Issue link: https://resources.mouser.com/i/1429079
mouser.com/vishay / 11 T oday's on-board charging (OBC) systems convert power from the AC grid into a DC voltage used to charge the high-voltage battery pack in either an electric vehicle or a plug-in hybrid vehicle. As the trend toward fast-charging systems continues, the power demanded from OBC has been steadily increasing from 5.5kW to 11kW, and now up to 22kW. This development, paired with a high-efficiency and power-density demand at a low system cost, has driven the need for 3-phase topologies. This 22kW OBC is designed to operate from a 3-phase input voltage (without a neutral) that ranges from 340V AC to 480V AC and provides an output voltage range from 250V to 500V with a maximum current of ~50A. The input stage (Figure 1 and Figure 2) utilizes a T-type Vienna rectifier, a common topology that is used for power factor correction (PFC), that meets the end requirements in terms of harmonics and reactive power, and allows the charger to operate over a wide input voltage range. This topology also works with a virtual neutral point that allows the DC bus to be split into two symmetrical stages. With this approach, it is possible to use 650V silicon MOSFETs for the main DC/DC stage instead of much more expensive 1200V silicon carbide (SiC) devices required with other topologies. The T-type Vienna rectifier allows for power factor correction (PFC), but like all boost-derived topologies, it cannot handle the inrush current drawn by the DC link capacitors (C1/ C2/C5/C6 in Figure 2) during the initial power-up. This bank of capacitors (either aluminum electrolytic or film, depending on the application requirements) is required to handle the high ripple currents generated by PFC switching. Although the PFC diodes (D1-D6 in Figure 2) have excellent surge capability and the three PFC inductors (L1-L3 in Figure 2) are in series between the AC input and the DC link capacitors, active control of the inrush current is still necessary. This function could be accomplished with a mechanical relay, but this device would be very large (to handle the 40A RMS current), and it would not meet the typical lifetime requirements of the OEMs. The Vishay VS-40TPS12LHM3 thyristors (TH1-TH4 in Figure 2) are connected as normally open bi-directional switches in two of the input legs. In parallel with the thyristors are the Vishay high energy absorption PTCEL inrush current limiters (PTC1/PTC2 in Figure 2). They are switched on when the charger is first energized and then switched off when the DC bus is charged to the appropriate level. It is also worth mentioning that the necessary X/Y filter and safety capacitors (for example, the Vishay AY2 series) are located in the fuse block and EMI filter section in Figure 2 that is not shown in detail. The Vishay VS-E5PH6012LHN3 diodes (D1-D6 in Figure 2) and SQW61N65EF MOSFETs (Q1-Q6 in Figure 2) work together to provide the active rectification. The 650V MOSFETs are configured as bi-directional switches and are switched at 30kHz (with low losses) because of the minimal reverse recovery current of the diodes, which are hyper-fast devices with soft recovery characteristics. This combination also reduces the induced noise back onto the AC power grid and simplifies the design of the input filter. Figure 2: The simplified schematic of the power factor correction stage (Source: Vishay)