Advanced EMI mitigation techniques for automotive convertersAdvanced EMI mitigation techniques for...

Texas Instruments 1 ADJ 1Q 2020

PowerAnalog Design Journal

Advanced EMI mitigation techniques for automotive converters

IntroductionWith the emergence of new trends in automotive electronics such as autonomous driving, car infotainment systems and hybrid to all-electric cars, electrical engineers are facing an onslaught of new challenges, particularly in designing automotive front-end power systems. Such systems directly impact the reliability of overall system design.

The demand for higher processing power, more communication, longer wire harnesses and miniaturized electronic system sizes make electromagnetic interference (EMI) caused by switch-mode power supplies one of the most formidable challenges in an automotive envi-ronment. This article provides an overview of the EMI problems caused by automotive switch-mode power supplies and the various techniques employed in TI’s converter products to help mitigate the issue.

Automotive DC/DC convertersIn a typical automotive power train, a wide-input front end DC/DC converter such as the TI LM53635 converts the battery voltage to a regulated voltage. The power requirement of such DC/DC converters can range from sub-1 W to a few kilowatts depending on the applica-tion. There are many unique features of auto-motive DC/DC converters: a wide input voltage range (3 V to 40 V) to protect the down stream device against voltage transients (for systems with a 12-V battery at the input), low quies-cent current requirements (sub-10 µA) for always-on systems, deep dropout conditions down to a 3-V input during cold-crank condi-tions, higher operating junction temperatures (minimum of 150°C), and the need to meet stringent EMI standards.

Causes of EMI in DC/DC convertersEMI is one of the most undesired effects of any DC/DC converter. The main sources of noise in a switch-mode power supply involve discontinuous input currents due to the switching of power field-effect transistors (FETs), the fast rising and falling rate of the switch node, and additional ringing along the

switching edges due to parasitic inductances in the power loop. Figure 1 shows the emissions generated due to discontinuity in the input currents of the switching converter. This creates spurs in the switching frequency and its harmonics that can couple to other equipment connected to the input line.

By Yogesh Ramadass, Director – Power Mgmt. R&D, Kilby Labs, Santa Clara Ambreesh Tripathi, Application Engineering Manager, CCP – BSR, Santa Clara

Figure 1. Emissions generated by the discontinuous input currents of switching converters

0 0.5 1 1.5 2 2.5 3 3.5 4

Inp

ut

Cu

rren

t (A

)

3.5

3

2.5

2

1.5

1

0.5

0

Time (µs)

0.15 1 10 30

Am

plitu

de (

dB

µV

)

80

70

60

50

40

30

20

10

0

–10

–20

Frequency (MHz)

AveragePeak

(a) Input current

(b) Switching noise

http://www.ti.com/adj



Figures 2 and 3 illustrate the emissions generated due to the high slew rates and the additional ringing on the switching node of the converter. Fast switch-ing edges lead to sustained harmonics even up to frequencies in the range of 100 MHz, which can then couple to other sensitive parts of the system. Switch-node ringing, meanwhile, can have energy content in the hundreds of megahertz switching frequencies, causing issues in systems employing radio-frequency designs.

Figure 4 summarizes the different frequency bands that are important while meeting the stan-dards set forth for automotive EMI compliance. Various circuit design, package and board layout techniques are typically employed for EMI mitigation in each of these bands. The next few sections will go into the details of some of these techniques and the improvement obtained.

Figure 2. Emissions generated by fast rise and fall times of the switch node

30 7050 9040 8060 100

Am

pli

tud

e (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (MHz)

Average Peak

Figure 3. Emissions generated by high-frequency ringing on the switch node

Sw

itc

h N

od

e (

5 V

/div

)

Frequency (50 ns/div) 30 100 1000

Am

pli

tud

e (

dB

m)

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

Frequency (MHz)

Average

Peak

(b) Emissions from ringing(a) Switch-node ringing

Figure 4. EMI bands of interest and mitigation techniques

ConductedCISPR 25 Class 5

Low Frequency

ConductedCISPR 25 Class 5

High Frequency

RadiatedCISPR 25 Class 5 - Rod

RadiatedCISPR 25 Class 5 - Bicon

RadiatedCISPR 25

Class 5 - Log

ConductedVery-Low Frequency

Passive Filter Slew Rate Control

Flip Chip on Leadframe Package

Pinout Optimization

Design Feature

Packaging Feature

External Addition

1kHz

100kHz

150kHz

20MHz

30MHz

108MHz

200MHz

1GHz

Spread Spectrum




Passive filter design and limitationsAs shown in Figure 1, input voltage ripple generated by discontinuous currents can conduct to other systems via physical contact of the conductors. Without control, excessive input and/or output voltage ripple can compromise operation of the source, load, or adjacent system. Traditionally, the input ripple is minimized by using a passive inductor/ capacitor (LC)-based EMI filter. The LC filter offers the required attenuation needed to meet EMI specifications. An input filter designer that is integrated into TI’s WEBENCH® online design tool not only calculates the LC filter values based on EMI compliance require-ments, but also ensures stable operation of the DC/DC converter by thorough impedance analysis to make sure that the output imped-ance of the input filter is much lower than the input resistance. However, there are limita-tions in using the passive input filters to mini-mize conducted EMI. Beyond the size and cost constraints imposed on the system, larger-sized inductors for input EMI filter design lose attenuation at frequencies greater than 30 MHz due to their lower self-resonance frequency.

Figure 5 shows the high-frequency (30 to 108 MHz) EMI plots of the LMR33630-Q1, a 2.1-MHz 5 V, 3-A output DC/DC converter at a 13.5-V input. The plots are taken on the same board, with the only change being that of the filter inductor. As shown in Figure 5b, the printed circuit board (PCB) having the induc-tor with higher self-resonance frequency offers much better EMI attenuation at high frequencies. Further, the passive filter does not provide any attenuation for EMI coupling paths that are not through conduction on the input line. To overcome these limitations and reduce the size and cost of the input passive filter, other techniques are becoming common.

Figure 5. Dependence of EMI performance on an input filter inductor’s self-resonant frequency

40 60 80 100

Am

plitu

de (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (MHz)

Average Peak

Inductor: 2.2 µH, XAL4030-472SRF: 52 MHz

30 60 80 100

Am

plitu

de (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (MHz)

Inductor: 600 nH, XAL4030-601SRF: 106 MHz

(a) Inductor with lower SRF

(b) Inductor with higher SRF




Spread spectrumOne common technique to deal with increased spectral emissions from a switching converter is to use spread spectrum. Spread spectrum is a way to reduce EMI inter-ference by dithering the switching converter frequency, which eliminates peak emissions at specific frequencies by spreading emissions across a wider range of frequencies. This has the effect of spreading the noise spectrum and reducing the fundamental energy, as shown in Figure 6. Equation 1 defines modulation index (Mf), which shows the correlation between carrier frequency (fC) modulation frequency (fM), and the modulation ratio (δ).

M

f

f

f

ffC

M

C

M=

×=

δδ, where

∆

(1)

It is possible to improve the spreading of noise (and thereby the attenuation) by increasing Mf, which is higher for a higher dithering bandwidth (∆fC) and a lower fM. Recent automotive switching converters switch at ~2.1 MHz to be above the AM frequency band. For these switching converters, ensuring that the low side of dithered frequency is higher than the AM band (1.8 MHz) will limit ∆fC. An even higher attenua-tion factor can be achieved by lowering fM. But if fM is lower than the resolution bandwidth of the spectrum analyzer test setting—per the automotive EMI stan-dard—attenuation will be less than the theoretical calculation. Thus, keep fM close to 9 kHz for optimal performance.

There are two broad categories for spread-spectrum (SS) algorithms. In the first category, the dithering happens in a pseudo-random fashion, where changes in the switching frequency are randomized within the dither bandwidth. In the second category, the switching frequency is dithered in a triangular fashion, where the switching frequency increases and decreases in steps between the lower and higher ends of the dither band-width. Figure 7 shows the frequency-vs.-time characteris-tics, along with the spur attenuation possible, for the pseudo-random and triangular modulation schemes.

Figure 7. EMI plots of a 2-MHz switcher on the same PCB

Time

Fre

qu

en

cy

(a) Pseudo-random modulation

Time

Fre

qu

en

cy

(b) Triangular modulation

Ambient

SS Disabled

Pseudo-Random

Triangular Linear

Triangular Random

45

40

35

30

25

20

15

10

5

00.1 1 10

Frequency (MHz)

Em

issio

n L

evel (d

Bµ

V/m

)

Input: 13.5 VOutput: 5 V at 1 A

(c) Emissions comparison

Figure 6. Spread bands of harmonics in modulated square signals

Harmonics

PossibleOverlap

dB

f

Spread Bandsof Modulated

Signal




Triangular modulation offers the benefit of improved spur attenuation at the expense of introducing large spurs at the modulation frequency. This additional spectral noise is further exacerbated when optimizing for Comité International Spécial des Perturbations Radioélectriques (CISPR) and Federal Communications Commission (FCC) standards, which result in the modulation spurs being placed in the audio band around 9 kHz. This effect is evident in Figure 8c, where a large spike is observed in the audible band when using triangular modulation.

To overcome this problem, in addition to the triangular modulation of the switching frequency, an advanced frequency modula-tion (AFM) scheme dithers the modulation frequency itself in a pseudo-random fashion. This leads to the frequency-vs.-time curve shown in Figure 8d. This solution combines the benefits of both triangular and pseudo-random modulation techniques of increased spur attenuation at the fundamental and harmonics while not being hamstrung by the large spike in audible frequencies. The AFM technique implemented in devices like the TPS55165 maintains high EMI attenuation across the 150- to 108-MHz band of interest while attenuating the modulation tone close to 9-kHz maximally. The spectral comparison in Figures 8c and 8d shows the benefits in the audible range.

Figure 8. EMI plots of a 2-MHz switcher on the same PCB

Time

Fre

qu

en

cy

Time

Fre

qu

en

cy

(b) AFM modulation(a) Triangular modulation

(c) Emissions without AFM

1 10 100

Am

plitu

de (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (kHz)

Large Spikein Audible

Band


(d) Emissions with AFM

1 10 100

Am

plitu

de (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (kHz)





Switch-node ringingIn addition to the discontinuous input current that causes conducted EMI at the input line, the high dv/dt loop in the DC/DC converter at the switch node generates EMI, and needs attenuating to meet the compliance limit. As the switching frequency of switch-mode power supplies increases, there is a general trend to shorten the rise and fall times of the switch node to reduce the switching losses. However, a switch node with very short rise and fall times maintains high energy content, even at high frequencies close to its 100th harmonic, as shown in Figure 9. The switch-node waveform of converters that offer short rise and fall times will cause much higher emissions. When choosing such a device, look for additional features in the device that can help mitigate EMI effects.

Switch-node ringing is caused by the parasitic induc-tance in the power loop resonating with the overall capaci-tance present in the switch node of the DC/DC converter and may range close to 100 MHz. At such higher frequen-cies, the differential filter at the input does not provide any attenuation; thus, different techniques are required to avoid the emissions. One primary technique is to inherently minimize the power-loop inductance. Newer products from TI like the LM53625-Q1 and LM53635-Q1 move away from wire-bond packages to leadframe-based packages (Figure 10), which help lower the power-loop inductance and in turn reduce the switch-node ringing.

Improving this further is the layout arrangement of the input capacitors of the DC/DC converter. By optimizing the pinout of the DC/DC converter so that there is symme-try in the layout of the input capacitors, the electromag-netic fields generated by the input power loops are contained within the symmetric loops, thereby minimizing the emissions to systems nearby.

Figure 9. EMI plots of square waveforms with different rise and fall times

Time (1 µs/div) Time (500 ns/div)

1 MHz Fundamental (1 V/div)

38 dB down at 33 MHz Down 40 dB at 375 MHz

1 MHz Fundamental (1.24 V/div)

(a) 20-ns rise and fall time—1-MHz square wave (FFT) (a) 1-ns rise and fall time—1-MHz square wave (FFT)

Figure 10. New package option helps lower power-loop inductance

Wirebond

Junction

Lead Frame

Board

HEATHEATDAP / Thermal Pad

Junction

Leadframe

SolderCopper Bump

Board

HEAT HEATNo DAP

(a) Standard wire bond QFN

(b) Flip chip on lead frame QFN




Figure 11 shows the improvement in the emissions obtained as a result of symmetric pinout. Figure 11a has only one input-voltage capacitor placed to the left of the converter, while Figure 11b is with two capacitors to provide the symmetric input path. A combination of improved packaging, along with symmetric pinout and board layout of the input capacitors, enables the power converter to meet the spectral standards even in the 30- to 108-MHz band.

Despite these techniques, there may be designs where the high-frequency (60- to 250-MHz) EMI may still not be within specified standards. One common way to mitigate and improve margin in the high-frequency range is to use a resistor in series with the boot capacitor of the switching converter. This helps slow down the switching edges leading to lower EMI, but comes with a penalty of lower efficiency and potential undervoltage lockout issues with the boot voltage.

Switching converters like the LM61440-Q1/LM62440-Q1 are designed to allow a resistor to select the strength of the high side FET’s driver during turn on. As shown in Figure 12, the current drawn through the RBOOT pin, the dotted blue arrows, is magnified and drawn through from CBOOT, the dashed red arrows. This current is then used to turn on the high-side power MOSFET without putting a resistor in the series path. With RBOOT short circuited to CBOOT, the rise time is rapid; switch-node harmonics will not roll off until above 150 MHz. If CBOOT and RBOOT are connected through 700 Ω, the slewing time increases to 10 ns typical when converting 13.5 V to 5 V. This slow rise time allows the energy in switch-node harmonics to roll off near 50 MHz under most conditions.

ConclusionThe rapid growth of electronics within automotive systems has put a tremendous strain on the design of the power converters used to supply end equipment within a car. The close proximity of sensitive systems makes design for electromagnetic compatibility a stringent challenge. Designers must take extreme care when designing power converters for automotive systems in order to comply with and meet the standards set forth by agencies to ensure that critical systems can safely operate in a noisy environment. Employing a combination of techniques with TI’s DC/DC converters ensures that your designs using TI components will pass the system standards without much rework.

Related Web sitesWEBENCH® online design tool

Product information:LM53625-Q1, LM53635-Q1, LM61440-Q1,LM62440-Q1, LMR33630-Q1, TPS55165-Q1

Figure 11. EMI plots of the LM53635-Q1 2-MHz switcher on the same PCB

40 60 80 100

Am

pli

tud

e (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (MHz)

Input: 13.5 VOutput: 5 V at 3.5 A

40 60 80 100

Am

pli

tud

e (

dB

µV

)

50

40

30

20

10

0

–10

–20

Frequency (MHz)

ReducedEmissions

Input: 13.5 VOutput: 5 V at 3.5 A

(a) One bypass capacitor CIN HF1

(b) Symmetric pinout with two bypass capacitors in parallel

Figure 12. True slew-rate control implementation in the LM62440-Q1

CBOOT

SW

RBOOT

VCC

VIN

HS FET

LS FET

HSDriver

LM62440-Q1

http://www.ti.com/adjhttp://www.ti.com/webenchhttp://www.ti.com/product/LM53625-Q1http://www.ti.com/product/LM53635-Q1http://www.ti.com/product/LM61440-Q1http://www.ti.com/product/LM62440-Q1http://www.ti.com/product/LMR33630-Q1http://www.ti.com/product/TPS55165-Q1


Analog Design Journal

E2E is a trademark and WEBENCH is a registered trademark of Texas Instruments. All other trademarks are the property of their respective owners.

TI Worldwide Technical Support

TI SupportThank you for your business. Find the answer to your support need or get in touch with our support center at

www.ti.com/support

China: http://www.ti.com.cn/guidedsupport/cn/docs/supporthome.tsp

Japan: http://www.tij.co.jp/guidedsupport/jp/docs/supporthome.tsp

Technical support forumsSearch through millions of technical questions and answers at TI’s E2E™ Community (engineer-to-engineer) at

e2e.ti.com

China: http://www.deyisupport.com/

Japan: http://e2e.ti.com/group/jp/

TI TrainingFrom technology fundamentals to advanced implementation, we offer on-demand and live training to help bring your next-generation designs to life. Get started now at

training.ti.com

China: http://www.ti.com.cn/general/cn/docs/gencontent.tsp?contentId=71968

Japan: https://training.ti.com/jp

A011617

Important Notice: The products and services of Texas Instruments Incorporated and its subsidiaries described herein are sold subject to TI’s standard terms and conditions of sale. Customers are advised to obtain the most current and complete information about TI products and services before placing orders. TI assumes no liability for applications assistance, customer’s applications or product designs, software performance, or infringement of patents. The publication of information regarding any other company’s products or services does not constitute TI’s approval, warranty or endorsement thereof.

© 2020 Texas Instruments Incorporated. All rights reserved. SLYT789

http://www.ti.com/adjhttp://www.ti.com/supporthttp://www.ti.com.cn/guidedsupport/cn/docs/supporthome.tsphttp://www.tij.co.jp/guidedsupport/jp/docs/supporthome.tsphttp://e2e.ti.comhttp://www.deyisupport.com/http://e2e.ti.com/group/jp/http://training.ti.comhttp://www.ti.com.cn/general/cn/docs/gencontent.tsp?contentId=71968https://training.ti.com/jp

IMPORTANT NOTICE AND DISCLAIMER

TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265Copyright © 2020, Texas Instruments Incorporated
http://www.ti.com/legal/termsofsale.htmlhttp://www.ti.com

Advanced EMI mitigation techniques for automotive convertersAdvanced EMI mitigation techniques for...

Documents

Transcript of Advanced EMI mitigation techniques for automotive convertersAdvanced EMI mitigation techniques for...