3 Key Technology to Enable Traction Inverters

A traction inverter combines functions on both the high voltage part of the powertrain architecture (HV battery side, 400V – 800+V) and the low voltage part (LV battery side, 12V – 48V). Thus, a traction inverter requires isolation in between devices on the low-voltage and high-voltage domain. TI’s capacitive and inductive isolation techniques, found in isolated gate drivers, digital isolators, isolated modulators and amplifiers and solid-state relays, incorporates reinforced signal isolation in a capacitive circuit that uses silicon dioxide for the dielectric. Figure 3-1 shows a block diagram of a traction inverter system. The isolation barrier (red dotted line) separates the low-voltage domain (LV battery side) and high-voltage domain (HV battery side).

In the low-voltage domain, a microcontroller (MCU) determines how to drive the power stages. The MCU sends pulse-width modulation (PWM) signals to the isolated gate drivers, which transfer the signals across an internal isolation barrier to the high-voltage side to drive the gates of the power stages. To amplify the signal to a strength necessary for the bulky power stages, the isolated gate driver requires a gate supply voltage from the isolated bias supply. The MCU runs the sensing and speed control in a closed loop and handles host functions to fulfill mandatory hardware and software security and safe code execution requirements. The MCU interfaces with the analog front end of the resolver or a Hall-effect sensor.

Implementing a safe power-tree keeps the MCU and critical power rails from losing power. A power-management integrated circuit (PMIC), connected to the LV car battery, powers the MCU. The LV battery voltage can be pre-regulated to a stable supply voltage before reaching the isolated bias supply. Additionally, for higher safety levels an HV to LV DC-DC supply can be added to provide voltage on the LV side by ORing, even when there is a failure on the LV battery or pre-regulation.

Figure 3-1 Traction Inverter System Block Diagram

Key functions in the high-voltage domain include:

• Power switches – Typically power modules based on silicon carbide (SiC) or an isolated gate bipolar transistor (IGBT), which are controlled by isolated gate drivers with protection and monitoring capabilities, such as internal temperature sensing and active short circuit (ASC) protection.
• Isolated gate driver – This IC takes a PWM signal input from the MCU on the LV side to drive the gates of the power modules with on the HV side. The isolated gate driver transfers this digital signal across the internal isolation barrier and uses the signal on the HV side to drive the gates of the power modules. At the same time, the isolated gate driver prevents hazardous DC or uncontrolled transient current flowing from the high-voltage domain.
• Isolated bias supply – A galvanically isolated power supply that takes the analog input from the LV side and transfers the input over to the HV side, providing the secondary supply to the gate driver. Here, the isolated gate driver combines the analog voltage with a PWM signal to drive the power stage gates.
• Isolated voltage and current sensing – To sense the DC link voltage and motor-phase current and maintain the correct torque applied to the motor, also used for the motor control loop.
• Position sensing – Vital for closed loop control meeting safety requirements. For position control, the sensors enables a known position and the positions throughout motion.
• Active discharge – To discharge the DC link capacitor voltage to a safe voltage. Active discharge is required for the type of motors that can generate back-electromotive force (EMF). United Nations regulation No. 94 of the Economic Commission for Europe of the United Nations requires that the voltage of the DC bus capacitor drops to a safe voltage (60V) in less than 5 seconds. Additionally, diagnostic circuitry is included to perform a self-tests on critical functions to prevent system failure.

The inverter control and safety schemes also vary with vehicle type. For example, a permanent magnet synchronous motor (PMSM) can be leveraged because the PMSM has high efficiency, low torque ripple, and a large speed range. PMSMs often use space vector PWM control, also known as field-oriented control. Other popular motor types in PHEVs and BEVs include induction motors, externally excited synchronous machines, and switched reluctance machines.

To reduce the potentially-costly, and geopolitically-precarious, rare earth materials used in a permanent magnet, the externally excited synchronous motors (EESM) are growing, not only for the secondary axle, but also for the primary. EESM machine types include conductive EESM and inductive EESM (iEESM). Commercial vehicles using EESM include the Toyota® Prius™, Chevrolet® Bolt EV™, Ford® Focus Electric™, VW® e-Golf™, BMW® iX3™, and so forth. In an EESM, the permanent magnet rotor is replaced with an electromagnet powered by a separate DC excitation current, for example in an H-bridge setup added to the traditional inverter structure. Additionally, this allows dynamic control of the rotor magnet flux, which is not possible with standard magnets.