With the increasing number of electronic devices in modern cars, there is an increasing need to adopt good designs to meet the requirements of the major electromagnetic compatibility standards. At the same time, the increasing level of integration also makes automotive designers urgently need system-on-chip ASICs and dedicated standard product solutions, which can replace multiple discrete components. This article discusses some of the electromagnetic compatibility and integrated circuit (IC) issues facing automotive designers.
The electronic equipment in modern cars continues to develop with continued strong momentum-engineers are developing increasingly complex solutions for automotive comfort, safety, entertainment, power transmission, engine management, stability and control applications. Moreover, advanced electronic equipment has also been increasingly used. As a result, even the most common cars today are equipped with electronic equipment that was exclusive to high-end cars a few years ago.
In the past, non-critical applications such as comfort and convenience have promoted the development of automotive electronics applications. Just like power windows or central locks, these electronic applications simply replace existing mechanical systems. Recently, the scope of automotive electronic equipment has been expanded to support key applications such as engine optimization, active and passive safety systems, and some advanced infotainment systems including global positioning systems.
We are currently entering the third stage of the development of automotive electronics. At this stage, electronic equipment not only plays a role in supporting key functions, but also controls these key functions-whether it is providing important driver information and controlling the engine, or anti-collision detection and prevention, wire control and driving and Intelligent climate control, etc. As you might imagine, these applications require low-cost, easy to install, and increasingly intelligent and stable electronic solutions.
Speed ​​and cost factors have promoted the birth of "universal" embedded hardware electronic platforms. These platforms can provide basic or common hardware functions, and can be customized to provide the functions required by different models in the same car series through special application software, and can even customize functions for different car manufacturers. System-on-chip (SoC) semiconductor solutions integrate multiple functions in an integrated circuit, which reduces the number of components and space requirements, while ensuring long-term reliability, which is extremely important for the development of a successful universal embedded electronic platform.
Electromagnetic compatibility
With the increasing number of automotive electronic devices and the more and more extensive application of complex electronic modules in various parts of automobiles, electromagnetic compatibility issues are becoming more and more a design challenge faced by engineers. The three main issues are:
(a) How to minimize electromagnetic sensitivity so that electronic devices are not affected by electromagnetic emissions from other electronic systems such as cell phones, global positioning systems or infotainment devices.
(b) How to protect electronic equipment from the harsh automotive environment, including transients in the power supply system and interference with large loads or inductive loads such as switching lights and starting motors.
(c) How to minimize electromagnetic emissions that can affect other automotive electronic circuits.
Moreover, with the increase of system voltage, the increase of vehicle electronic equipment and the increase of frequency caused by more high-frequency electronic equipment, these problems have become more and more challenging. In addition, many electronic modules now interface with low-power, inexpensive sensors with large linearity differences and large zero offsets. These sensors rely on small signals, and electromagnetic interference will be catastrophic for their normal operation.
Compatibility testing, pre-compatibility testing and standards
These problems mean that automotive electromagnetic compatibility testing has become a basic element of automotive design. Compatibility testing has been standardized among car manufacturers, their suppliers and various legislative bodies. The later the electromagnetic compatibility problem is discovered, the harder it is to identify the root cause, and the solution may become more limited and expensive. Therefore, it is a basic approach to consider electromagnetic compatibility issues at all stages of the process-from integrated circuit design and printed circuit board layout to module installation and final automotive layout design. To simplify this process, pre-compatibility tests that consider electromagnetic compatibility issues at the module and integrated circuit stages have been standardized.
Design integrated circuits and modules that meet electromagnetic compatibility requirements
For integrated circuits, there are three main electromagnetic compatibility standards: Electromagnetic emission standard-IEC 61967: Electromagnetic susceptibility standard for measuring radiated and conductive electromagnetic emissions in the range of 150 kHz to 1 GHz-IEC 62132: Measurement of 150 thousand Transient standard for electromagnetic immunity in the range of Hz to 1 GHz-ISO 7637: Electrical interference caused by conduction and coupling of road vehicles.
So, how do system designers ensure that their system chips and final modules meet the above standards? The traditional SPICE model is useless at this time, because the electromagnetic field is not compatible with the SPICE simulation environment. Because the size of the chip and the entire assembly is much smaller than the wavelength of the electromagnetic signal (the wavelength is 30 cm at 1 gigahertz, which is much larger than the size of the integrated circuit), at the level of the integrated circuit, it is accurate enough to use electromagnetic fields to model the electromagnetic field. It is worth noting that radiated emissions and sensitivity are not the main issues for integrated circuits; the main issues are conducted emissions and sensitivity to effective antennas on printed circuit boards and wiring harnesses.
Several techniques are used to ensure that the electromagnetic compatibility requirements are met. We will look at the two issues of electromagnetic emissions and electromagnetic sensitivity one by one.
Electromagnetic emission
Electromagnetic emissions are generated by high-frequency currents in the outer loop as an antenna. Sources of these high-frequency currents include flipping of core digital logic such as digital signal processing and clock drivers (synchronous logic generates large and sharp current peaks containing a large number of high-frequency components), analog circuit actions, and switching digital inputs / Output pins and high-power output drivers that provide high current peaks for printed circuit boards and wiring harnesses. To minimize the impact of these factors, designers should use low-power circuits wherever possible. This may include architectures that reduce or use adaptive supply voltages or distribute clock signals throughout the frequency domain. The number of switching elements in one clock cycle can also be reduced by turning off unused parts of the digital system. In addition to this, controlling the slope of the clock and driver signal rising / falling edges to slow the switching edges and provide soft switching characteristics also helps reduce electromagnetic emissions. Finally, designers should carefully study the external and chip layout methods. For example, differential output signals using "twisted pair" wires produce less electromagnetic emissions and are less sensitive to electromagnetic emissions. Ensuring that the power supply and ground are close to each other and using efficient power supply decoupling are also simple ways to reduce electromagnetic emissions.
Electromagnetic sensitivity
Rectification / pumping, parasitic devices, current and power consumption are the three most serious interference effects of low electromagnetic sensitivity. The high-frequency electromagnetic power is partially absorbed by the integrated circuit, which can cause some interference, including outputting high-frequency high voltage to high-impedance nodes and high-frequency large current to low-impedance nodes.
An important way to minimize the effects of electromagnetic sensitivity is to make the circuit symmetrical, thereby avoiding the possibility of rectification. The method is to use a differential circuit topology and layout. Even if small signals are required in applications (such as using sensors), the topology that can handle larger common-mode signals can help the system maintain a linear state in the case of large-scale electromagnetic signals. Filtering to limit the frequency input range of sensitive devices is another method often used, especially with on-chip filters. The use of high common-mode rejection ratio (CMRR) and power supply rejection ratio (PSRR) designs also allows the circuit to avoid rectification, reducing internal node impedance and placing all sensitive nodes on the chip. Finally, in order to avoid or control parasitic devices and currents, it is very important to use protection devices to limit the level of suppression beyond electromagnetic sensitivity. This helps to avoid rectification and maintain the signal level in a symmetrical state. It is also very important to minimize the substrate current and release the current at critical locations.
The latest semiconductor technology
Many designers are using mixed-signal semiconductor technology to provide the system-on-chip solutions needed in today's automotive applications. The latest high-voltage mixed-signal technology is particularly suitable for designs that require high-voltage output—such as driving motors or starting relays—and is combined with analog signal conditional functions and complex digital processing.
The I2T and I3T series developed by Almatis Semiconductor Corporation (AMIS) is a model of the latest high-voltage mixed-signal ASIC technology. The I3T80 is based on a 0.35 micron CMOS process and can handle a maximum voltage of 80 volts, allowing complex digital circuits, embedded microprocessors, memory, peripherals, high-voltage functions, and various interfaces to be integrated in an integrated circuit.
Figure 1: Several functions are integrated in a single chip manufactured using the I3T80 process
Figure 1 illustrates several functions integrated in a single chip manufactured using the I3T80 process, including sensor analog interfaces (one of the most common requirements for automotive applications), high-voltage drivers for motors and transmissions, and the use of embedded 16 / 32-bit ARM ( tm) Digital processing circuit of the processor core. For low power processing requirements, 8-bit embedded R8051 processors are also provided. As shown in the figure, other 'standard' IP modules that AMIS can provide include timers, pulse width modulation (PWM) functions, JTAG for simplified device testing, interfaces, and communication transceivers including CAN bus and LIN bus communication options. Finally, it should be pointed out that the I3T technology contains built-in protection functions to protect the application-specific integrated circuit from damage due to overvoltage or incorrect battery connection.
Figure 2: Comparison of electromagnetic immunity between AMIS-30660 and other competitive products
AMIS uses this mixed-signal technology and many of the reasonable design methods for electromagnetic compatibility described in this article to develop various special standard products (ASSP) for the automotive industry, including AMIS-41682 standard speed, AMIS-42665 and AMIS-30660 high-speed CAN transceiver. These devices provide an interface between the CAN controller and the physical bus, which simplifies the design and reduces the number of components in 12-volt and 24-volt cars and industrial applications requiring a maximum rate of 1 megabaud CAN communication. For example, AMIS-30660 fully complies with the ISO 11898-2 standard, and provides differential signaling capabilities for the CAN bus through the CAN controller's transmit and receive pins. Integrated circuits provide designers with 3.3-volt or 5-volt logic-level interface options to ensure compatibility with existing applications and the latest low-voltage designs. The carefully matched output signal minimizes electromagnetic emissions, eliminating the need for a common-mode choke; and the large common-mode voltage range of the receiver input (± 35 volts) ensures high electromagnetic sensitivity (EMS). Figure 2 shows the comparison of electromagnetic immunity between AMIS-30660 and other competing products.
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