Stop EMI from spreading in an EV design
Long the bane of design engineers, electromagnetic compatibility (EMC) continues to be a chief concern for electric vehicle (EV) and hybrid electric vehicle and (HEV) systems. Traditional internal combustion engine (ICE) vehicles are largely mechanical in nature, with electronics bolted to the mechanical power plant. EVs and HEVs, however, are very different.
Electrical power is converted into mechanical motion using high-voltage batteries, motors, and chargers. These high-voltage automotive systems can easily cause EMC issues. Fortunately, there are several tried and true techniques for reducing EMC in isolated systems, and many come with no additional cost.
The language of EMI
Before tackling EMI improvements, the basic terms used in standards and testing must be understood. EMC refers to both the immunity and emissions of a device, while electromagnetic interference (EMI) focuses only on the emissions of a device. CISPR 25, the most common EMC standard used for vehicles, specifies both EMI and immunity requirements.
Immunity is the ability of a device to operate correctly in the presence of interference. Reducing the EMI of a device usually improves its immunity to outside interference, so many designers focus primarily on reducing EMI and let immunity take care of itself.
Within CISPR 25, EMI is divided into conducted and radiated emission limits. The difference between the two is fairly intuitive. Conducted EMI travels from one device to another through the power, signaling, or other connected cables. Radiated EMI, on the other hand, travels through electromagnetic fields to interfere with another device. CISPR 25’s EMI standards ensure that the conducted and radiated emissions are below a specified threshold under specific test conditions to reduce the chances of vehicle systems interfering with one another.
Common mode is a common enemy
Central to any EMI discussion are differential-mode and common-mode currents. Since common-mode currents often cause EMI, the vast majority of circuits operate using differential-mode current instead. Figure 1 illustrates balanced differential signaling, which includes a dedicated conductor for the return current. Unfortunately, the return current often finds an alternative, longer path back to the source and creates a common-mode current.
Figure 1 Balanced differential-mode current features a dedicated conductor for the return current. Source: Silicon Labs
The common-mode current creates an imbalance in the two conductors that causes radiated emissions, as shown in Figure 2. Fortunately, many common-mode currents can be reduced with a few design improvements. Before exploring these methods, however, there are additional isolation challenges associated with high-voltage vehicle systems.
Isolation helps and hurts EMI
Isolation, and digital isolation in particular, is one of the fundamental technologies enabling the electric vehicle revolution. Isolation devices allow safe communication and signaling across the high-impedance barriers that separate high-voltage and low-voltage domains. The separation of these power domains creates a high-impedance path between the two circuits, as shown in Figure 3.
This high-impedance path creates a problem for common-mode currents induced by large changes in voltage present only on one side. These induced currents must find a path back to their source, and, thanks to the isolation barrier, the paths they take are often long, poorly defined, and of high impedance. The large loop areas of these paths lead to increased radiated emissions. Thankfully, this and other EMI issues can be reduced by using traditional EMI best practices with a few modifications specific to digital isolators.
Three simple methods for reducing EMI
Method 1: Select an isolator that minimizes transmissions
Digital isolators leverage CMOS technology to create isolation barriers and transmit signals across them. Signals are transmitted across these barriers using high-frequency RF signals. In many digital isolators, the default output configuration determines when an RF transmitter will be active. If the signal being sent by an isolator is typically high or low, simply choosing the matching default output state will minimize the transmissions, reducing EMI and power consumption.
Figure 4 illustrates the difference between a default low and default high isolator for an SPI bus configuration. With the proper digital isolator selected, the components around the isolation device may now be optimized for EMI.
Method 2: Select the correct bypass capacitors
Virtually every digital isolator specifies the use of bypass capacitors on the supply pins, and these have a tremendous impact on the EMI performance of the system. The bypass capacitors help reduce noise spikes on the power rails by supplying additional current to the device during transient loads. In addition, the bypass capacitors short AC noise to ground and prevent it from entering the digital isolator.
Ideally, the impedance of a capacitor decreases with frequency. In the real world, however, a capacitor’s impedance begins to increase at the self-resonant frequency due to the effective series inductance (ESL). As shown in Figure 5, reducing the capacitor’s ESL raises the self-resonant frequency and the frequency at which the capacitor’s impedance starts to increase.
In general, a smaller-sized capacitor, such as an 0402, will have a lower ESL because ESL depends on the distance between the two capacitor ends. Reverse-geometry capacitors provide an even lower ESL, as shown in Figure 6. Nevertheless, even with the lowest possible ESL, the placement of the bypass capacitor plays a critical role.
Method 3: Optimizing bypass capacitor placement
Proper placement of bypass capacitors is just as important as selecting ones with low ESL because traces and vias on the PCB introduce series inductances. The series inductance of a trace increases with length, making short and wide traces ideal. Also, the length of the return path to the ground pin of a digital isolator adds additional series inductance.
Simply rotating the capacitor to be close to both the supply and ground pins often reduces the return path length. Figure 7 illustrates ideal and non-ideal placement of bypass capacitors. Using these techniques to select low ESL capacitors and optimize PCB design will maximize the EMI reduction from the bypass capacitors.
These basic EMI reduction principles and techniques provide a foundation for designing automotive systems that can meet the stringent requirements of CISPR 25 and beyond. As more vehicle systems add sophisticated electronics and as electric vehicles become more advanced, EMI will continue to be a chief concern.
The need for isolation will also continue to increase as EV systems adopt higher voltages to drive greater efficiency. By considering EMI and applying best practices up front, high-voltage, isolated automotive systems will be ready to meet the EMI requirements of today and tomorrow.
Charlie Ice is a senior product manager at Silicon Labs, focusing on the company’s Power over Ethernet (PoE) product line.