SNVAA93 December   2023 LM65645-Q1 , LM70660 , LM706A0 , LM706A0-Q1 , LM70840 , LM70840-Q1 , LM70860 , LM70860-Q1 , LM70880 , LM70880-Q1 , LMR38020-Q1 , LMR38025-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Conducted Emission Model of Buck Converters
    1. 2.1 DM Noise Emission Model
    2. 2.2 CM Noise Emission Model
  6. 3Reducing Conducted EMI of Buck Converter in 48V DC Power rail
    1. 3.1 Bead Consideration
    2. 3.2 Layout Consideration
    3. 3.3 Power Inductor Consideration
    4. 3.4 Spread Spectrum
  7. 4Summary
  8. 5References

1 Introduction

With the increasing use of 48V DCDC power rails in automotive systems, the higher dv/dt and di/dt of buck converters can make it more challenging for a system to meet EMI specifications. Table 1-1 shows the peak and average limits of conducted EMI for CISPR 25, which covers a specific frequency range from 150 kHz to 108 MHz.

Table 1-1 Conducted Emissions Peak and Average Limits in CISPR 25
Levels in dB (μV)
Band Frequency (MHz) Class 1 Class 2 Class 3 Class 4 Class 5
Peak Average Peak Average Peak Average Peak Average Peak Average
Broadcast
LW 0.15 - 0.3 110 90 100 80 90 70 80 60 70 50
MW 0.53 - 1.8 86 66 78 58 70 50 62 42 54 34
SW 5.9 - 6.2 77 57 71 51 65 45 59 39 53 33
FM 76 - 108 62 42 56 36 50 30 44 24 38 18
TV Band I 41 - 88 58 48 52 42 46 36 40 30 34 24
Mobile Services
CB 26 - 28 68 48 62 42 56 36 50 30 44 24
VHF 30 - 54 68 48 62 42 56 36 50 30 44 24
VHF 68 - 87 62 42 56 36 50 30 44 24 38 18

Typically, the EMI noise in the frequency range of 150 kHz to 30 MHz is primarily comprised of the switching frequency and its harmonics. This type of noise can be reduced by using EMI filters. However, for the frequency range of 30 MHz to 108 MHz, which includes AM and FM radio, the EMI noise is mainly caused by near-field radiation. EMI filters are less effective in this frequency band due to the influence of parasitic capacitance and inductance. Therefore, more design considerations are necessary to meet the EMI limits, particularly in the FM band. This application focuses on conducted EMI and provides tips and tricks that were implemented on the LMR38020-Q1 EVM.