this converter can cater to wide varying battery voltage with good efficiency
performance, but with a fixed bus voltage this has a very limited range of operation. With capacitors in the
primary and secondary side of the transformer, the problem of saturation of the core of the transformer isprevented.

SiC

CLLLC Resonant Tank

SiC

SiC/GaN

SiC/GaN

280 ± 500 V
800 V
SiC

SiC
SiC/GaN

SiC/GaN
ISEC

IPRIM

Voltagesense

Current
sense

Current
sense

Gate driver

Gate driver

Current
sense

Voltagesense

Controller

VSEC/VPRIM

Flyback power 12 V
supplyLDO/Buck
3.3 V

Figure 4-4. DAB- CLLLC Converter
This converter is primarily suited for onboard charger applications but can be used at higher power levels upto
10 kW. But scaling to higher power levels and paralleling can be difficult as it requires a highly symmetrical tankstructure and synchronization of multiple modules which can be quite difficult. The Bidirectional CLLLC resonant
dual active bridge (DAB) reference design for HEV/EV onboard charger TI reference design contains details on
the implementation of this converter.

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4.5 Summary of DC/DC Topologies
Table 4-1 compares and summarizes the previously-discussed DC/DC topologies.
Table 4-1. Pros and Cons of DC/DC Topologies
LLC Converter

Phase-shifted Full
Bridge (PSFB)

Dual Active Bridge(DAB)

DAB in CLLC ModePeak device stress on primary andsecondary sideHigh

Mild low

LowestHigh

Transformer KVA ratingHigh

MediumLow

High

Power output to transformer KVA rating

Low

MediumHigh

MediumInput and output capacitor RMS currents

High

MediumLow

High

Operation

Unidirectional

Unidirectional

Bidirectional

Bidirectional

Conduction losses

High

MediumLowestMediumTurn ON switching loss

ZVS

ZVS

ZVS

ZVS

Turn OFF switching loss

Low (ZCS)

HighHigh (device turn off at Low (primary side
peak leakage inductor turn off decided bycurrent value)magnetizing inductor
current, secondaryside turn off is zerodue to ZCS)

Total losses

Low

HigherMediumLow

Control complexity

Moderate

Very simple

Simple to complex

Moderate

Wide Battery Voltage, Fixed Bus Voltage

No, needs
additional DC/DC
stage

Yes (with reduced
efficiency)

Yes (with reduced
efficiency)

Limited range

Paralleling Modules

Intensive

Easy

Easy

Intensive

Switching Frequency

Fixed/ High (Si /
SiC)

High

High

Very High18

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Upcoming Trends in DC Fast Charging Infrastructure

5 Upcoming Trends in DC Fast Charging Infrastructure
The state-of-the-art power conversion topologies previously discussed, are driving the need for bidirectional,
high-power density, and scalable converters. These are driving the industry toward the latest trends in EV
charging which are the concepts of portable chargers and Vehicle to Grid (V2G) technology. This section covers
these advances.

5.1 Portable Chargers
Few Automotive OEMs are exploring the possibility of how ubiquitous DC chargers can lead to elimination of EV
onboard chargers (OBC) in the years to come, making EVs more efficient and affordable. By pairing a portable(small form factor) DC charger with every EV, auto OEMs could be looking to eliminate OBC from EVs.
Eliminating OBC from the car has numerous benefits such as:• Reduced overall system cost, since Q100 qualified devices/system may not be required• Serviceability of EV chargers becomes easier, no more downtime for your EV due to a charger breakdown• Higher efficiency, that is, more miles per charge due to reduced gross vehicle weight• Most importantly, the rate of charge will no longer be limited by the power rating of the OBC (which is the casewhile using a L1 or L2 AC charger)
These chargers are expected to be portable, that is, hand-held with a small form factor, natural convection based
cooling (i.e. without fans). This may require manufactures to look at GaN/SiC based architecture that can switch
in the order for 100s of kHz to a few MHz so as to reduce the size of the entire charger along with low thermal
emissions. Figure 5-1 illustrates the block diagram of a portable EV charger.
DC Charging System Power Flow (without OBC)Electric Vehicle
AC

AC/DC Converter

HVDC
CAN/PLCBMS

Portable Charger
Grid

Battery Pack

Figure 5-1. Portable EV Charger

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5.2 Vehicle-to-Grid Technology (V2G)‘Vehicle to grid’ technology, also referred to as V2G enables energy stored in electric vehicles to be fed back into
the national electricity network (or 'grid') to help supply energy at times of peak demand. Over 90% of cars areparked at any given time, which is a lot of energy just sitting inside but doing nothing. This energy can be used
for balancing the grid so that there is enough electricity on the network when we all need it. This necessitates theneed for having bidirectional converters in EV charging stations. Figure 5-2 shows an electric vehicle connected
to a charging station where bidirectional converters inside the charging station enable the energy stored in thebattery to be used for stabilizing the intermittencies in grid.
V2G Unit
Grid
Electric Vehicle

AC/DC
Bidirectional
InverterHigh Voltage
Battery

Control Unit
Control Unit

Figure 5-2. V2G Technology

6 Conclusion
The ultimate choice of a power topology boils down to the intended use case of that specific EV Charger namely
the targeted power levels, efficiency & power density targets to name a few. While high power highway chargers(or fleet chargers) would look at complex multilevel three-phase PFC and DC-DC stage to pack more-and-more
power into them, the new emerging portable chargers could be addressed by a simple interleaved totem pole
PFC stage followed by a LLC DC/DC stage. With interest in new requirements like V2G, engineers may have tolimit their topology choices to those that support bi-directional power transfer between the Vehicle and the Grid.
No matter, your choice of use case the advancement in the field of power electronics in tandem withsemiconductor technology is ready to offer everything you need to build your next generation DC Fast EV
Charging Station.

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References

7 References
•
•
•
•
•

•
•
•
•
•

•

•
•

Vienna Rectifier-Based Three Phase Power Factor Correction Reference Design Using C2000 MCU
10-kW, Three-Level, Three-Phase T-Type Inverter Reference Design for Solar String Inverters
98.6% Efficiency, 6.6-kW Totem-Pole PFC Reference Design for HEV/EV Onboard Charger
High efficiency GaN CCM totem pole bridgeless Power Factor Correction (PFC) reference design
Design and Implementation of a Highly Efficient Three-Level T-Type Converter for Low-Voltage Applications,M. Schweizer, J. W. Kolar, IEEE Transactions on Power Electronics, Vol. 28, No. s2, pp. 899-907, February
2013
Bidirectional CLLLC resonant dual active bridge (DAB) reference design for HEV/EV onboard charger
Dual active bridge reference design for level 3 electric vehicle charging stations
Phase-Shifted Full Bridge DC/DC Power Converter
Two Phase Interleaved LLC Resonant Converter Reference Design Using C2000™ MCUs
R.W. A. A. De Doncker, D. M. Divan, and M. H. Kheraluwala, “A threephase soft-switched high-power-density
dc/dc converter for high-power applications,” IEEE Trans. Ind. Appl., vol. 27, no. 1, pp. 63–73, Jan./Feb.
1991.
M. H. Kheraluwala, R. W. Gascoigne, D. M. Divan, and E. D. Baumann, “Performance characterization of ahigh-power dual active bridge dc-to dc converter,” IEEE Trans. Ind. Appl., vol. 28, no. 6, pp. 1294–1301,Nov./
Dec. 1992.
A.K. Jain and R. Ayyanar, “PWM Control of Dual Active Bridge: Comprehensive Analysis and Experimental
Verification,” IEEE Trans. Power Electron., vol. 26, no. 4, pp. 1215-1227, April 2011.
Overview of Dual-Active-Bridge Isolated Bidirectional DC–DC Converter for High-Frequency-Link PowerConversion System Biao Zhao, Student Member, IEEE, Qiang Song, Member, IEEE, Wenhua Liu, Member,IEEE, and Yandong Sun

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