Modern electronic systems are becoming increasingly dense and highly integrated. This article introduces some proven methods for reducing electromagnetic interference (EMI) levels in power system design. Designers must be aware of serious EMI issues late in the design phase, otherwise, excessive financial and time costs may result.
This article discusses how to improve EMI in terms of filter size and cost, while reducing design time and complexity, especially in switch-mode power supplies (SMPS).
Switching effects in power metal-oxide-semiconductor field-effect transistors (MOSFETs) are a significant source of EMI in the system. This effect ultimately affects system reliability.
EMI primarily occurs due to discontinuous input current, high slew rate at switching nodes, and increased ringing at switching edges caused by parasitic inductance within the power supply loop.
EMI sources in SMPS
EMI problems cannot be resolved late in the circuit design phase because they can lead to wasted cost and time.
For example, consider the buck converter in Figure 1. It demonstrates how each component behaves within its respective frequency band.
Texas Instruments
1. The figure shows a typical example of an EMI source in an SMPS (provided by Texas Instruments).
To reduce the cost and size of SMPSs, designers are currently trying to increase the switching frequency, which will also improve efficiency. However, this shift to higher switching frequencies will lead to more EMI problems. Now, designers need the freedom to integrate EMI mitigation methods that do not affect the power supply design.
The aforementioned rules are well established in industry standards and specifications, such as CISPR 25 for the automotive industry and CISPR 32 for multimedia equipment. Interference will be limited to specific levels, and vulnerable circuits will be able to handle those levels of interference.
Methods to reduce interference
Designers must understand the applicable standards for each specific application. Knowing how to measure EMI is also important, as this knowledge will enable engineers to better understand how to reduce EMI.
In automotive designs involving long wiring harnesses, conducted EMI is associated with the increased number of wiring harnesses in modern vehicles. Some best practices for preventing or mitigating EMI are as follows:
shield
This is likely the best way to control coupled or radiated EMI. Shielded metal sheaths and coatings can be used. Additionally, cables should also be shielded to prevent external EMI from affecting sensitive components.
filter
Use filters to eliminate unwanted signals. Passive filters are commonly used to minimize EMI. Additionally, AC line filters can prevent unwanted signals from entering the power supply or other power circuitry. Integrated active EMI filters will help reduce differential-mode (DM) conducted radiation at the system input because they can act as very effective low-impedance shunts (Figure 2).
Texas Instruments
2. This is an active EMI filter with an inductor, compensation element and injection capacitor (Cinj).
Ferrite beads can also suppress high-frequency noise.
Grounding
Grounding typically provides a low-impedance path for EMI. When a system is properly grounded, EMI is diverted from critical devices, improving power quality. Ensure proper grounding is used on the printed circuit board (PCB) and keep trace lengths as short as possible and away from the PCB edges. Additionally, minimize loop area on the PCB to reduce radiated emissions.
Twisted-pair cables also help reduce common-mode noise.
More layout tips
Let's look at the PCB layout of a typical DC-DC buck converter, which is crucial for achieving optimal performance. PCB layout is a key factor in achieving the converter's best EMI performance. In a buck converter, the loop formed by the input capacitor and power supply ground is the most critical area. This loop will have large transient currents, which can lead to high transient voltages when they react with the trace inductance.
Designers must pay attention to the routing within this loop; these traces must be wide and short. The loop area must also be as small as possible to reduce parasitic inductance (Figure 3). Complete layout guidelines are provided in Reference 2.
Texas Instruments
3. The traces for the input current loop must be wide and short. The loop area should also be as small as possible to reduce parasitic inductance.
Avoid EMI interference
EMI in design can ruin a designer's day. EMI occurs when electromagnetic fields generated by electrical/electronic systems interfere with each other. This can cause severe circuit interruptions, leading to disruptions in nearby electronic equipment. As discussed in this article, there are several methods that can be used to minimize EMI.
