Design of Compact Active EMI Filters to Reduce the CM...

2018. 08. 03

Jingook KimUlsan National Institute of Science Technology (UNIST)

http://icemclab.unist.ac.kr

Design of Compact Active EMI Filters to

Reduce the CM Conducted Emissions

1

FR-PM-5 - EMI/EMC for Power Electronics Systems

http://icemclab.unist.ac.kr/

Outline

• Introduction

• Feed-Forward Voltage-sense Voltage-compensation

(FF-VSVC) AEF

• Voltage-sense Current-compensation (VSCC) AEF

• Other types AEFs

• Conclusion

2

A Passive EMI Filter for both CM and DM

● A typical passive EMI filter consists of a X-cap, CM choke, and Y-cap.● The leakage inductance of CM choke can be used as the DM inductance.● Each filter generates a large impedance mismatching.

X-cap Y-capCM choke

LISN EUT

3[ref] H. W. Ott, Electromagnetic Compatibility Engineering, 2009, Wiley

CM

DM

LISN EUT

LISN EUT

Low Z High Z

High Z Low Z

A good and large CM choke is bulky and costly.

Y-capacitor is limited by the safety regulation on leakage currents.

Limitation in a Passive CM EMI Filter

Attenuation of DM noise by using X-cap is relatively easier.

● For sufficient CM noise reduction, the passive EMI filter with a large CM choke, a large Y-capacitor, or multi-stage filters are necessary.

● Active EMI filters (AEFs) employing active circuit components are also proposed toreduce the low-frequency CM noise in a compact-size and low-cost.

4

Topologies of Active EMI Filter (AEF)

Voltage-sense Current-compensation

Current-sense Voltage-compensation

Voltage-sense Voltage-compensation

Current-sense Current-compensation

Voltage-sense Voltage-compensation Current-sense Current-compensation

[ref] Y. Son, S. Sul, "Generalization of active filters for EMI reduction and harmonics compensation," IEEE Trans. Ind. Appl., vol.42, no.2, pp.545-551, March./April 2006

Feed

-bac

kFe

ed-f

orw

ard

5

6




Voltage-Compensation AEFs

● The voltage-compensation AEFs behave asa series impedance, such as a CM choke.

● It should be used with other passive filtercomponents.

7




● The voltage-compensation AEFs behave asa shunt impedance, such as a Y-cap.

● It should be used with other filtercomponents.

Current-Compensation AEFs

8

● Immunity against high voltage transient (e.g. surge)– Protection circuits are required, but it should not affect the performance

● Power supply generation for AEF– A DC voltage for AEF can be separately made, but it increases cost and size.– A proper DC voltage is usually available from a gate switching control board.

● Stability in the target applications– Because AEF should be used with other filter components, the stability

condition depends on the condition of filter and EUT.

Critical Issues in applying AEF to AC lines

(AEF Type 1) : FF-VSVC AEF

● Dongil Shin, et al., and Jingook Kim, "Analysis and Design Guide of Active EMI Filter in a Compact Package for Reduction of Common-Mode Conducted Emissions", IEEE Trans on EMC, vol. 57, no. 4, pp. 660-671, Aug. 2015.

9





Voltage-sense Voltage-compensation Current-sense Current-compensation

Feed

-bac

kFe

ed-f

orw

ard

Input

Impedance

Impedance

Boosting

Noise

Attenuation

Two Types of VSVC AEF

A-

sLZZ in

1

+=

A-1

1

The attenuation performance ishighest at the unity gain. 1:1 turn ratio transformer unity gain amplifier

Input

Impedance

Impedance

Boosting

Noise

Attenuation

( )( )sLZAZ in ++1=

A+1

1n

n

Z A sL Z

Z sL Z

1

n

n

sL ZZ

A

Z sL Z

The attenuation performance increaseswith the voltage gain (A). High turn ratio transformer required High Gain amplifier required

Feed back type Feed forward type

10

Structure of the Designed CM FF-VSVC AEF

• Common-mode Feed-forward VSVC• The AEF output for voltage compensation is coupled to both power lines

through a transformer, and isolated from the high power voltage.

• The AEF input for voltage sensing is connected to both power lines through capacitors and a resistor.

11

Feedback Loop Gain of the AEF

Stability(Gain margin)

Impedance boosting(Feedback input impedance)v

in

in

in

t

oc A

sCZZ

sCZ

V

VLoopGain

1||

1||

11

1

Disconnect

● Stability and noise attenuation can be analyzed from the feedback loop gain.

Loop Gain

o

oinj

opinj

v

sCRsL

AsMA

1*

LISNinjcmin ZZZZ

1

*

12

Noise Attenuation by Impedance Boosting

• The line impedance of the CM choke ( ) is amplified by .

• The closed-loop gain (Av) should be close to 1 for high impedance boosting.

• The noise attenuation performance of the AEF is achieved by impedance boosting.

LISNinjcmin ZZZZ 1

*

Impedance boostingwhen Av=1

v

in

in

v

in

in

v

in

in

disconininA

Z

sCZ

A

Z

sCZ

A

Z

sCZ

LoopGainZZ

1

||1

||

1

1||

1

1||

1

1 11

11

11

,

Impedance boostingwhen Av=1

1

11

11

AEFw/o,

AEFw/,

||1

||

||1

||1

ZZsC

ZZZZ

ZsC

ZA

Z

i

iNA

n

in

nsCLISNLcm

n

inv

in

CM

CM

in

- Input Impedance

- Noise Attenuation Factor

1inZvA1

1

o

oinj

opinj

v

sCRsL

AsMA

1*

13

Design of FF-VSVC AEF (1) - Cin and Csen

220VAC

injL

CML

CMR

CMC

CML

CMR

CMC

injLoR

injL

oC

2R

1RsenC senC

nZ nI

injM

injM

inC

inC

SMPS

CMM

injM 1Z

1 11

w/o Cin

1

1

|| 2 21 1

22

2

in inn n

v v

Lcm LISN nLcm LISN n

n

Z ZZ Z Z

A ANA

Z Z Z ZZ Z Z

Z Z

≈

1ZZ n

≈

in

LISNLcm

inv

in

n

in

nsCLISNLcm

n

inv

in

sCZZ

sCA

Z

ZZsC

ZZZZ

ZsC

ZA

Z

NA

in

1

1

1

||1

||

||1

||1

1

1

11

11

Cinw/

1

1( || )

n

in

Z ZsC

1

1 1( || )

n

in Y

Z ZsC sC

Ysen

sCsC

11

• A large impedance of CM noise source degrades the noise attenuation.• Cin is required to decrease the effective impedance of CM noise source.• Both Cin and Csen should be smaller than the Y-cap regulation standard.

14

Design of FF-VSVC AEF (2) – Compensation Part

• Aamp is designed to be 1/kinj, and the ratio of Linj and Ro is determined by the minimum operation frequency fo.

• Small Ro and Linj are desired for compact size, but they should be sufficiently large for the OP amp output current not to exceed the limit.

oinj

ampinj

o

oinj

ampinj

vRsL

AsM

sCRsL

AsMA

1

2 2 1( )

2 2 2

o inj inj amp o inj

v o

o inj o o inj o

f L k A f LA f

f L R f L R

inj

oo

L

Rf

2

220VAC

injL

CML

CMR

CMC

CML

CMR

CMC

injLoR

injL

oC

2R

1RsenC senC

nZ nI

injM

injM

inC

inC

SMPS

CMM

injM 1Z

injamp kA /1

Design of Co , Aamp, fo, Ro, Linj

OPamp

oinj

o IRfL

Vmax,

2

Minimum operation frequency

Considering the max current of the OP amp

15

Design of FF-VSVC AEF (3) – Sensing Part

• R1 should be much larger than 1/sCsen even at the minimum operation frequency fo.

• The amplifier gain Aamp should compensate the coupling coefficient.

220VAC

injL

CML

CMR

CMC

CML

CMR

CMC

injLoR

injL

oC

2R

1RsenC senC

nZ nI

injM

injM

inC

inC

SMPS

CMM

injM 1Z

injamp kA /1

sen

amp

sCR

RA

12

2

1

2

1

12 10

2o sen

Rf C

Design of Aamp, R1, R2

inj

ampkR

RA

1

1

2

16

Design of FF-VSVC AEF (4) – Stability Check

• If the feedback system is unstable, the AEF does not work properly or even damaged.

• After the initial design, the stability should be confirmed by checking the gain margin.

• The gain margin increases, as R1 decrease and Ro increases.

2121)(

RC

CC

sen

senp

][2

1

)(

1||2

))((

1||2

12180

180

180180

1

180180

1

dBRMj

sCLjR

CjR

CCjR

ZZZGM

inj

o

injo

p

pin

LISNLinjLcm

2

1

1804

1

)(

11

RCMLMLC seninjinjcmcmin

Fine tune of Ro and R1 according to Gain Margin

17

VNA Measurement vs. Model

01

221

2

VV

VS

02

121

2

/1

VI

VY

18

CE Measurements of the AEF installed on a SMPS

• The OP amp supply is supplied by the SMPS board using regulators.

• Two CM Chokes are replaced by the AEF, and the space is greatly reduced.

• The AEF has the noise attenuation about 10 ~ 12dB.

• The performance of the (AEF + 1 CM choke) is comparable to 3 CM chokes.

• The size is greatly reduced!19

Surge Inflow Path to VSVC AEF

The varistors and GDT arrestor cannot block surge under their operating voltage.

Mostly, the varistors and gas discharge tube (GDT) arrestors are installed to primarily suppress the surge voltage and current.

However, they usually operate only for very high surge voltages over several kV.

Surge protection circuits for the AEF should be separately prepared against the surge voltage or current that the varistors and arrestors cannot suppress.

220VAC

injL

CML

CMR

CMC

CML

CMR

CMC

injLoR

injL

oC

2R

1RsenC senC

nZ nI

Varistors

injM

injM

inC

inC

Surge

SMPS

CMMGDT

IsurgeVsurge,sen

injM

Compensation part Sensing part

[Ref] Sangyeong Jeong, Dongil Shin. Jongpil Kim and Seokjoon Kim, and Jingook Kim, “Design of Effective Surge Protection Circuits foran Active EMI filter”, 2017 Asia Pacific EMC conference, Seoul, Korea, June 2017. 20

Overcurrent path to the compensation part

Inducted voltage and current can be generated through the injection transformer.

The current flowing to R2 is almost negligible since R2 is usually much higher than output impedance of Op-amp.

The current injection into the op-amp output stage can be limited by a high value of Ro, but higher Ro degrades the noise attenuation performance at low frequency range.

2R

outopR ,

oRoC

- +injL

1oIoutopI ,

)0(2 RIinjM

injLIsurge

VLinj -

+

2, RR outop 1oosurgeLinj IsLsMIV Limits the inflowing current,but degrades performance of AEF.

21

Surge Protection Circuit for Compensation Part

When the diode is Turn-off :

When the diode is Turn-on :

Linj

popo

po

br VRR

RV

2,1,

2,

br

po

popo

Linj VR

RRV

2,

2,1,

2,2,1,

,

po

br

popo

Linj

outopR

V

RR

VI

,2,1,

2,

Linj

popo

po

br VRR

RV

2R

outopR ,

2, poR1, poR

oC

- +injL

1oIdI

2oIoutopI ,

)0(2 RIinjM

injLIsurge

VLinj -

+

2,1, popoo RRR 2, RR outop

,0dI ,21 oo II

2,

2,

po

Cooutop

R

VII

Vbr : Diode break down voltage

VC : Diode clamping voltage

1

1

o

CLOo

R

VVI

br

po

popo

Linj VR

RRV

2,

2,1,

A transient voltage suppression (TVS) diode is

installed to limit the current and maintain the

design freedom of Ro.

The diode limits the op-amp current as shown in

the right expressions.

The resistor of Ro,p1 should be sufficiently robust

against a large voltage pulse.

22

Overvoltage path to the Sensing part

The sensing part has basically a high input impedance from the AC line, the current is not critical issue but an overvoltage can be induced at the op-amp input, Rop,in.

Most commercial op-amps include internal diodes on each pin for electrostatic discharge (ESD) protection, but the on-chip ESD-diode is not suitable for preventing surges.

External TVS diodes are necessary for reliability from the surges.

senC

inopR , 1RinopV ,

ccV

ccV

Vsurge,sen

inopR ,

ESD-diode inside OP-amp

Break-down voltage of external TVS diodeshould be lower than that of internal ESD diode.

23

Surge Protection Circuit for Sensing Part

The TVS diode can be connected to the node ‘n1’ (option1) and ‘n2’ (option2).

The voltage at the node n2 is larger than the node n1 so the voltage can be more effectively limited in the option 2. However, the junction capacitance of the diode at the ‘option2’ position can degrade the noise attenuation performance of AEF

R1 or R1,p1 should be strong against the overvoltage surge pulse.

senC

inopR ,1,1 pR

2,1 pRinopV ,

ccV

ccV

option1 option2

n2n1

Vsurge,sen

e.g. Anti-surge resistor

21 nn VV

(not recommend this placement since overcurrent can flow to the diode.)

24

2kV Surge Test Set-up

2kV common mode (LINE-GND)surge tests had been performed,and the waveforms are measuredusing an oscilloscope.

The impedance of the CE noisesource is modeled as a capacitorof 40pF, which represents theparasitic capacitance betweenswitching transistors and heatsinks in SMPS.

3.6kV GDT arrestor

Designed AEF

SurgeGen.AC

2.2uFX-cap

420V Varistor

-

+

Currentprobe

AEF PCB

(DUT)

Load model(40pF)

SurgeGenerator

Oscilloscope VoltageSupply for AEF

SupplyProtection

GND

25

Measured Waveform of Compensation Part

About 200mA limited

br

o

ooLinj V

R

RRV

2

21

2

,

o

broutop

R

VI

2

,

o

Coutop

R

VI

br

o

ooLinj V

R

RRV

2

21

Ro,p1 and Ro,p2 are designed as 100 and 30 , respectively, and the clamping voltage of bidirectional TVS diode is around 6V.

The output current of op-amp is limited to about 200mA when large VLinj is induced as shown in the above figures.

When VLinj is small and diode is turn-off, current level is similar. (last peak of current)

2R

2, poR1, poR

oC- +

injL

injM

injL Isurge

VLinj-

+

2,1, popoo RRR

Iop,out

LinjVoutopI ,

Peak values of VLinj are similar

Id

2kV Surge

26

Measured Waveform of Sensing Part

The op-amp input voltage and current arecompared according to the diode position.

The internal ESD-diode are turned on ataround 6V, while the external TVS diode withthe break-down voltage of 3.5V wasimplemented

R1,p1 and R1,p2 are 3.6k and 2k, respectively.

senC

inopR ,1,1 pR

2,1 pR

ccV

ccV

option1 option2

n3n2n1Iop,in

Vop,in

+

-

The internal ESD-diode turn onand inrush current increases

inopI ,

Time (msec)

-0.3

-0.2

-0.1

0

0.05

0 10 20 30 40 50

No Protect.

Input Protect.(opt.1)


inopV ,

0 0.5 1 1.5 2 2.5-15

-10

-5

0

5

10

15

Time (msec)V

olt

age

(V)

-0.5

Time (msec)0 10 20 30 40 50

-15

-10

-5

0

5

10

15

Vo

ltag

e (V

)

No Protect.



inopV ,

2kV Surge

27

Performance Degradation Effect of Diode

02

121

2

/1

VI

VY

104

105

104 105 106 107

Imp

edan

ce (

Oh

m)

w/o AEFw/AEF, no protect.w/AEF, Output protect.w/AEF, Output + input (opt.1) protect.w/AEF, Output + input (opt.2) protect.

Frequency (Hz)

101

102

103

104

105

10621/1 Y

• The 1/Y21 parameter measured by the vector network analyzer (VNA), which can demonstrate the boosting of the power line impedance by the AEF.

• Junction capacitance of 2.5pF can cause performance degradation at high frequency since the additional pole is added in the transfer function.

Results

Output + input (opt.2)protect.

Other cases.

Input protect.option1

Input protect.option2

Output protect.

Port1Port2

VNA Measurement Set-up

28

(AEF Type 2) : VSCC AEF





Feed

-bac

k

• Dongil Shin, Sangyeong Jeong, and Jingook Kim, "Quantified Design Guidelines of Compact Transformer -less Active EMI Filter for Performance, Stability, and High Voltage Immunity", IEEE Trans on Power Electronics, vol. 33, no. 8, pp. 6723-6737, Aug. 2018.

29

Voltage-sense Voltage-compensation Current-sense Current-compensation Feed

-fo

rwar

d

Structure of the Designed VSCC AEF

• A simple low-cost compact AEF of the VSCC type without transformers is proposed.

• The amplifier part is designed as a push-pull amplifier with two BJTs.

• The proposed AEF can enhance the Y-cap in the LCL filter. 30

Loop Gain and Impedance Analysis

,1

AEFin AEF

n t

V AZ

I A

||

A

A

LISNcminpAEF ZZZZ 2||||

LISNcminp

FBsen

f

BJT

mtZZZ

Z

Z

Z

Zg

ZZA

||1||

,

LISNcmn

FBsen

ZZZ

Z

||

,

3211

A

A

III

VZ

fob

AEF

AEF

Zg

ZZ

Z

ZZ

Z

m

BJT

BJTf

FBsen

||||

1

,

(Closed-loop transfer function) =

(Loop gain) =

(AEF impedance) =

31

Noise Attenuation by the AEF

• The effective Y-cap impedance, is much decreased compared to (Zsen||Zinj), which results in a large increase of the NA.

• However, ZAEF below 1kHz is rarely affected by the AEF, and the influence of the AEF on the safety requirements for the leakage current is very small.

inpAEF

inpAEFLISNcm

AEFwLISN

oAEFwLISN

AEFZZ

ZZZZ

I

INA

||

||2

/,

/,

inpinjsen

inpinjsenLISNcm

capYwLISN

capoYwLISN

CapYZZZ

ZZZZZ

I

INA

||||

||||2

/,

/,

32

Design Target of VSCC AEF

0 0

1 22sen

AEF e L

m m L

ZZ R R

Z g g sC

)(

1||,0

injsen

injsenlowAEFCCs

ZZZ

L

L

e

mbias

injsen

highAEF RsC

RgZ

ZZZ 2

21,0

ZAEF without the damping circuitsat the low frequency

at the high frequency

ZAEF0

biasinjsen

injsen

opRCC

CCf

2

• The values of ZAEF0,high and fop should be reduced as much as possible while maintaining the sum of Cinp, Csen, and Cinj to be under the safety limit.

• Cinp are used to screen out the noise source impedance, ZEUT .

No damping circuits

gm : current gain of the BJT

33

Design of Class AB Push-Pull Amplifier

• Re of 1~4 is necessary to stabilize the BJT bias and the thermal runaway of a BJT.• RL should be as small as possible while satisfying the BJT current limit. • Rbias can be determined from the target fop.• DC bias point of the BJT in the class AB amplifier is located slightly above the cutoff, and Rp can be

extracted by solving the KVL from the base to the emitter.

BJT emitter voltage at the sat. region

The amplifier maximum output current

the max limit of the BJT collector current

biasinjsen

injsen

opRCC

CCf

2

CM noise current In

nI,

2

2

DCce sat

e L

VV

R R

,maxc

I

eBCBE

p

biasB

DC

p

bias

p

RIIVR

RIV

RR

R

2

||2

2

2

DC bias analysis of the BJT

34

Damping Circuit Design for System Stability (1)

• In Case 1, after Cf and Rin are added, the filter becomes stable, but its NA performance is poor. • In Case 2, Lf and Cin are inserted at each damping branch to recover the filter performance, but the

resonance between Lf and Cf causes a risk of feedback instability.• In Case 3, Rf is also added to suppress the Q-factor of the resonance. Finally, both stability and

performance of the AEF are optimized.

• The resonance between the CM choke2 and AEF most likely causes the system instability.

• The damping circuits are essential for feedback stability of this AEF.

Resonance

35

Damping Circuit Design for System Stability (2)

• For the system to be stable, the phase of the loop gain, Abt, should remain below 180˚.• The damping circuits should make the filter stable, while maintaining the performance.

Unstable by the resonance

Cf

rf

LLL

f2

1||

4

1

fpres

fLf

C2

,

220

1

fsresffpres LfRLf ,, 2010

fpres

inCf

R,2

1

inpres

in

LL RfC

fCrf ,10

1

21||2

1

Design equations for Damping circuits

36

Effects of AEF Stability on CE Noises

• The CM noises were measured in both time and frequency domains, when the EUT was turned off and only the AEF turned on.

• Without the damping circuits, the oscillation due to the instability occurs. After applying the damping circuits, all the oscillations and harmonic peaks disappear.

37

Design Flow of the VSCC AEF with Surge Protection

38

• The AEF’s immunity against high voltage surge transients is tested and guaranteed.

CE Measurements of the AEF installed on an Inverter

• For a fair comparison, the value of CY is set as the sum of Csen, Cinj, and Cinp, which are utilized in the filter with the AEF.

• The AEF is implemented into a real 2.2 kW current resonant inverter, and the CE are reduced by 5dB to 25 dB at a frequency range from 150 kHz to 6 MHz.

• The AEF can be embedded inside a real product without increasing the size and cost.

39

Other developed AEFs

40

• Sangyeong Jeong, et al, Jingook Kim, "A Transformer-Isolated Common-Mode Active EMI Filter without Additional Components on Power Lines", Early Access Articles, IEEE Transactions on Power Electronics, 2018.

• Dongil Shin, et al, Jingook Kim, “A Balanced Feedforward Current-Sense Current-Compensation Active EMI Filter for Common-Mode Noise Reduction”, under revision, IEEE Transactions on EMC, 2018.

41

• E-mail : [email protected]• Webpage : http://emcoretech.com

makes compact Active EMI Filter.

Most of the critical issues havebeen resolved now.(Reliability, immunity, power, stability)

Conclusion

• An EMI filter employing the AEF can be smaller, cheaper, and lighter than a passive-only filter.

• The design guidelines for two types of compact AEFs were derived for performance, stability, and high voltage immunity.

• More reliable other new AEFs are also being developed.• The AEFs are ready to be practically utilized in real home

appliance products.

42

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