Electromagnetic Compatibility of a DC Power Distribution System for the ATLAS Liquid

Argon Calorimeter

G. BLANCHOTCERN, CH-1211 Geneva 23, Switzerland

Georges.Blanchot@cern.ch

11th Workshop on Electronics for LHC and Future Experiments

12-16 September 2005, Heidelberg, Germany

LECC 2004 G. Blanchot, CERN 2

Liquid Argon Detector

Front End CratePresence of B field.

High level of radiation.Requires 4.5 kW of low

voltage power that can only be produced in its vicinity.

Front End Power SupplyVery close to the FEC.DC/DC converter powered from the control room.

Barrel Calorimeter

LECC 2004 G. Blanchot, CERN 3

Power Distribution Scheme

Front-End Crate

DC/DC

F2(s)

AC/DC

F1(s)Vin

ZO1

ZI2

ZS

100 meters, shielded power cable

Back End AC/DC Converter, delivers 280VDC at 16A max.

Front End DC/DC Converters, deliver all low voltages to the front end crate.

Zo1 = AC/DC converter output impedance.ZS = Cable impedance.ZI2 = DC/DC converter input impedance.

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Some EMC Issues

StabilityWhen powering DC/DC converters from AC/DC converters over long distances, some instabilities can show up. The stability conditions are reviewed on the basis of specific measurements.

Noise PropagationLong power cables behave as transmission lines. The noise can be amplified or attenuated depending on the cable properties and the load conditions. Measurements are made to put in evidence resonance of noise currents.

Grounding SchemeWhere to ground the shield of the power cable and consequences of floating the return line on the emitted noise.

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Stability

Model:

Negative resistancern causes a phase shift of 180˚ at low frequencies that can make T = -1 if the input, output and cable impedances are not matched properly

Long cableUsually Z01 << ZI2. When the cable is long, Zs can become dominant and the instability can show up.

T

FF

ZZZ

FF

V

V

I

SOin

out

11

21

2

1

21

load

loadn I

Vr

Vin

DC/DC

F2(s)

AC/DC

F1(s)

ZO1

ZI2

ZS

Vout

Must be different of zero

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Measurement of Impedances

I12 DC/DC

F2(s)

AC/DC

F1(s)

Vin VI2

ZS

VOS

CableLOAD

RL

A reference current is injected on the power line (under working conditions) with a bulk

injection probe or an in-line transformer.

The voltages VOS and VI2 are monitored with an oscilloscope or better with a low frequency

spectrum analyzer

121 I

VZZ OS

SO 12

22 I

VZ I

I 12V

VT OS

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Stability figure

100

101

102

10-2

10-1

100

101

102

[kHz]

Oh

ms

ZO1

ZI2

Z01

+ ZS

103

104

105

100

101

[Ohm

s]

103

104

105-200

-150

-100

-50

0

50

100

150

200

[Hz]

[Deg

ree]

Bode diagrams allow the estimate the stability margin.The impedance of the cabledominates the outputimpedance, but stayslower than the input impedance where thephase shhift occurs.

Output Impedance

Input Impedance

Output+cable impedance

Phase → -180˚ below 2 kHz

Stability margin: 20 dB

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Stability figure

Nyquist ChartThe plot of T(s) in the complex plane for increasing frequencies must not enclose the (-1,0) point. The curves are a fucntion of the load applied.

The system appears again stable at full load.

Full load

No loadIm[T

]

Re[T]

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Noise Propagation

Dominant source of electromagnetic interferences (EMI): Common mode currents.

The EMI limits in ATLAS are stated in terms of maximum observable common mode current outside of the shield.

The common mode current is not a constant in long cables: the limit applies at the worst case location.

Range 9 kHz to 500 kHz 500 kHz to 100 MHz

Limit 45 dBμA 39 dBμA

ATLAS EMI emission limits

Equipotential structures

CMI

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Noise Propagation

Two conductors transmission line model.

Resonances and conversions between CM and DM noise: Common mode transfer function Common mode to differential mode conversion

I1

I2

R1 L10

R2 L20

C10 C20

C12

V2V1

ΔzSHIELD

POWER

RETURN

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Common mode to common mode transfer function

RLoad

Injected common mode current

Near end CM current Far end CM current

NearEnd

FarEnd

CM

CM

CMI I

IH

0.00E+00

2.00E+00

4.00E+00

6.00E+00

8.00E+00

1.00E+01

1.20E+01

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

H(I

CM

)

Load: 30 Ohms

Load: 217 Ohms

|H| = 20 dB at 1.5 MHz at nominal load. To comply with the ATLAS limits, the AC/DC converter must stay below 39 – 20 = 19 dBμA.

The gain is negligible at light loads.

100 m

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Common mode to differential mode transfer function

RLoad

Injected common mode current

Near end CM current Far end DM current

100 m

0.00E+00

5.00E+00

1.00E+01

1.50E+01

2.00E+01

2.50E+01

3.00E+01

3.50E+01

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

H(C

M t

o D

M)

Load: 30 Ohms

Load: 217 Ohms

|H| = 30 dB at 100 kHz at light load.

The gain is negligible at nominal load.

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Common Mode and EMI Emissions of the DC Power Link

AC/DC RLoad

1 2

EMI emissions test setup (B)

Near end EMI

Far end EMI

AC/DC RLoad

1 2Return line is grounded

Configurable shield grounding

Near end CM

Far end CM

CM emissions test setup (A)

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CM and EMI EmissionsNear End Common Mode

-50

-40

-30

-20

-10

0

10

20

30

40

50

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

Icm

(d

Bu

A)

Shield Grounded on Both Ends

Shield Grounded on the AC/DC Converter

LIMIT

Far End Common Mode

-60

-40

-20

0

20

40

60

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

Icm

(d

Bu

A)

Shield Grounded on Both Ends

Shield Grounded on the AC/DC Converter

LIMIT

Near End EMI Emissions

-50

-40

-30

-20

-10

0

10

20

30

40

50

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hzd

Bu

A

Shield grounded on Both Ends

Shield Grounded on the AC/DC Converter

LIMIT

Far End EMI Emissions

-50

-40

-30

-20

-10

0

10

20

30

40

50

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

dB

uA

Shield Grounded on Both Ends

Shield Grounded to the AC/DC Converter

LIMIT

Limit

Limit

NE

AR

EN

DF

AR

EN

D

The shield effectively carries back the CM current

The CM current gets amplified around 2 MHz as expected.

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Use of shield for EMI containment

The shield is an effective way to reduce Emi emissions from power cables:Large amounts of power can be transmitted over long cables with negligible EMI emisisons.

Grounding the shield on AC/DC converter only:Allows to slightly reduce the CM current. However it can’t return efficiently through the shield, resulting in higher EMI emissions.

Comparison of grounding schemes.This maximises the CM current. However as it returns efficiently through the shield, the lowest EMI emissions are achieved.

Best grounding scheme.Shield grounded on both ends to optimze the CM return path, even if this maximises the CM current.

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EMI Emissions at the Experimental Site

What if the load is the front end power supply and the front end crate, instead of a simple resistive load?The DC/DC converters will contribute new CM current along the link.

-200

0

200

400

600

800

1000

1200

1400

1600

1800

0.01 0.1 1 10 100

MHz

uA

Common Mode

EMI Emission

LIMIT

The CM current emitted by the front-end converter is huge (1,6 mA): 50 times more than the AC/DC converter.

The EMI emissions are contained by the shield, except at low frequencies: at 100 kHz a peak at 600 μA persists.

The shield current due to external couplings was measured to be lower than 200 μA .

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Grounding of the Return Line

Floating vs Grounded on the Near EndEMI Emissions

Floating and Grounded Return

-50

-40

-30

-20

-10

0

10

20

30

40

50

1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08

Hz

dB

uA

Floating Return

Grounded Return

LIMIT

Leaving the return line floating is a source of increased EMI emissions by more than 40 dB. The return line must be grounded to comply with the safety rule, but it is also a very effective way to reduce the noise in the experimental area.

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Conclusions

A stable, low noise power distribution was achieved for the Liquid Argon Detector.

The noise emitted by the AC/DC converter is within the ATLAS limits, except at 100 kHz with 600 μA with respect to the limit of 200 μA.

The front end converters are the dominant source of noise.

The use of a shielded power cable, grounded on both ends, is the most effective way to reduce the noise.

Grounding the return line is an effective way to reduce the EMI emissions.