Electric vehicles (EVs) represent a fundamental factor for the success of e-mobility, thanks to their reduced environmental impact and lower operating costs compared with traditional internal combustion engine vehicles. While waiting for the EV charging network to reach a capillarity similar to that of common gas stations, electric vehicles must be equipped with on-board charging circuits that ensure high efficiency and long range.
The recharging of electric batteries requires, first of all, a conversion of the electric power source from alternating current (available on the electricity distribution network) to direct current. The circuit topologies used to perform this energy conversion are quite standard, including half-and full-bridge rectifier circuits and the classic “totem pole” configuration.
A classic EV charger circuit comprises a current rectification stage followed by a DC/DC converter stage. The rectifier circuit, composed of diodes with nonlinear characteristics, has a rather low power factor and a large number of undesired harmonic components. A high level of efficiency can be achieved only through careful design of the power factor correction (PFC) circuit.
To improve the power factor and reduce harmonic distortion, a solution based on active power factor correction (APFC) is commonly adopted. APFC is essential for an active switching circuit that receives a rectified voltage at its input and boosts that value until it reaches a DC set value, checking that the line current maintains the desired sinusoidal waveform. In principle, in an ideal PFC circuit, the input current “follows” the input voltage, behaving like a pure resistor and without manifesting harmonics in the input current.
In high-power devices like EV chargers, capable of handling the power of several kilowatts, active PFC is implemented using boost converter circuits. The boost converter, shown in Figure 1, causes the input current to be stored in an inductor for a certain time interval. Subsequently, when the switch S opens, the energy can reach the C0 capacitor passing through the D diode. The inductor behaves like a current source in series with the input current, and therefore, the output voltage is always higher than the input: with 220- to 240-VAC input, more than 340 V is obtained in output (380 to 400 V is commonly used worldwide). Note that the PFC stage is always followed by a DC/DC converter, with isolation of the output from the input.
The circuit in Figure 1 can be improved by replacing diodes with MOSFETs, each of them acting both as boost switch and synchronous rectifier. However, high-voltage MOSFETs usually have poor body-diode reverse-recovery characteristics; therefore, bridgeless totem-pole circuits have not been very common so far. The recent market of devices based on wide-bandgap (WBG) semiconductor materials, such as silicon carbide (SiC) and gallium nitride (GaN), has allowed the adoption of circuits for the implementation of an EV charger.
With a forbidden band two or three times greater than that of silicon-based components, WBG devices can withstand voltages and electric fields of higher intensity (electrons require two or three times more energy to pass from the interdiction zone to the conduction zone). As a consequence, the breakdown voltage of WBG devices is much higher, while the on-resistance is much lower. In high-power electronic circuits, such as EV chargers, a high breakdown voltage simplifies the design and improves efficiency. A reduced on-resistance value represents a further advantage for high-voltage circuits because it allows both switching and power losses to be reduced, enabling a particularly compact footprint.
A further advantage of WBG devices is their ability to generate lower temperatures than silicon-based devices operating under the same conditions. In a circuit for high-voltage applications, a SiC component can withstand junction temperatures higher than 200°C, compared with about 150°C for a silicon equivalent. The use of WBG devices in EV chargers enables higher switching rates and better energy efficiency, which, in turn, translate into more compact modules that are simpler to cool.
Wireless EV charging uses an inductor, usually placed under the asphalt, and a receiver onboard the vehicle. Charging occurs automatically through magnetic plates that continuously recharge the batteries, whether the vehicle is standing still or in motion.
In the area of wireless power transfer, engineers need solutions with high-power and high-efficiency GaN-based approaches. GaN Systems’ broad portfolio of transistors provides high-power wireless charging solutions to design smaller, cheaper, and more efficient power systems for demanding applications.