3-Phase PFC in EV and HEV Chargers
The need to address the issues related to climate change and the containment of carbon dioxide emissions is promoting the use of electric (EV) and hybrid (HEV) vehicles on a global scale. These types of vehicles, capable of reducing or even eliminating polluting emissions, are characterized by the presence of one or more electric motors and high voltage (HV) batteries. An automotive battery can be considered as HV if its rated voltage is greater than 60V. HV batteries are essential for storing the amount of energy needed to move the vehicle and power auxiliary and accessory circuits. Charging can be performed via onboard chargers or via external DC converters (fast charging). Besides batteries, DC-DC converters and inverters are also required to power the powertrain and other vehicle subsystems, such as heating, ventilating, and air conditioning (HVAC). In Figure 1 we can observe the block diagram of a typical EV and HEV powertrain.
The first stage is the power factor correction circuit (PFC), which in this case is connected to the 3-phase high voltage mains. This front end is often implemented by using a Vienna rectifier, a PWM-based approach which offers several advantages in all applications where high efficiency, low switching losses and high EMI/RFI immunity are required. The PFC is followed by a resonant LLC DC-DC primary converter, coupled to a secondary DC-DC converter (or rectification stage) which can finally charge HV batteries.
EV/HEV charging
The progressive abandonment of fossil fuels and the migration to electric and hybrid vehicles, inevitably entails a greater demand for recharging stations. Fast and ultra-fast charging stations, able to deliver up to 250 kW DC power, perform the conversion from alternating to direct electrical power outside the vehicle, an obligatory solution given the high power involved, in addition to the weight and cost involved by the required components. The same block diagram in Figure 1 is applicable to fast and ultra-fast charging stations. In particular, in Figure 2 we can observe the PFC stage, implemented through a classical scheme, known as Vienna rectifier.
In order to achieve high efficiency, keeping the switching losses low, high power MOSFETs, IGBTs, power integrated modules (PIMs) and SiC devices are normally employed, providing a complete solution for implementing a 3-Phase PFC stage. The Vienna rectifier proves particularly suitable for the implementation of PFC front ends in which there is a unidirectional flow of power (from AC to DC), high power density and low voltage stress across the switches. The Vienna AC to DC rectifier provides a power factor (PF) that is very close to the unit value, a sinusoidal current and low total harmonic distortion (THD).
Power discretes
Due to their superior electrical characteristics with respect to traditional silicon-based devices, SiC power discretes provide the best solution for implementing high efficiency PFC front-end. ON Semiconductor offers a wide selection of SiC MOSFET drivers, suitable for applications such as PFC, high performance inverters and high-power motor drivers. The NCP71705, specifically designed to drive SiC MOSFET transistors, delivers the maximum allowable gate voltage to achieve the lowest possible conduction losses. By providing high peak current during turn−on and turn−off, switching losses are also minimized. For improved reliability, dV/dt immunity and even faster turn−off, the NCP51705 can utilize its on−board charge pump to generate a user selectable negative voltage rail. For isolated applications, the NCP51705 also provides an externally accessible 5 V rail to power the secondary side of digital or high speed opto-isolators. The boost stage of a high power AC to DC converter benefits from the usage of efficient Schottky diodes, such as the CoolSiC™ Schottky diode 650 V G6, Infineon’s sixth generation of silicon carbide (SiC) Schottky diodes, with a maximum blocking voltage of 650 V. CoolSiC™ G6 was designed to improve efficiency and to enable compact power supply designs. The CoolSiC™ Schottky diode 650 V G6 is a state-of-the-art SiC diode, offering the lowest forward voltage and obtaining the highest efficiency. The benefit of the forward voltage reduction is visible in the lower conduction losses. The lower forward voltage of the CoolSiC™ Schottky diode 650 V G6 enables lower conduction losses, which contribute to achieve higher efficiency and lower junction temperature of the device. ROHM Semiconductor provides several silicon carbide devices suitable for high-power automotive grade applications. The SCS220AGHR, for instance, is a Schottky Barrier Diode delivering a reverse (breakdown) voltage of 650V which far exceeds the upper limit for silicon SBDs. this SiC SBD is compliant to the AEC-Q101 automotive standard and it features high-speed switching with ultra-small reverse recovery time. This minimizes both reverse recovery charge and switching loss, contributing to end-product miniaturization. The AEC-Q101 offers a continuous forward current of 20A, a reverse current in the range from 4 to 140 uA and a total power dissipation of 130W. The device is available in a TO-220AC package and it is suitable for applications such as on-board chargers, wireless chargers, EV chargers and converters.
