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Simple Ideas to Make EMI Issues a Thing of the Past

David BournerSenior Applications EngineerDecember 2016

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Orientation

› The system you are designing is complex – the many different parts have to work as one

– Power connectors, cables

– Modules

– Circuit boards

– Components: Analog , digital, electromechanical

– Backplane, chassis, enclosure

› Design time, components and resources are always limited

› Control of CE - conducted emissions - easy to overlook

› Maximum permitted energy levels extremely small

› RE - radiated emissions - are measured across many more decades of frequency compared with CE – RE suppression applies to the complete system application

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Aspects of EMI Control in Switched Mode Power Systems

› Elements of a SMPS

› Causes of “noise” in power trees

› Noise energy management principles

› Some noise control practices at work

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Let’s look at a SMPS (Switched Mode Power System)

Output filtering,

Hold-up cap bank

External OCL

Load switch

Load

Surge and transient protection

HoldupCap bank

ConductedEmissions Filtering

DC-DC Converter

Output / remote sensing

Redundant switching circuitry

Input sensing OVP/ OCP circuits

Source(s) and Redundant switching circuits

CM and DM noise current control area Bold arrows show DC

power flownoise energy flowsare different

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DCM - DC Module: From Module to Application

DCM (DC Module)Evaluation board

New Vicor Packages

VIA: Vicor AdaptorChiP: Converter housed in Package

Double-clamped ZVS power cell

+OUT

-OUT

Proprietary

Buck-boost control

+IN

-IN

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Voltage Across and Current In DCM Primary Winding

› ZVS eliminates the discontinuitiesseen in hard-switched converters

› Resulting frequency spectra show this

No need for external clampsor snubbers – but other partswill need to be added, externalto converters

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EMI Noise Spectrum: Classifying Noise in a SMPS

Control loop bandwidth not visible

Switching fundamental term + harmonics

RE 30 MHz – 1GHz

UNFILTERED EMI PEAK SCANS - CISPR 22 - 270 VDC, 10% of Full Load, Red LeadMODEL #MDCM270P280M500A40

CE 150 kHz – 30 MHz

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EMI Coupling Mechanisms

› Inductive coupling 𝑽𝒊𝒏𝒅 = −𝑳.𝒅𝒊

𝒅𝒕

– Voltages induced with inductive coupling increase with frequency, permeability and proximity

› Capacitive coupling 𝑰𝒄𝒂𝒑 = 𝑪.𝒅𝒗

𝒅𝒕

– Currents induced in adjacent conductorsincrease with frequency, permittivity and proximity

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Keys to Control of EMI

› Switching causes noise - couples in two modes:

– Common mode and differential mode

› Confine noise currents within smallest possible loop areasUse:

– Y caps for common mode control

› X caps for differential mode control

› These are selected and connected in such a way as to preserve safety in accordance with various classifications e.g. Y1, Y2

Note the central area of the block diagram (slide #4)

Apply noise control at the converter itself

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Simplified Model of the Converter Input Power Port

› Common-mode ACvoltages Vcm1, Vcm2and a differential mode AC voltage source Vdm

› Cs1 and Cs2: parasitics

› Small loop area currentpathways requiredfor both the common anddifferential mode currents

Cx

Cy1

Cy2

shield plane

Cs1

Vcm1

~ Vdm

~

Vcm2

~

Front-end ofDC-DC converter

Cs2

Suppression of pervasive noise must be effective

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EMI Control Concepts

› There is a critical order to EMI control component placement. As much of the CE noise is to be conducted back to its origin. Block and bypass noise from the converter input port toward the inputpower source

– X and Y capacitors are HF energy suppressors – place them close to converter

– Surge and transient protection may affect X and Y cap action

› The negative impedance of the converter - if unchecked - will produce input power bus instability

› Input filter damping helps dissipate EMI noise energy and assures stable operation

On the input side of the system we note that:

The input holdup caps provide bus stability due to ESR

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Noise current pathways change with frequency

AC source

VIAs going throughsubstrate, to solidground plane

trace on topside of board

lower end of each via makes good electricalcontact to the ground plane, the brown solid layer

At LF --- circuit completed through straight segment of ground plane between the

via contacts there, through minimum resistance path

At HF --- circuit completed in image traces in the ground plane, through

minimum inductance path

resistive load

~

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Glimpse of CE for a standalone PRM/VTM power tree

PRM switches at ~1.3 MHz

VTM switches at ~ 1.6 – 1.9 MHz Common mode input noise spectrum at PRM black power terminal

Note that the oscilloscope traces are measured in limited (20 MHz) bandwidth

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28V PWR SOURCE

+

_

M-FIAM7B

+

_

EMI GND

FIAM BASEPLATE

+

_

+

_

PRM

L2

R

VTM

+

_

+12 VDC

DC_RTN

MP028F036M12AL / MV036F120T100 TEST FIXTURE CE CONTROL ARRANGEMENT

28V RTN

lL1a

lL1b

28V lL3

SHIELD PLANE (COPPER BENCH TOP)

CY1(a,b) CY2(a,b)CY3(a,b)

CX1CY4(a,b)

CX2CX3

Notes ---------------------------------------------------------------------------------------------------

Y and bypass caps

CY1(a,b), CY2(a,b), CY4(a,b): 4.7 nF HV safety caps VishayVY1472M63Y5UQ63V0 or equivalent

CY3(a,b): 4.7 nF 250v a.c. rated part Vicor part number #01000

X-caps

CX1: 1000uF 63V rated ALEL paralleled with two 2.2 uF 50V rated ceramic capsCX2: two paralleled 10uF 25V rated ceramic caps, parallel 4.7nF HV cap addedCX3: four paralleled 10 uF 25V rated ceramic caps

-----------------------------------------------------------------------------------------------------------------

Inductors (all based on the Coilcraft SLC7530D-101ML power inductor)

L1a, L1b one winding each for common-mode choke implementationL2, L3 series connection of each winding in the part

Detuning resistor

R 1206 sized 10 W resistor for detuning

An example of a line-up for controlling CE

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Simple lab-based CE check outcomeTest setup – prototype built up with FIAM, PRM (Pre-regulator) and VTM (isolated Voltage Transformation Module)

Zero input test FFT CM FFT spectrum PRM black power terminal

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Implementing RE Control

Detecting noisesources inthe system in the nearH field

Cable harness design and placement;Use twisted wire,ribbons or coaxial cable.Apply CM filtering to harnesses to minimize RE.

Grounding of heatsinkturns it into a shield

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Checklist for Addressing EMI Concerns

› Design for EMI from the beginning; know what performance you require

› Select components, circuits with EMI in mind

› Plan your PCB layout

– Board stack-up: ground planes to be located close to matched power and signal trace layers

– Assign placements for filters, SMPS modules, analog/digital circuits and returns

› Determine cable harness design – select connectors – consider their placement

› Plan Grounding Strategy at component, circuit, module, circuit card and system level

› Filters: verify ratings, use correct components, keep input & output routes separate

› Shielding: select materials appropriate to noise spectrum, target usage: look for gaps, openings and deploy conductive gaskets (critical gaps < λ/20 guide)

› Use the schematic as tool to document EMI control design

– Apply a flow check to ensure assignment of appropriate in-circuit control measures

– Ensure that the input filtering design is conducive to input power bus stability

› Carry out EMI pre-assessments continuously as the design progresses

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Acknowledgments - Question Session

Special thanks to the very patient Vicor folks who helped me out with this projectScott LeeHannes SchachenmayrBob PauplisChris SwartzArthur RussellHarry VigJoe AguilarAnkur PatelPeter MakrumMike DeGaetanoVamshi Domudala

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IEEE PDH Information

› Code: 1206SOL

› Link for form: http://fs25.formsite.com/ieeevcep/form36/index.html