Solving High-Temperature Isolation Design Challenges Using Grade 0 Digital Isolators

As the automotive industry continues to adopt 48V systems in hybrid electric vehicles (HEVs), the need for signal isolation in vehicle networks becomes more important. Without reliable and effective protection of low-voltage circuits, the characteristics and advantages of high voltage are greatly reduced.

However, understanding the need to isolate high voltage event signals in 48V vehicles is only half the battle. Unlike pure electric vehicles (EVs), HEVs use a traditional internal combustion engine (ICE) in addition to the battery system. The high temperatures generated by the ICE often exceed 125°C. To be able to operate reliably in such an environment, automotive systems and their component parts must be able to withstand the high temperatures defined in the Automotive Electronics Council (AEC)-Q100 "Failure Mechanisms Based on Stress Test Qualification of Packaged Integrated Circuits".

The AEC-Q100 standard outlines the specifications that integrated circuits (ICs) designed for use in automotive systems must meet for reliable operation. Because automotive systems are often subject to temperature variations, a key specification of the AEC-Q100 standard is the ambient operating temperature range of the integrated circuit. AEC-Q100 outlines the operating temperature range of automotive-grade ICs based on different temperature grades, as shown in Table 1 .

Table 1: Automotive grades defined by AEC-Q100

As the widest temperature range defined by AEC-Q100, Grade 0 devices are usually designed for high temperature systems (such as 48V HEVs) because these vehicles can occasionally reach temperatures above 125°C due to the use of ICE.

Since electric vehicles do not have an ICE, the ambient operating temperature usually does not exceed 125°C in most cases, so devices rated as Grade 1 are sufficient to solve the problem.

Let’s look at some use cases to better illustrate the benefits of Class 0 devices when isolating in-vehicle network signals, specifically for digital isolators . Digital isolators are often used between different voltage domains (e.g., 48V and 12V ) to protect low-voltage side circuits from high-voltage side circuits and reduce the impact of high-voltage common-mode noise on low-voltage side signals.

The starter/generator shown in Figure 1 is an example where a Grade 0 digital isolator, such as the ISO7741E-Q1, can reduce design complexity while adding signal protection in a high-temperature environment. In the starter/generator, a digital isolator and a Grade 0 controller area network flexible data-rate (CAN FD) transceiver, such as the TCAN1044EV-Q1, can transfer data from the 48V side of the system to the 12V side. The 48V electrical system is in close proximity to the ICE; therefore, any temperature rise on the 48V system affects the isolator located at the edge of the interface between the 48V and 12V sides. The temperature of these systems can rise from 125°C to 150°C in a short period of time and varies by vehicle manufacturer, usually limited by mission profile or operating temperature profile.

Figure 1: Digital isolators protect the low-voltage side of a 48V starter/generator system

Other applications that may benefit from higher temperature grade digital isolators include water pumps, cooling fans, soot sensors, and traction inverters in 48V hybrid vehicles. Most of these systems use digital isolators along with transceivers (in most cases CAN, CAN FD, or Local Interconnect Network [LIN] communication protocols) as the communication interface. Figure 2 shows a heating, ventilation, and air conditioning (HVAC) compressor module with an isolator for communication from the high-side MCU to the low-voltage side communication interface board.

Figure 2: Digital isolators protect the low-voltage side of a 48 V HVAC compressor module

If a digital isolator is used at a temperature that exceeds its operating limits, it can cause degradation of system timing specifications, or it can cause a loss of communication if the isolator stops functioning. Neither of these situations is desirable for a critical system like a starter generator. The standard approach to ensuring communication at all times is to use a liquid and air cooling system that can reduce heat and keep the IC temperature below its operating limits. However, an elaborate air cooling system can result in increased cooling system design cost, space, and weight. Using integrated circuits that can handle higher ambient operating temperatures can reduce the burden on the cooling system, making it simpler and more cost-effective.

Most qualified automotive digital isolators, including the ISO7741-Q1, meet the Grade 1 temperature range of -40°C to 125°C , which is suitable for many automotive applications. However, in high-temperature systems, similar to the use cases discussed in this article, Grade 0 devices such as the ISO7741E-Q1 will provide HEV/EV designers with an alternative digital isolation solution. This solution can reduce the bill of materials and shorten time to market without compromising system performance.