How can power converters be designed to minimize EMI?

Switch-mode power converters are highly efficient and cost-effective devices. However, they must be carefully designed and integrated into systems to avoid electromagnetic interference (EMI) problems. This article reviews some EMI sources in power converters and examines a few design considerations for DC/DC converters and DC/AC inverters in power conversion, renewable energy, and motor drive applications.

While power converter EMI can be minimized and controlled, it usually can’t be eliminated. It’s inherent in systems with discontinuous input currents, fast voltage and current slew rates, and challenges related to parasitic inductances. The increases in switching frequencies enabled by advanced gallium nitride (GaN) and silicon carbide (SiC) power switches can intensify those challenges (Figure 1).

Figure 1. Discontinuous input currents (blue) and fast switching slew rates (red) are common sources of switching power converter EMI. (Image: Texas Instruments)

Controlling EMI in switching power supplies begins with the basics: optimizing the PCB layout, following good grounding practices, minimizing loop areas, and applying shielding as needed. Filters on the input and output can further reduce EMI.

While conventional passive filtering is often sufficient, some designs can benefit from active filtering. An active EMI filter works like noise-canceling headphones: it detects noise, generates a signal with the opposite phase, and injects the generated signal into the line to eliminate the noise signature.

Design techniques like soft switching and slew rate control are common approaches to controlling EMI. In soft switching, the converter’s power switches turn on and off when the voltage or the current is zero, called ZVS and ZCS, respectively.

Soft switching reduces EMI, stresses the switching transistors, improves reliability, reduces switching losses, and increases efficiency. Power converters and inverters that use soft switching are sometimes referred to as resonant designs.

Figure 2. The EMI noise signature using spread spectrum modulation (blue) has a much lower peak than conventional modulation (red). (Image: MDPI eneregies)

Slew rate control adjusts the dV/dt rate of the switching transistors to minimize EMI generation. It’s usually implemented by adding some impedance in the gate-drive connection to slow the transition. Using slew-rate control can reduce emissions at the ringing frequency up to 10 dB, but with increased power dissipation (and lower efficiency) due to the longer transition times.

Spread spectrum modulation is another advanced method of controlling and reducing EMI in switching power converters. By varying (spreading) the switching frequency, EMI is spread across a wider bandwidth instead of concentrated at a single frequency (Figure 2).

Spread spectrum modulation can be implemented in several ways. Frequently used variations include frequency hopping spread spectrum (FHSS), which hops between frequencies in a predetermined pattern to provide EMI reduction but at the expense of control complexity. A simpler approach is to use a direct sequence spread spectrum (DSSS) that generates a pseudo-random frequency pattern but can be less effective in reducing EMI.

Variable frequency drives are different

Motor drive circuits have many of the same EMI issues as other power converters, but the high output voltage can magnify them. This is especially true for the cables that deliver the high-frequency power from the variable frequency drive (VFD) to the motor.

Figure 3. VFD cables are complex assemblies that must be properly specified and installed to minimize EMI. (Image: KEB Automation KG)

Some important considerations when specifying and integrating VFD cables include:

Shielding is the primary consideration. Several options include a braided copper mesh or a foil layer, copper tape, and armor that can be made of aluminum or steel wire. These shielding options provide varying EMI performance levels and can be combined. The use of armor adds protection from physical damage.
Grounding is necessary for maximum shielding effectiveness. Both shield ends are usually grounded to provide a low-impedance path for noise currents.
Conductor sizes and insulation materials should be optimized to minimize signal reflections and further reduce EMI generation.

Summary

Switching power converter designers have an array of tools for controlling EMI. These range from basics like filtering and PCB optimization to more sophisticated techniques like slew rate control and spread spectrum modulation. Designers of industrial VFDs must also optimize the specification and integration of cabling between the drive and motor.