Solving High Temperature Isolation Design Challenges with Class 0 Digital Isolators

“As the automotive industry continues to adopt 48V systems in hybrid electric vehicles (HEVs), the need for signal isolation in in-vehicle networks becomes even more critical. If the low voltage circuit is not reliably and effectively protected, the characteristics and advantages of high voltage will be greatly reduced.

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Author: Neel Seshan

As the automotive industry continues to adopt 48V systems in hybrid electric vehicles (HEVs), the need for signal isolation in in-vehicle networks becomes even more critical. If the low voltage circuit is not reliably and effectively protected, the characteristics and advantages of high voltage will be 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 conventional internal combustion engine (ICE) in addition to a battery system. The high temperatures produced by ICE typically exceed 125°C. To operate reliably in such an environment, automotive systems and their components must be able to withstand high temperatures as 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. Since automotive systems are often subject to temperature variations, a key specification for the AEC-Q100 standard is the integrated circuit’s ambient operating temperature range. AEC-Q100 outlines the operating temperature range of automotive-grade ICs according to different temperature grades, as shown in Table 1.

Table 1: Automotive grades as defined by AEC-Q100

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

Since EVs do not have an ICE, the ambient operating temperature typically does not exceed 125°C in most cases, so a Class 1 rated device is sufficient to solve the problem.

Protecting Low Voltage Circuits with Class 0 Digital Isolators

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

The starter/generator shown in Figure 1 is an example where a Class 0 digital isolator, such as the ISO7741E-Q1, reduces design complexity while adding signal protection in high temperature environments. In the starter/generator, digital isolators and a Class 0 Controller Area Network Flexible Data Rate (CAN FD) transceiver such as the TCAN1044EV-Q1 can transmit data from the 48V side to the 12V side of the system. The 48V electrical system is in close proximity to the ICE; therefore, any temperature rise on the 48V system will affect 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 for a short period of time, usually limited by the duty curve or operating temperature curve, which varies by automaker.

Figure 1: Digital isolators protect the low 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 as well as transceivers (CAN, CAN FD or Local Interconnect Networks in most cases) [LIN] communication protocol) 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-side communication interface board.

Figure 2: A digital isolator protects the low-voltage side of a 48 V HVAC compressor module

If a digital isolator is used at temperatures above its operating limits, it may degrade the system timing specifications, and if the isolator stops functioning, it may result in a loss of communication. Neither is desirable for critical systems like starter generators. The standard way to ensure communication at all times is to use liquid and air cooling systems that reduce heat and keep IC temperatures below their operating limits. However, a well-designed 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 offload the cooling system, making it simpler and more cost-effective.

Most qualified automotive digital isolators, including ISO7741-Q1, meet the -40°C to 125°C Grade 1 temperature range and are 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 reduces bill of materials and reduces time to market without compromising system performance.