57 E-Mobility Engineering | September/October 2025 Fast charging | Deep insight For a 250 kW commercial charger, the two-stage approach is the pragmatic industry standard. However, threelevel topologies are becoming more popular as the power levels increase beyond 250 kW. This allows the same transistors to be used to achieve the higher conversion. A three-phase two-level active pulsewidth modulation (PWM) rectifier uses a full bridge converter topology with silicon IGBT or SiC MOSFET switches with a DC link capacitor and active control of AC currents. This gives good control of the power factor and the THD with straightforward control of the current loops and a relatively low component count. However, this has higher switching losses if using silicon IGBTs at high frequency and large dv/dt stress on the DC link. Using SiC MOSFETs reduces losses and size. A three-level Vienna rectifier is an alternative for the PFC front-end. This uses a three-level neutral-pointclamped style rectifier configured for three-phase PFC. Using three levels reduces the switching stress and the input current ripple with lower EMI and lower blocking voltage per device. This is favoured for 100–500 kW class chargers. However, it is slightly more complex balancing the zero-switching point. When compared with a boost-type PWM rectifier, the Vienna topology uses multilevel switching (three levels), which reduces the inductance value requirement and reduces the voltage stress on the switches by half. This improves efficiency and power density. The L3 DC charger consists of a three-phase AC mains feeding an input EMI filter and surge protector and an input contactor or circuit breaker. This feeds a front-end PFC/ active rectifier with a bulk DC link capacitor for the high-voltage DC bus, typically 700–1000 V DC. This feeds an isolated DC–DC converter that regulates to battery voltage and provides galvanic isolation. For a battery voltage of up to 1000 V, a 250 kW fast charger has to handle 250 A. For a 400 V bus, that same 250 kW charger needs 625 A, which means thicker, heavier cables, more losses and more cooling required. Power architecture The aim for the PFC is to smooth out variations in the supply to achieve consistency of over 0.98 (where the maximum is 1) with low total harmonic distortion (THD) of under 5–8% to meet grid standards. This has to handle a wide input AC range of typically 380–480 V AC three-phase with high reliability, low input current ripple and good thermal behaviour. In the most common two-stage converter, the PFC produces a regulated high-voltage DC bus (≈750–1000 V), then isolated DC–DC converter(s) are used to step to battery voltage. This has an easier control system and allows for modularity. Isolation can also be provided in the second stage with good EMI separation. However, the two conversion stages mean more losses and a slightly lower theoretical peak efficiency, but each stage can be optimised. A single converter performs the PFC from the grid with isolation and regulation in one stage, typically using a matrix converter or bridgeless highfrequency isolated topology. This potentially has higher efficiency and fewer power stages but with a more complex control and is harder to meet standards and reliability targets at 250 kW. This makes it less common for large public chargers. The fast-charger architecture (Image courtesy of onsemi) A modular architecture for fast-charger stations (Image courtesy of Texas Instruments)
RkJQdWJsaXNoZXIy MjI2Mzk4