How do automotive electric compressors cope with the challenge of high pressure?

The compressor is a part of the car air conditioner. It compresses the refrigerant into high-temperature and high-pressure gas, which then flows through the condenser, throttle valve and evaporator for heat exchange to achieve heat exchange between the inside and outside of the car. Traditional fuel vehicles use the engine as the power and drive the compressor through a belt. New energy vehicles are separated from the engine and powered by batteries. The inverter circuit drives the brushless DC motor, which drives the compressor to rotate and achieve the heat exchange function of the air conditioner.

The electric compressor is a core component of thermal management in electric vehicles. In addition to improving the environmental comfort (cooling and heating) in the vehicle cabin, it plays an important role in the temperature control of the electric drive system, which is crucial to the battery's service life, charging speed and cruising range.

Figure 1: The electric compressor is a core component of thermal management in electric vehicles

Electric compressors need to meet increasing demands, including low cost, smaller size, less vibration and noise, higher power levels, and higher energy efficiency. These demands are inseparable from the design of compressor drive circuits and the selection of excellent devices.

The functions of the electric compressor controller include: driving the motor (inverter circuit: including ASPM module or discrete device with gate drive, voltage/current/temperature detection and protection, power conversion), communicating with the host (CAN or LIN, receiving start/stop and speed signals, sending operating status and fault signals), etc. ON Semiconductor has corresponding solutions in each circuit (Figure 1). This article focuses on the inverter circuit ASPM module solution.

Figure 2: Electric compressor drive circuit control block diagram

ASPM automotive grade intelligent power module

Automotive Smart Power Module (ASPM) is a modular solution that integrates power semiconductor devices, drive circuits and control circuits, designed to provide efficient, reliable and compact power conversion and control.

Figure 3: ASPM27 (left) and ASPM34 (right) from ON Semiconductor

Advantages of ASPM automotive-grade intelligent power modules

The power chip and IC chip of the ASPM module are directly soldered to the copper lead frame, then covered with ceramic, and finally cast in epoxy resin. Compared with the discrete solution, the parasitic inductance is greatly reduced, the number of components in the overall design and the area required for the PCB board are reduced, high insulation withstand voltage is provided, and good heat dissipation performance can be maintained.

Figure 4: ASPM internal structure

cost

In terms of cost, if you compare the device cost of ASPM modules and discrete devices alone, the cost of modules will be higher. However, in terms of the overall system cost, considering PCB, mechanical installation, quality and performance costs, the higher the system power, the more advantages of using ASPM modules.

Thermal properties

Figure 5: Thermal performance advantages of ASPM

In the design of electric compressors, heat dissipation is a key factor, which directly affects the current carrying capacity of the module. Therefore, the heat dissipation characteristics of the package are crucial in determining its performance. There is a trade-off between heat dissipation characteristics, package size, and isolation characteristics. The key to good packaging technology is to optimize the package size while maintaining excellent heat dissipation performance without sacrificing isolation level.

Taking the 650V ASPM27 series as an example, these modules use DBC (copper clad laminate) substrate technology, which brings good heat dissipation performance. The power chip is directly mounted on the DBC substrate, so that the heat can be more effectively transferred from the chip to the outside, thereby improving the heat dissipation efficiency and reliability, which is crucial to maintaining the long-term stability and service life of the power module under high current operation.

Because temperature directly affects the performance, reliability and life of the product, most designers want to know the temperature of the power chip accurately. However, since the power chip (such as IGBT, FRD) inside the package works under high voltage conditions, it becomes more difficult to directly measure its temperature. In the past, due to cost and technical reasons, designers often did not directly measure the temperature of the power chip, but used an external NTC thermistor to detect the temperature of the module or radiator. Although this method is simple, it cannot accurately reflect the temperature of the power component itself. In the 1200VASPM34 series, a major design innovation is to integrate the NTC thermistor and the power chip on the same ceramic substrate to achieve temperature sampling inside the module.

In this way, the actual temperature condition of the power chip can be reflected more accurately, so that developers can clearly know the internal temperature margin of the module and take corresponding measures in system control. For example, at low speed, the system heat dissipation is not good, resulting in excessive module temperature. The frequency can be appropriately increased to enhance heat dissipation; or the frequency can be appropriately reduced at high frequency and high power or over-temperature shutdown protection can be performed. The switching frequency of the ASPM module of ON Semiconductor is designed to be as high as 20kHz or more (ASPM27-V3 can reach 40kHz, and the IGBT switching speed of FS4 is faster and the switching loss is lower), which can easily meet the speed sampling requirements of existing electric compressors below 15,000 rpm.

Figure 6 ASPM27 internal circuit diagram

Power density

Compared with the discrete IGBT solution, ASPM greatly reduces the line inductance, and there is no need to consider the electrical safety distance between discrete devices; the insulation between the pin and the heat dissipation surface is up to 2500V, and there is no need to add additional insulating gaskets like IGBT. It is easy to install and has high reliability.

Figure 7: Power density of ASPM solution vs. discrete IGBT solution

reliability

The ASPM module integrates optimized protection circuits and drivers that match the IGBT switch characteristics, which can greatly shorten the circuit matching and development time for developers. By integrating undervoltage protection and short-circuit protection functions, system reliability has been greatly improved. The built-in high-speed HVIC has the ability to resist dv/dt and negative voltage, providing an IGBT driving capability without optocoupler isolation. The integrated HVIC allows the use of a single-power drive topology that does not require a negative power supply.

Figure 8: HVIC has the ability to resist dv/dt and negative pressure

To achieve higher reliability, the CTE mismatch between different materials can be minimized. ON Semiconductor's ASPM modules are certified by AEC-Q and AQG324, and discrete devices are certified according to AECQ100/101. We can also consider performing some special reliability tests based on customer-specific requirements.

Trends and Challenges

The concept of margin needs to be considered when selecting power devices for electric compressors in high-voltage environments to ensure that there is sufficient safety margin to cope with voltage fluctuations and transient events under various conditions.

The margin is usually based on the following considerations:

Steady-state voltage margin: Under normal working conditions, considering factors such as voltage fluctuations and load changes, the actual operating voltage is usually designed to be lower than the nominal withstand voltage of the power device. For example, if the maximum voltage of the battery system is 400V, a 650V withstand voltage device provides a voltage margin of 250V.

Transient voltage margin: In the case of switching operation or grid abnormality, momentary voltage spikes may occur. At this time, the margin is used to ensure that the device will not be broken down under these short but strong voltage shocks.

Reliability margin: During long-term operation, the voltage resistance of power devices may gradually decrease due to factors such as temperature and aging. Therefore, providing sufficient voltage margin can help extend the life of the device and improve the reliability of the entire system.

When 650V withstand voltage power devices are used in systems with peak voltages close to their rated values, designers need to carefully evaluate whether the voltage margin is sufficient to ensure that the power devices can operate safely and stably under all expected operating conditions. With the development of electric vehicle technology, the battery voltage platform continues to rise. The peak voltage of some car companies' 400V platform has reached more than 500V. When the original 650V ASPM module has insufficient margin in new applications, it will drive the market and technology to develop higher withstand voltage levels such as 750V ASPM modules.

On the 800V platform, due to the relatively small size of passenger car compressors, PCB design is relatively difficult when using 1200V modules. Since the internal space of the miniaturized compressor is limited, it is necessary to ensure that there is sufficient electrical safety distance between key components when designing the high-voltage PCB layout, which is a challenge for high-density packaged power modules.

The module generates greater losses when working at high voltage, requiring an efficient heat dissipation solution, while the miniaturized design may limit the design of the heat dissipation area and heat dissipation path, increasing the complexity of the thermal management design. High voltage levels mean a higher risk of electromagnetic interference, requiring more detailed PCB routing design and shielding measures to comply with relevant electromagnetic compatibility standards.