58 September/October 2025 | E-Mobility Engineering Power simulations are crucial to confirm the assumptions on working voltage and currents, losses, cooling requirements, selection of power and passive components, among others. Once an implementation plan is ready, control simulations, including the power parameters, are carried out to confirm that the control loops are effectively executed with the power design. Control processor The control processor is a key component for the fast charger to handle the conversion algorithms. The controller is usually a microcontroller that has been optimised for real-time processing from companies such as Texas Instruments, Microchip, Infineon and STMicroelectronics. For example, the TMS320F28004x real-time microcontroller from Texas Instruments can be used for controlling a Vienna rectifier. The controller is part of the C2000 family with additional features and software libraries that make development of a Vienna rectifier topology smaller, lower cost and easier. Actuating the signals for the control of the power devices on the Vienna rectifier can be challenging, especially when due to size constraints the switching frequencies are over 100 kHz. The flexibility in the PWM module on the controller allows designers to generate these signals easily, while an on-chip trigonometric math unit (TMU) accelerates trigonometric operations in the control loop. This TMU is similar to a floating-point instruction set and is an IEEE-754 floating-point math accelerator tightly coupled with the CPU. However, where the floating-point unit provides generalpurpose floating-point math support, the TMU focuses on accelerating several specific trigonometric math operations that would otherwise be quite cycle-intensive. These operations include sine, cosine, arctangent, divide and square root. A Vienna rectifier uses hysteresisbased controllers, which are more complex to design. Sine triangle-based PWM has also been shown to work for Vienna rectifier control. Furthermore, with the development of an average current control model, the adoption of the rectifier has accelerated. Still, this type of control can be quite challenging to design because of the need to execute multiple loops, together with fine-tuning and switching at higher frequencies. There are several types of Vienna rectifiers, but the most popular for L3 charging stations is the Y-connection variant (see below) . A bridgeless interleaved three-phase PFC/interleaved boost stage uses an interleaved boost PFC per phase or per leg to reduce input ripple. This reduces the ripple for improved THD, with the possibility of thermal spreading if implemented as parallel modules in a modular charger design. However, this needs more inductors and components with more complex parallel design. Onsemi has developed a reference design that covers a wide output voltage range, which is able to charge EVs with both 400 and 800 V batteries, and is optimised for the higher voltage level. The input voltage is rated for EU 400 VAC and US 480 VAC three-phase grids, and the power stage delivers 25 kW over the 500–1000 V range. Below 500 V, the output current is limited to 50 A, derating the power in alignment with profiles of DC charging standards such as CCS. For a modern 250 kW DC fast charger, SiC MOSFETs are the preferred choice for both the front-end PFC and the isolated DC–DC (or at least on the primary side of the DC–DC). They enable higher efficiency, smaller magnetics, reduced cooling and simpler bidirectional operation – all key for compact, high-power chargers. Silicon IGBTs remain reasonable where initial unit cost is the overriding constraint or for conservative, legacy module reuse. Development process Feasibility studies helped validate the initial requirements and assumptions of the design, integrated as part of the system design that encompasses hardware, software, thermal management and mechanical design, prototyping and validation. All the essential system variables and most critical compromises and trade-offs for the solution happen during the feasibility studies. Two of the main design activities are the power simulations with SPICE models and the control simulation using MATLAB and Simulink. Deep insight | Fast charging A Vienna rectifier for a PFC front-end in a fast charger (Image courtesy of Microchip)
RkJQdWJsaXNoZXIy MjI2Mzk4