EMC Design Guidelines for Switched-Mode Power Supplies

September 21, 2021 Cadence System Analysis

Key Takeaways

  • High power density, discontinuous input currents, high slew rate currents and voltages in switching devices and diodes, and additional ringing caused by parasitic impedance are some of the reasons for EMI in switched-mode power supplies.  

  • EMC standards set by FCC, CISPR, etc. help guide the compatibility of the switched-mode power supplies used in consumer, automotive, and industrial electronics.

  • High-frequency emissions can be mitigated by reducing the slew rates of the switching edges and by using effective gate driver designs for switch-mode converters.

SMPS

 Switched-mode power supplies are some of the most common circuits in modern electronics

Switched-mode power supplies are some of the most common circuits in modern electronics. However, in switched-mode power supplies, the switching of FETs creates electromagnetic interference (EMI), which is so detrimental that a great amount of consideration must be given to EMI reduction techniques. One thing that can help mitigate EMI is adhering to electromagnetic compatibility (EMC) design guidelines during the initial design phase. By following these guidelines, designers can ensure their power supply can handle EMI within the acceptable limits. 

EMI in Switched-Mode Power Supplies

Switched-mode power supplies utilize switching devices to supply regulated DC. While this type of power supply offers more advantages than linear regulated power supplies, it often faces EMI issues. Here are a few reasons why EMI often shows up in switched-mode power supplies:

  • High power density
  • Discontinuous input currents
  • High slew rate currents and voltages in switching devices and diodes
  • Additional ringing caused by parasitic impedance 

The EMI emitted from switched-mode power supplies can be low-frequency, mid-frequency, or high-frequency emissions. These emissions can form one of two types of EMI:

  1. Conducted EMI - Couples to the power supply output through parasitic impedances, power connections, and ground connections.
  2. Radiated EMI - Happens over radiations. 

EMC in Switched-Mode Power Supplies

The EMC of a switched-mode power supply defines the ability of the power supply to perform its predefined function in the presence of internally and externally-generated EMI. There are EMC standards set by FCC, CISPR, etc. to guide the compatibility of the switched-mode power supplies used in consumer, automotive, and industrial electronics. The EMC can be improved with the help of filters, gate drivers, and certain circuit board layout modifications.

Here are some EMC design guidelines to help mitigate EMI in switched-mode power supplies.

EMC Design Guidelines 

Most switched-mode power supplies use high frequency switching to reduce the size of a transformer. High-frequency digital signals and switchings evoke most radiated EMI problems in a switched-mode power supply. Here are some layout modifications that can be put in place on the digital circuit side of the switched-mode power supply to reduce EMI.

Isolate the Power and Control Circuit 

In switched-mode power supplies, gate drivers switch FETs. The control signals are digital signals, which need to be guarded against the power circuit signals. It is important to keep separate ground planes for power and control circuits. 

Control the Rise and Fall Time of Digital Pulses 

As the high slew rate of currents and voltages boosts EMI emissions, it is beneficial to add a series resistor to digital signal traces to control the rise and fall time. Adding capacitors in parallel to the signal trace can also help in limiting the sudden rise and fall of currents and voltages. There are slew-rate controllable driver ICs available, which can increase the EMC of switched-mode power supplies. 

Use Filters and Gate Driver Designs

Low-frequency emissions, around the 30 MHz frequency range, can be minimized using passive EMI filters at the input of switch-mode converters. High-frequency emissions can be mitigated by reducing the slew rates of the switching edges and by using effective gate driver designs for switch-mode converters.

There are several other EMC design guidelines to decrease EMI problems in switched-mode power supplies. Filters, shielding, decoupling capacitors, and snubbers can all help reduce EMI as well. Cadence's software offers PCB design tools that can identify the degree of EMI susceptibility of a switched-mode power supply.  

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