By Osamah Ahmad, product marketing, C2000 real-time MCUs, and Kevin Stauder, system engineer, automotive systems, Texas Instruments

After a decade of rapid growth, global hybrid electric vehicle (HEV) and electric vehicle (EV) market sales increased a record 43 percent in 2020 over 2019. This much-anticipated global HEV and EV market surge occurs because of region-led initiatives like China’s New Infrastructure Plan, Europe’s End of Life Vehicle program, and American automakers like Tesla.

As illustrated in Figure 1, J.P. Morgan estimates that the market share of battery EVs (BEVs), plug-in hybrid EVs (PHEVs), and hybrids may represent as much as 59% of all vehicle sales by 2030.

For decades, the internal combustion engine (ICE) has run the vehicle and its heating, ventilation, and air-conditioning (HVAC) systems. Still, the transition to HEVs with small combustion engines or full EVs with no combustion engine requires additional components. One of these components is a brushless-DC (BLDC) motor, a type of DC motor that rotates the air-conditioning compressor instead of having the engine do so (Figure 2).

Additional HVAC system components for HEVs and EVs, such as a positive temperature coefficient heater or a heat pump, will help with battery thermal management by moving heat from the battery to the cabin, ultimately improving efficiency. In this article, however, I’ll focus on the design challenges of HVAC compressor subsystems within HEV and EV heating and cooling systems and discuss how real-time control can address those challenges.

Components within the HVAC compressor module

Figure 3 is a block diagram of an AC compressor BLDC module powered by a high-voltage battery.  Various transistor types, including field-effect transistors, insulated-gate bipolar transistors (IGBTs), and silicon carbide transistors, can be used in the control module. There are corresponding gate drivers to control power to the BLDC motor.

HVAC compressor designs need to be simple and compact (using a single converter and transformer to generate both voltages), and offer good performance. Discrete insulated gate bipolar transistors (IGBTs) with gate driver integrated circuits (ICs) simplify designs by allowing for minimized printed circuit board trace length and component count. Real-time microcontrollers (MCUs) that can handle complex algorithms enable sensorless (velocity and torque) motor-control applications and improve the cost to design.

Let’s review the challenges when designing HVAC compressor systems.

Reliable sensorless control

A reliable HVAC compressor system should offer high startup torque, automatic motor parameters, diagnostic algorithms, low noise, oscillation suppression, and high efficiency. Many MCUs, such as C2000 real-time MCUs, are programmable through C2000 InstaSPIN-FOC™ control software to enable low-effort motor control for sensorless applications. C2000 InstaSPIN-FOC software, which is free, significantly lowers the barrier to entry for motor-control developers by bringing the ability to identify, tune and fully control a three-phase motor in minutes. This enables designers to address their key design challenges through reliable sensorless control easily.

Low audible noise and low EMI

HEV and EVs are susceptible to audible noise that stands out in an otherwise quiet vehicle without engines. In ICE vehicles, the engine is already audible such that any noise coming from the HVAC system is minuscule by comparison. This noise negatively affects the HEV and EV user experience, especially when heating or cooling the cabin.

HEVs and EVs are also susceptible to EMI because of the BLDC motor and electronics needed for the compressor. Whether electronically or audibly, the new components brought in for a compressor in HEVs and EVs should not disturb the existing system or the user’s driving experience.

Real-time MCUs address these concerns by combining on-chip hardware enablers combined with real-time control software. TI’s real-time MCU software uses a field-oriented control-based current waveform that enables accurate position and angle estimation, lowers noise and vibration, and allows reliable motor control.

Functional safety

An HVAC compressor system that fails while the vehicle is in motion is not necessarily considered a critical safety hazard. Some vehicles integrate thermal management of the battery and cabin with a single compressor, however. Functional safety is a concern for such scenarios, as the battery could catch fire if the compressor fails. An overtemperature condition for the motor could cause a similar, dangerous scenario. Automotive Safety Integrity Level (ASIL) B is the typical functional safety requirement for these systems, with some designs requiring up to ASIL D.

A real-time MCU development flow should comply with functional safety standards such as International Organization for Standardization 26262:2018-5, and enable safety mechanisms that provide sufficient diagnostic coverage to meet the application’s safety goals. Meeting these safety goals and requirements will ultimately protect critical functions in the vehicle from failing.

Real-time MCU considerations

With the evolving trends and varying requirements coming from original equipment manufacturers across the globe, having the ability to leverage a compatible platform helps enable application scalability. In cases where Grade 0 operating temperatures, functional safety compliance, and varying power levels are priorities, a range of cost and performance within an MCU family is important, in addition to features such as:

• A scalable pin count.
• Integrated flash memory.
• Functional safety features that help enable up to ASIL D in automotive systems.
• Key communication interfaces for automotive applications include Controller Area Network (CAN), Controllers Area Network-Flexible Data Rate (CAN-FD), Local Interconnect Network (LIN)
• Premium analog-to-digital converters, pulse-width modulators, and central processing unit million-instructions-per-second performance for real-time control.
• Automotive-qualified devices, with Grade 0 options.
• Easy-to-use software, example designs, and system expertise.

Conclusion

EVs and HEVs are currently in the process of explosive growth. The HVAC compressor subsystem in these vehicles requires new components that bring forth design challenges such as reliable sensorless control, application scalability, and ease of development. With the help of real-time MCUs, you can smoothly navigate the move from ICE to HEV and EV HVAC systems.

For more information on other subsystems and details about new trends in heating and cooling control modules in 48-V, 400-V, or 800-V HEVs and EVs, see the white paper, “How to Design Heating and Cooling Systems for HEVs/EVs.”