EMI Filter Design for an Isolated DC-DC

Boost Converter

Ishtiyaq Ahmed Makda (PhD Research Fellow)

Morten Nymand (Supervisor)

Maersk Mc-Kinney Moller Institute, University of Southern Denmark, Odense,

Denmark

Outline:

Introduction

DC-DC Converter in Electric Vehicles

System Definition

Common Mode EMI Filter Design and important

outcomes

Differential Mode EMI Filter Design

Conclusion

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Introduction:

One of the most critical issues for the environment today is pollution

generated by hydrocarbon combustion

However, today it is one of the main sources for transportation

But, hybrid electric vehicles (HEV) and full electric vehicles (EV) are

rapidly advancing as an alternative

One of the key blocks inside HEV and EV is the DC–DC converter

This converter has to be capable of handling the energy transfer

from the low voltage DC bus and the high voltage DC bus (used for

the electric traction)

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EV Drive System:

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Some design considerations are essential for automotive applications:

• Reliable

• Light weight

• Small volume

• High efficiency

• Low electromagnetic interference (EMI)

Ultra High Efficiency Converter:

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Primary Parallel Isolated Full Bridge Boost Converter

(Invented by Morten Nymand)

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Vital Features:

• Input voltage range= 30 to 60Vdc

• High Output Voltage > 360Vo

• Maximum efficiency is 98%

• Power ranges from 1.5 to 10kW

• Power loss reduced by factor 4 as

compared to the state-of-the-art

• Reduced converter size and

weight

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Electromagnetic Interference (EMI)

Conducted Emission Radiated Emission

Differential

Mode (DM)

Noise

Common

Mode (CM)

Noise

Common Mode (CM) & Differential Mode (DM) Noise:

Differential-Mode Noise

Flows on Line and take

return path from Neutral

Currents flowing around

loops

Easy to understand

Ref: http://www.hottconsultants.com/pdf_files/APEC-

2002.pdf

Common-Mode Noise Flows on Line and Neutral

simultaneously and take return

path from chassis

Involves parasitic

Currents flow around loops

usually involving parasitic

capacitances

More difficult to understand

The noise source and current

path must first be visualized and

understood before a solutions

can be determined

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Common Mode (CM) noise – Generation mechanism,

modelling and filter design

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Motivation:

• In high current applications, transformer needs extensive interleaving of primary and

secondary windings to reduce the proximity effect

• Such a transformer exhibits large capacitive coupling between primary and secondary

windings

• This large capacitive coupling is known to create large CM noise in the converter

• Therefore we are analyzing the impact of large coupling capacitance on CM noise

• However, surprisingly enough, this study shows that this inherently large

capacitive coupling has very little influence on the CM noise current propagation

in the converter

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Major CM Noise Source (Transformer Coupling Capacitance):

Transformer Coupling Capacitance (CSPS) as a

major CM noise source

Transformer Winding Arrangement

How Big is the Transformer Coupling Capacitance?

CSPS of one transformer is 3.7nF

Two transformers in parallel, so CSPS is 7.4nF

State 1: When all primary switches are ON and the energy is being stored in the inductor L1

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CM Noise Voltage Generation Mechanism:

Equivalent Circuit of Converter during State 1

• Both transformer primary windings are clamped to primary return

• Average primary winding voltages during this state is therefore zero

• All secondary rectifier diodes are reverse biased and OFF – energy is supplied to the load by C1 capacitor

• Potential of Secondary winding float – coupled to secondary return potential only through parasitic junction

capacitances of diodes and heat sink

• Both transformer secondary windings have the same potential

State 2: When only two diagonal primary switches in each bridge are ON

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CM Noise Voltage Generation Mechanism (Cont.):

Equivalent Circuit of Converter during State 2

• Two series connected transformer secondary windings are connected to output voltage Vo

• Average voltage of the two series connected secondary windings (at node ‘A’ w.r.t secondary return) is half of the

output voltage, i.e. Vo/2

• Due to turns ratio of two transformers, the average potential on each of the two primary windings will be at half of

the reflected output voltage w.r.t. primary return, i.e. Vo/4n

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CM Noise Model (Cont.):

Cpar,PS is the transformer coupling capacitance (in nF) which is in series with the

parasitic diode capacitances (in off-state)

NEWS: Total effective capacitance is a few hundred of pF and hence CSPS

has a very little influence on the CM noise propagation in the converter

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CM Noise Suppression:

Two noise suppression measures has been taken:

CM noise model with 1st noise suppression measure CM noise model with two noise suppression measure

Ceff = CSPS−1 + CSD

−1 −1= 358 pF (4)

Validity of the CM Noise Model:

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Experimental Setup:

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Power Converter (DUT)

Current Probe

LISN

EMI Test Receiver

Digital Function Generator

Digital Oscilloscope

Input power source

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Experimental Results (Cont.):

(a) (b)

(c)

(a) CM noise without capacitors Y1, Y2 and LCM

(b) CM noise with capacitors Y1 and Y2 only

(c) CM noise with Y1, Y2 and LCM

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Conclusion for CM Noise Filter:

• A CM noise model for an isolated full-bridge boost converter has been presented

• Since the inherently large transformer coupling capacitance (in nF) is in series

with output rectifier diode parasitic capacitances (in pF), the effect of the large

transformer coupling capacitance is effectively eliminated

• Large transformer turns ratio further reduces the magnitude of the injected CM

noise voltage in low input voltage high power isolated boost converters

• Therefore, despite of a much larger transformer coupling capacitance, low-

voltage high-current converters have lower CM noise and thus requires less

common mode attenuation

Differential Mode (DM) noise filter design

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Filter Design Process

1. Measurement of inductor ripple current and calculation of amplitude odd

harmonics

2. Calculation of attenuation requirement at worst case frequency

3. Cutoff frequency calculation and determination of filter order

4. Sizing of filter capacitor

5. Inductor calculation

6. Filter damping branch

7. Selection of components

8. Simulation and experimental results

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Filter Order and Cut-off frequency

Since 75dB of attenuation is required, 4th order LC filter

is employed

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DM EMI Filter Hardware

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Experimental Results

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Thank you for your attention

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