Designing Vienna Rectifiers for EV Chargers
OEMs developing EV chargers are choosing Vienna rectifiers for efficient power conversion. Unlike conventional rectifiers that draw distorted currents from the grid, Vienna rectifiers excel with their ability to draw clean, sinusoidal currents. This translates into a unity power factor (zero phase angle), meaning they can achieve higher power efficiency and reduce losses while minimizing the strain on the grid.
Vienna rectifiers use a three-level switching structure. This not only reduces switching losses but also minimizes the output voltage ripple, ensuring a smoother flow of power to the EV battery. Compared to other topologies, Vienna rectifiers boast higher efficiency, lower electromagnetic interference (EMI), and a simpler design with fewer components.
Overview of a Vienna rectifier topology
EV DC fast chargers require three-phase power for their operation. To enhance the overall quality and suppress harmonic currents, power factor correction (PFC) is necessary. The Vienna rectifier topology is popular due to its operation in continuous conduction mode (CCM), the use of multilevel (three-level) switching, and the lower voltage stress on the power components.
At its heart, the Vienna rectifier resembles a three-phase diode bridge, a common circuit for converting AC to DC power. However, it takes this concept a step further by integrating a boost converter. This additional stage allows for precise control over the output voltage.
The schematic shows a three-phase circuit. Six diodes are used: a pair for each cycle. The diodes provide first line rectification, converting each negative half-cycle to positive. Following this initial conversion, three power switches come into play. Pulse-width modulation is used to turn the switches on and off, controlling the current and the voltage at the output stage.
The Vienna rectifier is classed as a three-phase boost converter, changing three AC mains inputs into a single, high-voltage DC output. The current on each input is kept in phase with its voltage by varying the switching frequency of the MOSFETs controlling each phase.
The classic Vienna rectifier
The circuitry for each phase comprises a boost inductor, a pair of rectifying diodes, and a series of interconnected power transistors. The switches are linked to the midpoint of a capacitive divider, one capacitor for the positive half-cycle, one for the negative. Two diodes per phase are required to conduct the input voltage.
In a conventional single-phase H-bridge rectifier, current only flows when the input voltage is greater than the output voltage across the capacitor. This leads to short peaks of current that generate harmonics.
The conduction path of each phase passes through either one of the two diodes on each phase when the associated MOSFETs is OFF, or through the MOSFETs when they are conducting. The pulse width modulation (PWM) signals controlling the MOSFETs are adjusted based on the current feedback. Changing the on/off times of the MOSFETs removes the instantaneous current pulses that result from only using rectifying diodes. Removing the pulses in a continuous conduction configuration results in near-sinusoidal current waveforms.
When should you use a Vienna rectifier?
The Vienna rectifier stands out as an excellent solution when it comes to converting three-phase AC electricity. It provides a range of advantages that make it an attractive option for various applications, especially electric vehicle charging.
One of the Vienna rectifier's key strengths lies in its ability to achieve near-perfect power factor correction (PFC). Unlike conventional rectifiers that distort the AC waveform, the Vienna rectifier draws clean, sinusoidal current from the grid. This translates to a significant reduction in harmonic distortion, simplifying the filtering requirements downstream. This saves on component costs and minimizes electromagnetic interference, ensuring cleaner and more efficient power transfer.
Furthermore, the Vienna rectifier operates in continuous conduction mode (CCM). This means that the current flowing through the circuit never drops to zero, minimizing transient spikes and voltage fluctuations. This translates to smoother power delivery and less stress on the components, leading to increased reliability.
Efficiency is another hallmark of the Vienna rectifier. Its unique switching mechanism minimizes energy losses, making it an environmentally friendly choice, minimising operating expense and reducing the heat generated within the system. This translates to smaller cooling systems, allowing a lower profile and a more aesthetically pleasing design for EV chargers.
While inherently it allows only the unidirectional flow of power from the grid to the DC load, the Vienna rectifier topology can be improved, to support bidirectional operation by adopting the three-phase T-type topology. This allows the EV to feed energy back into the grid or even the homeowner's electrical system.
Despite its several advantages, the control system of the converter can be intricate. Implementing complex switching patterns requires sophisticated control algorithms, which may pose a challenge for some applications. However, with advancements in control technology, the Vienna rectifier continues to solidify its position as a powerful and versatile topology in the world of power conversion for EV charging.
Vienna rectifier trade-offs
The design of EV chargers requires a trade-off between cost-effectiveness, energy efficiency and operational reliability. To achieve the design goals, the selection of the appropriate rectifier component is paramount. Therefore, a thorough understanding of the compromises associated with various rectifier topologies is crucial.
The primary factor that determines the cost of an EV charger design is usually the switching technology. IGBTs represent a cost-effective option, while silicon carbide (SiC) MOSFETs provide greater efficiency at a higher price point. The higher switching frequencies achievable with SiC devices permit the use of smaller and less expensive passive components, such as inductors.
Although the cost of choosing SiC MOSFETs, coupled with the additional driver circuitry required for its operation, may seem like a significant price increase over silicon MOSFETs, the full advantages of using silicon carbide devices are clear when considering the full cost of the application and long-term cost of operation/ownership.
Reliability and efficiency are closely linked. Although SiC provides increased efficiency and natural resistance to higher temperatures, the reliability of an EV charger is determined by heat management. Despite having lower efficiency, IGBTs can be used with larger heatsinks to provide satisfactory performance. Heatsinking increases both the size and cost of fixed EV chargers, but the impact on weight is insignificant. Frequently, the size and cost of heatsinking are determined by the precise heat dissipation requirements, rather than the switch technology employed.
Designers must take a balanced approach when selecting where to use SiC MOSFETs. A combination of silicon switches in slow legs plus SiC devices in fast legs will often provide the best price vs efficiency/ reliability ratio.
Vienna rectifiers require sophisticated control systems to manage their complex switching patterns. While modern microcontrollers can deliver robust control, complexity increases with switching frequency. Elevated switching frequencies demand processors with faster execution speeds and the use of more intricate control algorithms. This, in turn, can impact the development time and cost associated with the EV charger design.
Vienna rectifiers achieve a commendable compromise by delivering satisfactory performance at a moderate cost, with the advantage of bi-directional power flow. While they may not be the optimal selection for very high-power applications, their versatility and near-unity power factor (PFC) make them a popular choice for EV chargers. The selection of the best rectifier hinges upon the relative importance of cost-effectiveness, energy efficiency and operational reliability, all of which are directly influenced by the specific application requirements and the target market.
Conclusion
Vienna rectifiers provide an appealing combination of characteristics for EV chargers. The design of the rectifier, however, is driven by a number of trade-offs. The main factor to be considered is the choice between prioritizing efficiency or price.
This crucial decision determines the selection of switch technology, with SiC MOSFETs chosen for optimal efficiency and IGBTs chosen for cost reduction. This choice will impose restrictions on variables such as the switching frequency, the choice of magnetic components, and the complexity of the control electronics.
Engineers can optimize the performance of Vienna rectifiers in EV charger designs by understanding the trade-offs and tailoring the design to meet their specific application requirements.