Application Example: Onboard Charger (OBC)
An OBC converts grid AC voltage to an isolated DC output voltage for charging the high-voltage DC battery. The current measurements in OBCs typically range from 10 A to 60 A at AC input voltages from 85 VRMS to 265 VRMS and at DC output voltages from 200 V to 1000 V.
Figure 1 shows a simplified block diagram of a two-stage OBC. A PFC converter followed by an isolated DC/DC converter is the preferred topology for an OBC design. A non-isolated PFC boost converter ensures the rectified line current follows the rectified line voltage. This front-end PFC stage creates an intermediate DC bus with a relatively large ripple. An isolated DC/DC stage then provides galvanic isolation and a well-regulated, high-bandwidth output voltage with minimum current ripple flowing into the battery.
The microcontroller (MCU) or digital signal processor (DSP) receives feedback signals (voltage, current, temperature, and so forth) from the PFC and DC/DC stages and generates pulse width modulated (PWM) signals for controlling the field-effect transistors (FETs). Normally the MCU or DSP is referenced to a different ground domain (referred to as the cold side) and is isolated from the high voltage domain (referred to as the hot side). These feedback and control signals to and from the MCU or DSP must be isolated as well.