Three key elements leading to next-generation EV traction inverter innovation

The function of the traction inverter is to convert the high-voltage DC of the electric vehicle battery into the AC required by the electric motor. The traction inverter controls the motor's speed and torque. Its efficiency directly affects the output power, heat dissipation performance and driving range of the electric vehicle.

Figure 1 shows some of the most important chips or modules of a traction inverter: microcontroller (MCU), isolated gate driver, and isolated bias supply.

Figure 1: Electric vehicle traction inverter block diagram

To maximize the reliability and efficiency of your traction inverter, you must address the design challenges associated with these three components. Let’s look at innovations that can help alleviate these challenges.

Real-time control MCU

Reducing the size and weight of electric vehicle traction inverters can increase driving range and reduce costs, which requires continuous innovation of MCUs. One way to reduce size and weight is to spin the motor at higher speeds (>20 kRPM). Doing so requires optimizing the control loop for low latency, from the analog-to-digital converter (ADC) readout to field-oriented control ( FOC) calculation, and pulse width modulation (PWM) technology.

TI has many accelerators and features to achieve low control loop latency, including some hardware accelerators designed specifically for traction inverters. The two accelerators on the AM263P4-Q1 MCU are the resolver-to-digital converter (RDC) and the trigonometric math unit (TMU).

The RDC converts the sine/cosine feedback from the resolver sensor into speed and position. This calculation is performed in dedicated hardware to speed up the conversion process and offload this functionality from the main core. Other diagnostic features built into TI MCUs include sin 2 + cos 2 = 1 checks. Combining these diagnostics with two redundant RDCs creates an optimized solution for the traction inverter, which must meet the automotive safety integrity level ASIL D.

The TMU coprocessor runs in parallel with each core and offloads trigonometric math functions from the main core, while also providing up to 8x speed improvements. The increased speed provides significant control loop advantages for traction inverters, as most traction inverter control loops use the FOC algorithm to implement the Clarke and Park transforms, which require trigonometric mathematical functions.

The RDC and TMU accelerators (including the control subsystem) enable real-time control latencies within 3 µs, enabling control of high-speed traction inverter motors well above 20 kRPM, thereby reducing system size and weight.

gate driver

As traction inverter power levels approach 500 kW, improving efficiency (reducing energy losses throughout the drive cycle) is a major consideration in gate driver design. Other design requirements include power density, weight, height, functional safety and cost.

To improve efficiency, silicon carbide (SiC) field-effect transistors (FETs) are widely used in the industry. At the same time, switch-powered isolated gate drivers are becoming more sophisticated and now include isolated ADC sensing, multiple overcurrent protection modes, bias supply monitoring, gate monitoring, programmable safe states, built-in self-test, and more. A new feature is "real time", i.e. gate drive strength as a function of time.

Depending on the system's safety requirements, having functional safety-compliant gate driver integrated circuits (ICs) can help support the system's ISO 26262 compliance. For example, gate drivers can help ensure ASIL-D fault detection rates of ≥99% for single faults and ≥90% for latent faults.

Modern gate driver ICs turn the SiC FET on and off as quickly as possible through a voltage slew rate control method (transient voltage), minimizing the time component (dt) and reducing turn-on and turn-off energy, thereby reducing overall switching losses. This ability to control and vary gate drive current strength significantly reduces switching losses, but at the cost of increased transient overshoot of the node during switching, as shown in Figure 2.

Figure 2: Controlling SiC slew rate by varying gate driver IC drive strength

Real-time variable gate drive strength provides ultimate flexibility in optimizing traction inverter designs to increase efficiency and mitigate transient overshoot.

From the perspective of the electric vehicle battery charging cycle, SiC transient overshoot reduction and efficiency optimization are possible, and three-quarters of the charging cycle can be used for efficiency improvement; see Figure 3.

Figure 3: Battery peak voltage and efficiency zone during state of charge

The functional safety-compliant UCC5880-Q1 gate driver can be configured via the 4MHz bidirectional Serial Peripheral Interface (SPI) bus or three digital input pins (you don’t want to use the SPI bus to set the drive strength at power-up.), Efficiency gains are achieved through dual outputs, split output architecture and variable current strength.

bias supply

Electric vehicles require traction inverters for efficient power conversion to achieve longer battery runtime with each complete discharge cycle. The isolated gate driver bias supply achieves high efficiency by minimizing the conducted power losses of the SiC power module. Setting the gate voltage to the maximum allowed level reduces drain-to-source on-resistance (RDS(on)) while ensuring reliability (Figure 4). When the current in the traction inverter approaches and exceeds 400 A, it is important to reduce R DS(on), as higher resistance will lead to excessive conducted power losses.

Figure 4: SiC gate-source voltage limitation and conduction losses during switching cycle

TI's isolated DC/DC bias modules minimize conduction losses, while the UCC5880-Q1 minimizes switching losses. For SiC or IGBT drivers, both positive and negative gate drive rail voltages can be easily adjusted to the most efficient gate voltage setting with an error of 1.3% over full operating conditions (voltage, safe operating area power, temperature and process). Closed-loop feedback provides high-precision regulation of gate drive voltage control, maximizing the safety and efficiency of SiC or insulated gate bipolar transistor modules.

TI's isolated DC/DC bias modules provide a high-density solution that integrates an isolated power transformer, primary-side bridge, secondary-side bridge and control logic. The small package footprint enables efficient, small drive solutions in multi-phase traction inverters, reducing the PCB area by more than 30% to less than 4 mm in height and eliminating more than 30 discrete components, thereby reducing failure rates ( Figure 5).

Figure 5: TI isolated bias module vs. flyback power supply size and component comparison

in conclusion

TI's products enable next-generation traction inverters to operate up to 5% more efficiently and enable higher speeds, higher power density, lower height, lighter weight and lower system costs . As the electric vehicle market accelerates, traction inverters will also receive continued attention.