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AC-DC Power Stage Technologies for Electric Vehicle Charging Systems

  • Contents

Overview: This article explores various AC-DC topologies, control strategies, and technical specifications crucial for enhancing efficiency and performance in chargers. It also addresses current challenges and advancements in the field.

 

To achieve a significant reduction in the volume and weight of electric vehicles, off-board chargers must be used for both fast and ultra-fast DC charging. The topologies and control strategies of AC-DC for off-board chargers as shown in Fig. 1 are covered in this article, focusing on technical specifications, current developments, and challenges.

 

circuit-topology-of-ac-dc-power-stage(a-f)

Fig. 1: Circuit topology of AC-DC power stage (a-f) Source: IEEE Access

 

The topologies shown here work well with fast DC charging. The rated power of the rectifiers can be increased to satisfy the demand for fast DC charging with an adaptable and appropriate design.

 

Three-Phase Buck-Type Rectifier

For an AC-DC rectifier in an electric vehicle charging station, there are critical requirements, such as

  • Power factor correction (PFC)
  • Low THD
  • High efficiency
  • High-power density

 

Merits

Because it can provide all of the above properties, the three-phase buck-type rectifier (TPBR) as shown in Fig. 1(a) is an appropriate option for the AC-DC power stage. Furthermore, when compared to boost-type three-phase rectifiers, TPBR offers an

  • Inherent inrush current free starting
  • Broader output voltage control range
  • Phase-leg shoot-through protection
  • Overcurrent protection circuit during short circuit

 

Demerits

 

 

Swiss Rectifier

The Swiss rectifier (SR), a variant of TPBR, is illustrated in Fig. 1(b).

 

Merits

TPRB, with eight switches compared to six switches, offers

  • Greater efficiency
  • Lower common-mode noise
  • Lower conduction
  • Lower Switching loss

 

Because of its circuit nature, SR allows for the implementation of DC-DC converter control techniques. Furthermore, space vector pulse width modulation (SVPWM) may be avoided for SR, making control simpler.

 

Interleaving SRs provides advantageous features like

  • Reduces current and voltage ripple
  • Reduces filter requirements
  • Increases power
  • High bandwidth
  • Reliability

 

Demerits

One of its main drawbacks is that SR only permits unidirectional power flow. However, to enable vehicle-to-grid functioning, bidirectional SR can be constructed at the expense of additional electrical components and a complex structure.

 

Vienna Rectifier

When compared to a three-phase boost PFC rectifier, the three-phase Vienna rectifier (VR) in Fig. 1(c) operates similarly, but the power flow is unidirectional.

 

Three-phase VR is made up of

  • Three boost inductors at the input
  • Six fast rectifier diodes
  • Six switches (two per leg)
  • Two split capacitors at the output

 

VR utilizes a bipolar DC bus design, which improves power flow capability. On the other hand, input current distortion must be avoided by correcting the voltage imbalance in the bipolar DC bus topology.

 

The power losses of several VR topologies were analyzed, and the structure shown in Fig. 1(c) had the fewest losses.

 

As seen in Fig. 1(d), the switches are used in place of the diodes to guarantee bidirectional power flow. Another name for this architecture is a three-phase, three-level T-type rectifier.

 

Merits

VR is commonly employed in high-power applications because of its

  • Straightforward control mechanism
  • High power density
  • High power efficiency
  • Unity power factor
  • Reduced-number switches
  • Low THD
  • Neutral connection-free construction
  • There is no need for a dead zone switching drive since the voltage stress on the switches is half that of the DC link voltage.

 

Demerits

 

VR frequency is reduced to about 250 kHz for an improved balance between high-power density and efficiency utilizing standard PCB technology. If this limit is exceeded, input current distortion could result, which would lower the quality of grid power.

 

Three-Phase Boost-Type Rectifier

A three-phase six-switch boost rectifier (TPSSBR) is shown in Fig. 1(e). It has

  • Three inductors connected in series with a three-phase input AC source
  • Six switches on three legs.

 

Inductors are used to increase the input current voltage and decrease its harmonic content. The top and bottom switches are switched in a complementary manner.

 

Merits

The three-phase boost rectifier is a good fit for the AC-DC power stage of the EV charger because of its

  • Straightforward design
  • Continuous input current
  • Bidirectional operation
  • High-output DC voltage
  • Low current stress
  • Few switches
  • Straightforward control scheme
  • Low THD
  • High efficiency

 

Demerits

 

This topology also preserves gentle switching, prevents bridge short-through issues, and guarantees automated step-up operation. Zero-voltage transition (ZVT) and zero-current transition (ZCT) TPSSBRs can also be used to provide soft switching as shown in Fig. 1(f).

 

Multilevel AC-DC Converter

Researchers frequently use the multilevel converter (MLC) architecture, which generates alternating voltage levels from many lower levels of direct current voltages.

 

There are three main types of MLC:

  • Neutral Point Clamped (NPC) MLC
  • Flying Capacitor (FC)
  • Cascaded H-Bridge (CHB)

 

Merits

An MLC converter's fundamental method of operation is to use switches, capacitors, and voltage sources to create a staircase waveform at the output.

 

Because MLC can supply high power with higher efficiency and power density, it is a preferred option for the AC-DC power stage in EV fast and ultra-quick charging applications.

 

Some of the distinctive features of an MLC are

  • Less voltage stress on the switches in high-voltage applications
  • Low EMI
  • Reduced voltage transition between levels
  • Low THD
  • Smaller dv/dt
  • Minimization of magnetic components to allow superior performance

 

Summarizing the Key Points

  • The article discusses advanced AC-DC power stage technologies tailored for electric vehicle chargers, emphasizing efficiency and performance improvements.
  • It gains a thorough understanding of the crucial role that topologies, control strategies, and technical specifications play in optimizing on-board charging systems.
  • It explores the dynamic evolution of fast and ultra-fast DC charging solutions, addresses current obstacles, and showcases technological advancements.
  • It also showcases the latest developments in onboard chargers that contribute to reducing the volume and weight of electric vehicles, meeting the growing demand for efficient charging solutions.

 

Reference

Safayatullah, M., Elrais, M. T., Ghosh, S., Rezaii, R., & Batarseh, I. (2022). A Comprehensive Review of Power Converter Topologies and Control Methods for Electric Vehicle Fast Charging Applications. IEEE Access, 10, 40753–40793. https://doi.org/10.1109/access.2022.3166935

Rakesh Kumar, Ph.D.

Rakesh Kumar holds a Ph.D. in electrical engineering, specializing in power electronics. He is a Senior Member of the IEEE Power Electronics Society, Class of 2021. He writes high-quality, long-form technical articles for global B2B semiconductor brands. Feel free to reach out to him at rakesh.a@ieee.org! Checkout his complete portfolio @muckrack.com/rakesh-kumar-phd | @linkedin.com/in/rakesh-kumar-phd

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