Why Isolation is Critical in Traction Inverters
Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs) require a traction inverter to convert dc energy from the high-voltage battery or dc link bus to the three-phase ac energy used to drive the traction motor. Traction inverters are typically capable of transferring power in the 20 to 100 kW range, with switching voltages in the 200 V to 800 V range and currents in the hundreds of amperes.
The entire system can be monitored and controlled via an automotive bus such as CAN (“Comm” in the diagram below). The CAN bus is isolated with digital isolators like Si86xx and digital isolators with integrated dc-dc power converters like Si88xx.
The traction inverter function is accomplished using six switches: one high-side and one low-side switch for each phase. Isolation separates the controller domain from the high-voltage ac domain. The gate drive for these switches uses isolated gate drivers, such as Si8239x, Si823x, Si823Hx, Si827x., and Si828x. The output side of the isolated gate drivers require a supply (typically 15 to 30 Vdc for IGBTs) that is isolated from the low-voltage domain and can be provided by the built-in dc-dc converter in the Si828x driver family.
Getting optimal efficiency from the system may require peak ac voltages that exceed the vehicle’s HV battery voltage, so traction inverters are often preceded by a bidirectional dc-dc boost converter. This boost converter often contains two switches that require isolated gate drivers. The system voltages can be sensed before the booster and traction inverter and a measurement taken with a isolated sensor (“Sns” in the diagram below).
HEVs also include a generator that converts mechanical energy from the internal combustion engine (ICE) into electrical energy to recharge the battery when the ICE is running. A block diagram for the generator’s ac-dc converter looks the same as the traction inverter, with the motor (“M” in the diagram below) replaced with a generator and the power flowing from generator towards the HV battery. Isolated gate drivers are needed to drive the switches.