Electromagnetic Compatibility of a DC Power Distribution System for the ATLAS Liquid Argon Calorimeter G. BLANCHOT CERN, CH-1211 Geneva 23, Switzerland 11th Workshop on Electronics for LHC and Future Experiments September 2005, Heidelberg, Germany

LECC 2004G. Blanchot, CERN2 Liquid Argon Detector Front End Crate Presence 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 Supply Very close to the FEC. DC/DC converter powered from the control room. Barrel Calorimeter

LECC 2004G. Blanchot, CERN3 Power Distribution Scheme Front-End Crate DC/DC F 2 (s) AC/DC F 1 (s) V in Z O1 Z I2 ZSZS 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. Z o1 = AC/DC converter output impedance. Z S = Cable impedance. Z I2 = DC/DC converter input impedance.

LECC 2004G. Blanchot, CERN4 Some EMC Issues  Stability When 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 Propagation Long 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 Scheme Where to ground the shield of the power cable and consequences of floating the return line on the emitted noise.

LECC 2004G. Blanchot, CERN5 Stability  Model:  Negative resistance r n 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 cable Usually Z01 << ZI2. When the cable is long, Zs can become dominant and the instability can show up. V in DC/DC F 2 (s) AC/DC F 1 (s) Z O1 Z I2 ZSZS V out Must be different of zero

LECC 2004G. Blanchot, CERN6 Measurement of Impedances I 12 DC/DC F 2 (s) AC/DC F 1 (s) V in V I2 ZSZS V OS Cable LOAD R L A reference current is injected on the power line (under working conditions) with a bulk injection probe or an in-line transformer. The voltages V OS and V I2 are monitored with an oscilloscope or better with a low frequency spectrum analyzer

LECC 2004G. Blanchot, CERN7 Stability figure  Bode diagrams allow the estimate the stability margin. The impedance of the cable dominates the output impedance, but stays lower than the input impedance where the phase shhift occurs. Output Impedance Input Impedance Output+cable impedance Phase → -180˚ below 2 kHz Stability margin: 20 dB

LECC 2004G. Blanchot, CERN8 Stability figure  Nyquist Chart The 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 load Im[T] Re[T]

LECC 2004G. Blanchot, CERN9 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. Range9 kHz to 500 kHz500 kHz to 100 MHz Limit45 dBμA39 dBμA ATLAS EMI emission limits Equipotential structures

LECC 2004G. Blanchot, CERN10 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 I1I1 I2I2 R1R1 L 10 R2R2 L 20 C 10 C 20 C 12 V2V2 V1V1 ΔzΔz SHIELD POWER RETURN

LECC 2004G. Blanchot, CERN11 Common mode to common mode transfer function R Load Injected common mode current Near end CM current Far end CM current |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

LECC 2004G. Blanchot, CERN12 Common mode to differential mode transfer function R Load Injected common mode current Near end CM current Far end DM current 100 m |H| = 30 dB at 100 kHz at light load. The gain is negligible at nominal load.

LECC 2004G. Blanchot, CERN13 Common Mode and EMI Emissions of the DC Power Link AC/DC R Load 12 EMI emissions test setup (B) Near end EMI Far end EMI AC/DC R Load 12 Return line is grounded Configurable shield grounding Near end CM Far end CM CM emissions test setup (A)

LECC 2004G. Blanchot, CERN14 CM and EMI Emissions Limit NEAR END FAR END The shield effectively carries back the CM current The CM current gets amplified around 2 MHz as expected.

LECC 2004G. Blanchot, CERN15 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.

LECC 2004G. Blanchot, CERN16 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. 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.

LECC 2004G. Blanchot, CERN17 Grounding of the Return Line  Floating vs Grounded on the Near End 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.

LECC 2004G. Blanchot, CERN18 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.