EMI studies of different switched converters: setup and lessons learnt Power Working Group G. Blanchot – 7/4/2008

Typical sLHC System Front-end System S(f) Bulk Power Supply DC/DC Converter EMI Coupling EMI Emission System noise Noise on Mains Contributors to overall system (detector) noise: The system itself: thermal noise, cross talk and internal couplings within the system. EMI couplings from other sources onto the cables and boards. EMI emissions of the DC-DC converter on the cables. EMI emissions of the bulk power system and ancillary systems. Conducted noise from the mains.

Requirements on the Mains Front-end System S(f) Bulk Power Supply DC/DC Converter EMI Coupling EMI Emission System noise Noise on Mains Bulk power supplies are off-the-shelve commercial pieces The must comply with EU regulations for what concerns EMC (CISPR11): Emission of conducted noise on the mains (CISPR11). Emission of radiated noise from the envelope (CISPR11). Emission of harmonic currents on the mains (IEC 61000-3-2). Immunity against disturbances on the mains (IEC 61000-6-2). There are no mandatory compliance to EMC requirements for what concerns the output of the bulk power supply: Specific requirements are needed.

Requirements for the DCDC Converter Front-end System S(f) Bulk Power Supply DC/DC Converter EMI Coupling EMI Emission System noise Noise on Mains The converter is a component No standards are applicable to components: The EMC aspects must be studied for the targeted system. Particularities are: Radiation hard devices limit the applicable technology. Magnetic field tolerance forbids the use of ferromagnetic cores. Material budget imposes integrated solutions. Long input cables (>30 m). Susceptibility of the front-end is often not known but usually large.

Requirements for the System Front-end System S(f) Bulk Power Supply DC/DC Converter EMI Coupling EMI Emission System noise Noise on Mains The overall performance is characterized by the system noise Each system has its own requirements and particularities: Susceptibility to conducted and radiated noise can very between systems. Usually, peak of susceptibility between 1 MHz and 50 MHz: Conducted noise dominates.

Characterizing system immunity The characterization (measurement) or the specification (if the system is under development) are the first steps for the succesful design of a low noise front-end system. Immunity against conducted noise (ripple, CM currents) The systems must be designed such that they tolerate a specified level of CM current and ripple on the power lines. The susceptibility can be measured by means of standard characterization methods: bulk injection. The information obtained is useful to specify the power distribution system. Immunity studies are rarely found, but are essential. Also they are difficult to carry on and need specific instruments: probes, generators, amplifiers, couplers, spectrum analyzers will all be available at CERN for this purpose. CMS Tracker immunity study (F. Arteche, 2005)

Understanding LVPS noise External EMI External EMI External EMI Bulk LVPS Internal EMI DC/DC Internal EMI System S(f) Internal EMI System noise The DC/DC converter can be a major contributor to the system noise, but not necessarily To insure objectivity of the studies, a reference test setup is needed. Required independence from the system and from the bulk power source. Well defined test conditions and measurement methods for reproducible and comparable results. The reference performance of a converter can then be compared to the measured immunity curves of the system. The best power conversion system can then be chosen on a scientific basis. Different converters can then be compared. Tests with a particular system cannot provide any objective evaluation of a converter, because it is a function of the system immunity which is often not known.

EMC at design stage Vin=12-24 V Vout=1.5-3V Iout=1-2A Rad-hard technology Inductor Power dissipation EMI (dV/dt) EMI (dI/dt) EMI (dψ/dt) Challenges designing a low noise converter Vout/Vin and switch speed will set dV/dt (E field). Iout and switch speed will set dI/dt (H field). Careful layout: Control of noise “islands”: dV/dt areas, and noise loops: dI/dt loops. Selection of components, minimize parasitics, good decouplings. Technological issues (PCB stack up and materials, thermal management).

Marcelo Perez, UTFSM Valparaiso – Chile, HELEN program. Reference test setup Ground plane Reference return path for CM currents Cables lay on the plane. Source, load and filters earthed to the plane. LISN: Line Impedance Stabilization Network Calibrated, standardized filter. Filters noise from bulk LVPS. Provides reference impedance seen by the converter towards its source of power, over the whole frequency range of interest. ISL LISN DC/DC ISL: Impedance Stabilized Load Calibrated load: DM (load). CM (output to earth plane). Splitter Marcelo Perez, UTFSM Valparaiso – Chile, HELEN program.

Line Impedance Stabilization Network V(LISN) ICM+DM LISN Provides a standardized voltage measurement of the symmetric and asymmetric noise between line and earth (ICM + IDM). The impedance is calibrated from 100 kHz to 100 MHz. Above 1 MHz: Z=50 ohms. To measure accurately the CM current only, a calibrated current probe is used. Alternatively, a CM/DM splitter can be used (less accurate). CM/DM splitter Current probe

Impedance Stabilized Load CM ZT CM ZIN DM ZIN Reference load DC: nominal load (2Ω, 2.5V/1.25A). CM: 50Ω between each line and earth above 150 kHz to 100 MHz. DM: 50Ω between each line and earth above 150 kHz to 100 MHz, 100 Ω between lines. Symmetrical construction, calibrated curves.

Characterization of a DC/DC ISL6540A (prototype 1) 10V to 2.5V, 1.25A. 58 dBuA Several prototypes were tested: Buck prototypes 1 & 2. ST1S10 evaluation board. Enpirion prototype (CMS, Katja’s talk).

Characterization of a DC/DC ISL6540A (prototype 2) 10V to 2.5V, 1.25A. 60 dBuA The new layout did not help to reduce the noise: the noise is mainly set by the topology of the converter and its switching properties. ST1S10 Evaluation board Low Noise Integrated converters allow to achieve low noise levels in the frequency range where the susceptibility peak usually occurs.

Characterization of a DC/DC (2) ISL6540A (prototype 2) 10V to 2.5V, 1.25A. The emission of noise takes place on all the ports: output and input. The amplitude of the noise depends of the load: - Dynamic parameters change with the load: ripple, dV/dt, dI/dt. In absence of load, the CM noise current is radically reduced in prototype 2. ISL6540A (prototype 2) 10V to 2.5V, 0A.

Characterization of a DC/DC (3) ISL6540A (prototype 1) 10V to 2.5V, 0A. In absence of load, the second prototype has better performance than the first one. Proto2 is characterized by a better layout: smaller dv/dt islands=reduced stray capacitances to earth, shorter power paths=smaller dI/dt couplings. An integrated converter (integrated switches and control) will minimize the dV/dt island (stray capacitances) further down. ISL6540A (prototype 2) 10V to 2.5V, 0A.

Characterization of a DC/DC (4) ST1S10 evaluation board, nominal current The integrated ST converter has its noise spectrum disappearing entirely in absence of load. Integrated converters seem to have better noise performance that those using discrete parts. However at full load the switch frequency peaks remains large. ST1S10 evaluation board, 0A.

Spatial Scans Complementary tools are being set up, one of them being a near field scanning table Narrowband or broadband scans. Real time display. At switch frequency the sensitivity is very low: accurate tuning is required, ongoing discussions with manufacturer to optimize the settings. EM Scanner (being set up): Spatial scan of near H field emission at board level will allow identifying hot spots on prototypes.

Conclusion To achieve low noise systems: Need to address noise and susceptibility of all the involved components: bulk power supply, DC to DC converter, front-end system. Reference test methods are used to quantify the amount of noise of the power converters, allowing for comparisons. The noise is contributed by dV/dt and dI/dt, the latter being the dominant coupling for integrated converters. The noise at full load must be as low as reasonably possible: The bulk supply must have low noise emissions as well, this must be specified at procurement. The FE system must be designed with some degree of immunity against noise currents. The magnetic field tolerance difficults the filtering of the noise: Can only count on layout and topologies to minimize the noise.