EMI Basics
High-voltage batteries, electric motors, and chargers convert electrical energy into mechanical motion. These high-voltage automotive systems are prone to causing EMC (Electromagnetic Compatibility) problems. Fortunately, there are several reliable technologies available to reduce EMC in isolated systems.
Before embarking on improving EMI, it's essential to understand the basic terminology used in standards and testing. EMC refers to a device's immunity and emission characteristics, while EMI focuses solely on the device's emission values. CISPR 25 is the most common EMC standard used in vehicles, specifying 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 can generally improve its immunity to external interference, so many designers focus primarily on reducing EMI and optimizing immunity.
In CISPR 25, EMI is divided into conducted and radiated emission limits. The difference between the two is quite intuitive. EMI is conducted from one device to another through power lines, signal lines, or other cables. Radiated EMI, on the other hand, propagates through electromagnetic fields, thus interfering with other devices. The CISPR 25 EMI standard ensures that conducted and radiated emissions are below specified thresholds under specific test conditions to reduce the chance of vehicle systems interfering with each other.
Common mode is the biggest problem
At the heart of any EMI discussion are differential-mode current and common-mode current. Since common-mode current typically causes EMI, the vast majority of circuits operate using differential-mode current. Figure 1 illustrates a balanced differential signal, including a dedicated conductor for the return current. Unfortunately, the return current often finds an alternative, longer path to its return source, resulting in a common-mode current.
Figure 1. Path of the return current for balanced differential mode current.
Common-mode current creates an imbalance in the two paths, leading to emitted radiation, as shown in Figure 2. Fortunately, common-mode current can be reduced through some design improvements. However, other isolation challenges exist for high-voltage vehicle systems before exploring these methods.
Figure 2. Common-mode current displayed in a balanced differential signal system.
Isolation helps reduce EMI
Isolation, especially digital isolation, is one of the fundamental technologies driving the electric vehicle revolution. Isolation devices allow for secure communication and signal transmission across high-impedance barriers separating high-voltage and low-voltage domains. This separation of power domains creates a high-impedance path between the two circuits, as shown in Figure 3.
Figure 3 shows that isolation creates a high impedance between the two grounds in the system, effectively eliminating the electrical connection between them.
This high-impedance path poses a problem for common-mode currents, which are caused by voltage changes on only one side. These induced currents must find a path back to their source, and due to the isolation barrier, the chosen paths are often long, difficult to define precisely, and have high impedance. The large loop area of these paths leads to increased radiated emissions. Fortunately, this problem, along with other EMI issues, can be mitigated by using conventional EMI practices and making some modifications for digital isolators.
Three simple ways to reduce EMI
Method 1: Select the isolator that minimizes transmission.
Digital isolators utilize CMOS technology to create isolation barriers and transmit signals across these barriers. High-frequency RF signals are used to transmit signals across these barriers. In many digital isolators, a default output configuration determines when the RF transmitter is activated. If the signal transmitted by the isolator is typically high or low, simply selecting a matching default output state will minimize transmission, thereby reducing EMI and power consumption.
Figure 4 shows that the default high digital isolator has less internal RF transmission for the bus transmission shown.
Figure 4 illustrates the difference between the default low isolator and the default high isolator configured via the SPI bus. By selecting an appropriate digital isolator, the components surrounding the isolated device can now be optimized for EMI.
Method 2: Select the correct bypass capacitor
Almost every digital isolator specifies the use of bypass capacitors on the power supply pins, which has a significant impact on the system's EMI performance. Bypass capacitors help reduce noise spikes on the power rails by providing additional current to the device during transient loads. Furthermore, bypass capacitors short-circuit AC noise to ground and prevent it from entering the digital isolator.
Ideally, the impedance of a capacitor decreases with frequency. However, in the real world, due to the effective series inductance (ESL), the impedance of a capacitor begins to increase at the self-resonant frequency. As shown in Figure 5, reducing the ESL of a capacitor increases the self-resonant frequency, and the capacitor's impedance begins to increase.
Figure 5. Actual capacitor model and the relationship between impedance and frequency in non-ideal capacitors.
Typically, smaller capacitors (e.g., 0402) have lower ESL because ESL depends on the distance between the ends of the two capacitors. As shown in Figure 6, reverse geometry capacitors offer even lower ESL; however, even with the lowest possible ESL, the placement of the bypass capacitor plays a crucial role.
Figure 6 shows that the ESL provided by the reverse geometry capacitor (right) is lower than that of the standard capacitor (left).
Method 3: Optimize the location of the bypass capacitor
Properly placing bypass capacitors is just as important as selecting low-ESL capacitors, because traces and vias on the PCB introduce series inductance. The series inductance of a trace increases with length, so short and wide traces are ideal. Similarly, the length of the return path to the ground pin of a digital isolator adds additional series inductance.
Simply moving the capacitor closer to the power and ground pins typically reduces the length of the return path. Figure 7 illustrates the ideal and non-ideal locations for bypass capacitors. Using these techniques to select low-ESL capacitors and optimize the PCB design will minimize the EMI of bypass capacitors.
Figure 7 compares the ideal and non-ideal locations of the bypass capacitor.
These fundamental principles and techniques for reducing EMI provide the basis for designing automotive systems that meet CISPR 25 and higher requirements. As more vehicle systems incorporate complex electronics and electric vehicles become increasingly sophisticated, EMI will remain a primary concern.
As electric vehicle systems adopt higher voltages to improve efficiency, the need for isolation will continue to grow. By taking EMI into account and applying best practices in advance, high-voltage isolated automotive systems can meet today's and future EMI requirements.
