Reducing Conducted EMI in a Buck Converter for 48 V Automotive Applications

Abstract

This application note discusses some of the challenges of EMI (electromagnetic interference) in buck converters particularly in automotive systems where the power rail is increasing to 48V DC. The higher rates of change in voltage (dv/dt) and current (di/dt) in the buck converter make it more difficult to meet conducted EMI compliance. This application note discusses the composition and generation of conducted EMI noise in buck converters, using an emission model. To address these EMI related concerns, several methods and techniques were implemented to help designers mitigate conducted EMI issues in a buck regulator. Layout tips, inductor selection, ferrite bead optimization and spread spectrum are discussed in this article.

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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
Band	Frequency (MHz)	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.