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3 Ways MCU Can Solve 800V Electric Vehicle Traction Inverter

Jan 04, 2023      View: 324

The electric vehicle (EV) traction inverter is the heart of the EV. It converts DC power from high-voltage batteries into multi-phase (usually three-phase) AC power to drive the traction motor and control the regeneration of energy from braking. Electric vehicle electronics are moving from 400V to 800V architectures, which promise to enable.


- Fast charging - delivering double the power at the same current.

- Increased efficiency and power density through the use of silicon carbide (SiC).

  • Weight reduction by using thinner cables to reduce the current required for the same power rating at 800V.

electric vehicle traction inverter


In the traction inverter, the microcontroller (MCU) is the brain of the system, performing motor control, voltage and current sampling via an analog-to-digital converter (ADC), calculating magnetic field orientation control (FOC) algorithms using magnetic cores, and driving power field effect transistors (FETs) using pulse width modulation (PWM) signals. For MCUs, the shift to 800V traction inverters poses three challenges.


- The need for lower latency real-time control performance.

- Increased functional safety requirements.

- The need for fast response to system failures.


Real-Time Control with Lower Latency


To control the torque and speed of the traction motor, the MCU uses a combination of peripherals (ADC, PWM) and computational cores to complete the control loop. With the move to 800V systems, traction inverters are also moving to wide-bandgap semiconductors (e.g., SiC) because of their greatly improved efficiency and power density at 800V.


This control loop delay becomes a priority in order to achieve the higher switching frequencies required by SiCs. Low-delay control loops also allow engineers to run motors at higher speeds, thereby reducing motor size and weight. To understand and reduce control loop delays, you must understand the control loop signal chain and its various stages, as shown in Figure 1.


control loop signal chain

Figure 1: Control Loop Signal Chain


To achieve excellent real-time control performance, you must optimize the entire signal chain, including hardware and software. The time taken from ADC sampling (input from the motor) to writing the PWM (output to control the motor) is the basic measure of real-time control performance. Starting with ADC sampling, the inverter system needs to sample accurately and quickly, i.e., achieve a high sample rate, at least 12-bit resolution, and low conversion time.


Once sampling is available, it needs to be transferred to and read by the processor through the interconnect, with optimized bus and memory access architecture to reduce latency. In the processor, the core needs to use the FOC algorithm to calculate the next PWM step based on the phase current, speed and position of the motor.


To minimize computation time even more, the kernel needs a high clock rate and must execute a specific number of instructions efficiently. In addition, the kernel needs to execute a range of instruction types, including floating-point, trigonometric, and integer math instructions. Finally, the core again uses a low latency path to write the updated duty cycle to the PWM generator. Applying deadband compensation to the PWM output will prevent shorts when switching between the high-side and low-side FETs and is best applied at the hardware level to reduce software overhead.


Traction inverter control loop delays as low as 2.5μs for TI MCUs and less than 4μs for the AM2634-Q1. This level of control loop delay will be targeted for future designs that include SiC architectures.


Increased Functional Safety Requirements


Since traction inverters provide the power to control the motor, they are inherently functionally safe and critical systems. Since 800V systems have the potential to provide higher power, torque, speed (or all three), traction systems need to be functionally safe to meet Automotive Safety Integrity Level (ASIL) D requirements. A key part of the functional safety system is the MCU, as it needs to make intelligent decisions to safely respond to system failures. Therefore, the use of an ASIL D certified MCU is an important safety element.


To make it easier for engineers to meet system safety requirements specific to traction inverters, TI MCUs offer additional features. For example, phase current feedback indicates information about motor torque, which makes these signals critical to safety. As a result, many engineers prefer redundant sampling of phase currents, which means the MCU must have multiple independent ADCs.


Fast Response to System Failures


Another challenge for engineers is the ability to quickly put the motor in a safe state in the event of a failure, such as a current renewal. In the AM2634-Q1 device, fault common inputs (for overcurrent, overvoltage or high-speed faults) go to an innovative programmable real-time unit (PRU).


The firmware executing in the PRU correctly evaluates and responds to the fault type and executes the desired PWM protection sequence, as shown in Figure 2, and then places the PWM directly into a safe state as needed. These operations occur in as little as 105ns. In addition, because the firmware is user-programmable, engineers can add additional custom logic as necessary to meet their application requirements.


pmw

As more and more electric vehicles are produced, design trends will shift to SiC and 800V technologies with the need to improve motor control performance and meet the functional safety requirements of traction inverters. As the world moves toward electrification, innovations in performance and efficiency are critical to help automotive engineers design the next generation of electric vehicles.


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