How to deploy electromagnetic interference mitigation technology in automotive electronic systems?

Here will introduce EMI filtering and other system technologies that can be integrated into the system architecture to minimize conducted and radiated RF EMI interference. These should help designers pass EMI standard tests in their respective regions.

Emission Standards

The automotive industry and individual car manufacturers must meet various electromagnetic compatibility (EMC) requirements. Compliance with EMC standards (CISPR 25 for automotive applications) is essential in product design. For example, two requirements are to ensure that electronic systems do not emit excessive EMI or noise, and are not affected by noise emitted by other systems.

The effort required to achieve compliance affects product development costs and time to market. CISPR 25 is one of the most stringent international emission standards for vehicles and equipment with radio interference characteristics. These limits and measurement methods are designed to protect the on-board receiver from interference generated by components, such as switching regulators in power supply designs.

Designers need to fully understand CISPR and other emission standards before starting power supply design.

Types of electromagnetic interference

In order to reduce EMI in the design, engineers must first understand how EMI propagates into the design. There are two main categories of EMI—conducted and radiated—which can lead to longer design time to market and increased costs. Design work to reduce EMI to pass the EMI standards of the region or country/region where the product is used is critical to creating a successful functional design.

Conducted EMI is usually coupled through cables and physical conductors, such as power connections, parasitic impedances, and ground connections. Due to the electric field (capacitive coupling) or magnetic field (magnetic coupling), radiated EMI is coupled through the air through a radio transmission source.

Switching regulators are usually one of the main culprits of EMI generated inside or outside automotive systems.

The origin of electromagnetic interference

Electronic circuits usually have current flowing from the power source to the load and back to the power source in the loop. Loops have inductance and varying currents through components, wires, or PCB traces. When the current changes in the loop, it will produce a proportional voltage. This loop has self-inductance, and due to the current demand in the load, the current rate of change is di/dt. When the current changes rapidly in the loop, a voltage spike will occur.

To minimize spikes, designers can reduce the loop area, which will reduce loop inductance. The power IC can use two input loops in parallel, effectively reducing the parasitic loop inductance by half 5 (Figure 1). Designers can use bypass capacitors placed strategically close to ICs and other equipment to minimize EMI.

Figure 1. This schematic shows a simplified synchronous buck converter with critical loops and traces for EMI identification.

A good ground plane will provide a low impedance path for components such as bypass capacitors. Designers can keep noisy switching nodes or oscillators as far away as possible from sensitive nodes on the PCB. A good ground area or plane can also be used as a shield or physical isolation from noisy areas or components such as switching nodes/power transistors, high di/dt capacitors and inductors.

Some other methods will also help reduce radiation in the loop. An example is a design using a discrete buck regulator with switching power FETs. The drive signal to the FET can be slowed down by adding a gate resistor, which may help meet strict automotive emission standards. The disadvantage of this method is that the design now loses some efficiency, adds a component, and increases the space occupied by the circuit board.

EMI in automotive wiring harness

Advanced automotive electronic control technology has led to the addition of electronic equipment in the vehicle. The frequency and power in the vehicle gradually increase, creating a denser electromagnetic wave atmosphere. This will greatly promote EMI in the vehicle, thereby interfering with electrical/electronic equipment and possibly damaging electrical/electronic components.

Automotive wiring harnesses are one of the biggest contributors to EMI in automobiles, and they may also be affected by EMI.

Designers can take some measures to minimize the impact of EMI by shielding the source equipment and their respective wiring harnesses. By adding improved filters, conducted and radiated EMI in longer wiring harnesses can be minimized. Careful planning of the wiring harness also helps to place low-power circuits close to the signal source and high-power interference circuits close to the load.

Improved grounding technology will also help reduce EMI in automotive wiring harnesses. Shielding the wiring harness and connecting it to the body is a good way to reduce EMI interference.

Reduce radiated and conducted EMI in the car

Figure 2 shows the EMI bands and mitigation techniques of interest.

Figure 2. This image illustrates the EMI bands of interest and possible mitigation techniques.

Radiated EMI in automotive non-isolated power converters

Radiated EMI is caused by common mode noise in the vehicle power cable, which radiates into the vehicle space. This noise is mainly radiated by the non-isolated power converter through the switching power supply device in the power converter. Higher switching frequencies in modern power supplies and efforts to reduce the physical size of power converters are the main contributors to EMI in automobiles.

Conducted EMI in automotive buck converters

Designers may find that passing the FM band limitation in CISPR 25 Class 5 is very challenging. That’s because the EMI filter will deteriorate at high frequencies. Near-field coupling will also reduce the performance of the EMI filter, because high-frequency noise will generate strong magnetic fields and electric fields, and these magnetic fields and electric fields will be coupled to the input of the EMI filter.

Some solutions a designer might want to try include:

Reduce the noise source by adding start-up resistors or buffers, or reducing the switching frequency (this will reduce the high frequency harmonics of the noise source).

Reduce the parasitic effect of the power switch (SW) capacitance by placing as little PCB SW copper as possible, and also consider heat dissipation.

Adding a shielded enclosure will reduce electric field coupling.

Add filter components-common mode chokes can be added, but this will increase system cost.

The use of EMI filters and careful layout at the input of the buck converter will also help. The iron shielding box can be used as a last resort.

Components that help minimize EMI

Usually, the simplest component is the most important. Chip ferrite beads can be designed into electronic systems to achieve full current processing up to 85°C. The small size of ferrite beads allows them to provide EMI protection even in the most densely populated PCBs.

EMI suppression film capacitors comply with AEC-Q200 (Revision D) and IEC 60384-14: 2013/AMD1: 2016 Class IIB quality standards, and can be used as EMC filters for automotive power inverters.

EMI in electric vehicles

Electric powertrain (EPT) is a major contributor to broadband, high-level EMI. It will invade vulnerable electronic and radio frequency systems, such as systems in connected vehicles, infotainment, advanced driver assistance systems (ADAS), and autonomous driving systems. EMI management is particularly important in these systems.

EMI in the Internet of Vehicles (V2X)

Through wireless networks using 5G and V2X technologies, future electric vehicles will transmit, communicate, and process more data through low-voltage networks than today’s vehicles. The automotive industry is driving the development of battery capacity, range, engine power, and fast charging technology, all of which use high current and power levels. These high power/current levels will emit powerful electromagnetic fields that need to be addressed in the architecture of all electrical components.

Due to the presence of power inverters in the EPT, EMI mitigation is essential for the reliable and safe operation of low-voltage networks with potentially sensitive electronics and radio frequency units. Inverters operate at high power and fast switching frequencies, which can generate fast voltage and current transients, which are the main sources of conducted and radiated EMI.

In V2X automotive communication applications, passive components also play an important role. No matter how complex the semiconductor, without EMC components, transient protection, high-frequency connectors and antennas, V2X would not be possible.

Summarize

Switching power supplies in automobiles require some form of input filtering to pass EMI standards, such as CISPR 25 or other regional EMI regulations. This article also discusses other forms of minimizing EMI in automobiles, and it is likely that they will need to be integrated into many design architectures to obtain standard approval.

There are many ways to help suppress the erosion of EMI on automotive electronics. Most designers use more than one method, and some use multiple or all methods. Time to market is critical to the design of these automotive power supplies, and EMI testing must be performed after the overall design is fully completed.

As we develop from gasoline-powered vehicles to electric vehicles, and then to autonomous vehicles, we need to modify and add new and innovative EMI mitigation technologies to pass compliance tests in a timely manner. As we set foot in the future of automotive electronics, more innovative technologies will emerge.