Why AC-DC Converters Matter in Fast Charging Stations

Article By : Saumitra Jagdale

AC-DC converters are the future of EV charging stations.

Electric vehicles (EVs) constitute a major segment of the promising technologies for achieving a sustainable transport sector in the future. Here, AC-DC converters are the backbone components for expanding and improving the EV transportation.  

Understanding the functionalities of AC-DC converters, types of charging stations, problems facing conventional two-level AC-DC converters, and the significance of using multi-level converters is essential to ensuring successful use of AC-DC converters in EV charging stations. 

Figure 1: Here is how an AC-DC charger operates. (Source: IES-Synergy)

Significance of AC-DC converters 

Primarily, an outlet delivers AC power whereas EV batteries function with DC power for charging the battery, thus there is a need for an AC-DC conversion of AC power to DC power. It’s a major component of an EV battery charger and acts as an input current shaper for power factor correction and harmonic reduction. 

Figure 2 illustrates a simple AC-DC converter, where four general-purpose rectifier diodes are used to rectify the AC input.  

Figure 2: Schematic shows AC-DC converter circuit and rectifier diodes in it. (Source: Multisim)

The functionality of the transformeris to step down the 230 V AC supply to 13 V AC. The arrangement of the circuit is a full bridge as it consists of four diodes. The rectifier will rectify both the positive and negative peaks of the AC signal. A filter capacitor is added after the bridge converter to smooth out the output voltage. Additionally, a Zener regulator is connected in reverse bias before the output to regulate the output voltage.

Types of charging stations 

As rechargeable batteries are the power source for EVs to function, it’s critical to understand some parameters of charging stations. Most fundamental parameters like power efficiency, compact architectures, and fast charging will determine the overall productivity of charging stations. 

EV charging stations are classified into three levels: Level I, II, and III.   

A Level I charging station constitutes the segment of smaller battery sizes. The charging duration of a Level I charging station is approximately 8-10 hours; however, this duration may vary depending on the energy capacity of the battery. It only uses AC charging and there is an onboard charger as the charging component is located inside the EV. 

The charging duration on Level II is approximately less than half the time of Level I. Furthermore, Level III fast charging stations use an external charger (off-board) to supply high voltage. Level III is capable of charging an EV in as little as 20 to 30 minutes, while Level I or Level II charging stations can charge the vehicle in four to eight hours.  

Requirements for enabling fast charging stations 

The power range of fast charging is above 50 kW, which is considered high according to industry standards. So, there is a need for a larger AC-DC converter to supply this extra power to enable fast charging. Thus, high-power charging is best carried out where AC-DC converters are built into the charging station and not installed inside the vehicle due to size constraints. 

Another requirement of a fast-charging station, defined by the Society of Automotive Engineers (SAE) standard, “is the galvanic isolation between the distribution grid and the battery pack.” There are two different architectures available for achieving this: either through a low-frequency (LF) transformer at the input side or through the implementation of a high-frequency (HF) transformer included in the DC stage by means of isolated DC-DC converters. 

AC-DC converter drawbacks 

The drawback of conventional AC-DC converters like 2L voltage source converters (VSCs) is that they have limited power ratings and high harmonic pollution. To avoid such drawbacks, hybrid filters are used, but these filters also increase the cost of the system.  

Additionally, 2L VSCs have an undesirable high switching frequency.  

Importance of multi-level converters  

To overcome the drawbacks of AC-DC converters, 2L AC-DC converters should be replaced by multi-level converters (MLCs). MLCs have demonstrated many advantages, such as low harmonic and low voltage stress, and high-power capability. These converters can reduce the switching element and produce the multi-level output with a single-phase T-type converter.  

The several levels of MLCs achieve a smoother output waveform, which reduces the harmonics and the output filter size. The major types of MLCs are neutral-point clamped (NPC), cascaded H-bridge (CHB), and flying capacitor (FC).  

Among them, the most popular is cascaded H-bridge, which has the capability of utilizing different DC voltages on the individual H-bridge cells. That results in splitting the power conversion amongst higher-voltage, lower-frequency and lower-voltage, higher-frequency inverters. 

Figure 3:  A detailed view of one phase of a three-level neutral point clamped inverter. (Source: ScienceDirect)

The above circuit diagram represents one phase of a three-level NPC inverter. This family of multi-level power converters are characterized by the use of clamping diodes to guarantee proper voltage sharing across power switches. 

Future of EV charging

The transformation from combustion engines to EVs is a long-term process. Many oil companies are already claiming a stake in EV networks by creating charging stations or promoting products aimed at EV maintenance.  

To help EVs reach ubiquity, governments and EV charging companies must ensure the availability of fast-charging infrastructureWithout an efficient charging infrastructure, EV uptake would be a slow-rolling process.

 

This article was originally published on EE Times.

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