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Serial Powering vs. DC-DC Conversion - A First Comparison

Serial Powering vs. DC-DC Conversion - A First Comparison. Tracker Upgrade Power WG Meeting October 7 th , 2008. Katja Klein 1. Physikalisches Institut B RWTH Aachen University. Outline. Compare Serial Powering & DC-DC conversion under various aspects Power loss in cables

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Serial Powering vs. DC-DC Conversion - A First Comparison

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  1. Serial Powering vs. DC-DC Conversion - A First Comparison Tracker Upgrade Power WG Meeting October 7th, 2008 Katja Klein 1. Physikalisches Institut B RWTH Aachen University

  2. Outline • Compare Serial Powering & DC-DC conversion under various aspects • Power loss in cables • Local efficiency • Compatibility with services • Power supplies • Bias voltage • Safety • Slow control • Start-up • Scalability • Flexibility • Potential to deliver different voltages • Process considerations & radiation hardness • Interplay with FE-chip • Interplay with readout & controls • Noise • Material budget • Space • Test systems • Discussion Serial Powering vs. DC-DC Conversion

  3. The Basic Ideas • Serial powering • Powered from constant current source • Each module is on different ground potential  AC-coupled communication • Shunt regulator and transistor to take excess current and stabilize voltage • Voltages are created locally via shunt and linear regulators • Parallel powering with DC-DC conversion • Need radiation-hard magnetic field tolerant DC-DC converter • One converter per module or parallel scheme • 1-step or 2-step conversion Vdrop = RI0 Pdrop = RI02 Conversion ratio r: r= Vout/ Vin ! << 1 Pdrop = RI02n2r2 Serial Powering vs. DC-DC Conversion

  4. The Buck Converter The “buck converter“ is simplest inductor-based step-down converter: Convertion ratio g > 1: g = Vin / Vout Switching frequency fs: fs = 1 / Ts Serial Powering vs. DC-DC Conversion

  5. The Charge Pump • Capacitor-based design • Step-down: capacitors charged in series and discharged in parallel • Conversion ration = 1 / number of parallel capacitors • Low currents Serial Powering vs. DC-DC Conversion

  6. Implementation Examples Serial powering: PP with DC-DC conversion: Atlas pixels, Tobias Stockmanns Stefano Michelis, TWEPP2008 • Two-stage system • Diff. technologies proposed for the two stages • Analogue and digital power fully separated • Power for optical links ~ integrated • HV not integrated • Regulators on-chip or on the hybrid • AC-coupled communication with off-module electronics • Power for optical links not integrated • HV not integrated Serial Powering vs. DC-DC Conversion

  7. What Conversion Ratio do we need? Conversion ratio needed for parallel powering with DC-DC converters? • Total tracker current estimate • Current strip tracker: 15kA; current pixel: 1.5kA • Geoffs strawman: strips: 25kW/1.2V = 21kA; pixels: 3.2kA; trigger layers: 10kA • Currents increase roughly by factor of 2 in this strawman • Power loss in cables • Goes with I2 increase by factor of 4 for same number of cables (2000) • Total power loss inverse proportional to number of power groups • Can compensate with (conversion ratio)2 • Material budget • Saving in cable x-section scales with I • Total material independent of segmentation • Of course want to reduce as much as possible With conversion ratio of ¼ we would be as good as or better than today. SP: current fixed; cable material & power loss depends only on # of cables! Serial Powering vs. DC-DC Conversion

  8. Power Losses in Cables • Power losses in cables lead to decrease of overall power efficiency  expensive • ... increase the heat load within the cold volume  cooling capacity must be higher • Consider system with n modules: Pdet = nI0V0 • Voltage drop on cables & power loss Pcable calculated within each scheme • Efficiency = Pdet / Ptotal = Pdet / (Pdet + Pcable) DC-DC, r = 1/10 SP DC-DC, r = 1/5 • PP with DC-DC conversion • Eff. goes down with n. Need more cables or lower conversion ratio • Equal to SP if conversion ratio = 1/n • Serial powering • Eff. increases with n. Since 10-20 modules can be chained, efficiency can be very high! Serial Powering vs. DC-DC Conversion

  9. Local Efficiency • Serial powering • Constant current source  total power consumption is contant! • Current of chain is fixed to highest current needed by any member • Current not used by a module flows through shunt regulator • Linear regulator: voltage difference between dig. & analog drops across it • Local power consumption is increased! • Estimated increase for - Atlas pixels (NIM A557): 35% - Atlas strips (NIM A579, ABCD): 18% • PP with DC-DC conversion • All DC-DC converters have inefficiencies • switching losses • ESR of passive components • Ron of transistor etc. • Typical values (e.g. comm. buck): 80-95% • Efficiency goes down for low conv. ratio! • Trade-off betw. eff. & switching frequency • In two-step schemes, efficiencies multiply • Estimates (St. Michelis, TWEPP2008): • Step-1: 85-90% • Step-2: 93% • Total: 80-85% • This needs to be demonstrated Serial Powering vs. DC-DC Conversion

  10. Compatibility with LIC Cables • Constraints from recycling of current services: • 2000 LICs with two LV conductors & common return each  Not realistic to split return to obtain 4000 lines •  Stay with 2000 LV power lines (“power groups“) • LV conductors certified for 30V and 20A • Twisted pairs (HV/T/H/sense) certified for 600V • 256 PLCC control power cables • Adapt at PP1 to (lower mass) cables inside tracker • Serial powering • Current is small • 30V allows for chains with more than 20 modules •  looks compatible • PP with DC-DC conversion • 30V is largely enough • For any reasonable segmentation and conv. factor currents should be lowere.g. 20 chips a 53mA per module  1.2A / module 20 modules per rod  24A /rod r = ¼  I = 6A •  looks compatible Serial Powering vs. DC-DC Conversion

  11. Power Supplies • Assume that power supplies will be exchanged after 10 years • PP with DC-DC conversion • Standard PS: ~15V, ~10A (radiation & magnetic field tolerant?) • Any sensitivity of converter to input voltage ripple? • No sensing needed (local regulation)? • Serial powering • Constant current source • Not so common in industry (e.g. CAEN) • Atlas: PSs developed by Prague group (developed already their current PSs) • No sensing Serial Powering vs. DC-DC Conversion

  12. Bias Voltage • Power is not a problem (currents are very low) • Up to now: independent bias lines for 1-2 modules • Might not be possible anymore when current cables are re-used • Note: T/H/sense wires are equal to HV wires • Serial powering • Not yet well integrated into concept • Derive on-module via step-up converters? In Atlas, piezo-electric transformers are discussed. • Or independent delivery using todays cables • PP with DC-DC conversion • Same options as for SP Serial Powering vs. DC-DC Conversion

  13. Safety (I) • Serial powering • Open leads to loss of whole chain • Shunt regulators/transistors to cope with this • Several concepts are on the market (next page) • Connection to module can break  bypass transistor on mothercable- high V, high I  rad.-hardness? - must be controlable from outside • Real-time over-current protection? • Real time over-voltage protection? • PP with DC-DC conversion • Open connections • Converter itself can break • Shorts between converter and module • If PP of several mod.s by one converter: risk to loose several modules at once • Fermilab expressed interest to perform a systematic failure analysis Serial Powering vs. DC-DC Conversion

  14. Safety (II) Shunt regulators + transistors parallel on-chip (Atlas pixels)+ redundancy- matching issue at start-up Regulator with lowest threshold voltage conducts first all current goes through this regulator spread in threshold voltage and internal resistance must be small One shunt regulator + transistor per module+ no matching issue- no redundany- needs high-current shunt transistor- must stand total voltage One reg. per module + distributed transistors+ no matching issue+ some redundancy- feedback more challenging Serial Powering vs. DC-DC Conversion

  15. Slow Control • Module voltage(s) • Module current(s)? • Bias current • Serial powering • Slow control IC or block on hybrid • Could be used to communicate with linear regulator and turn to stand-by • Ideas to sense module voltage in Atlas pixels: - sense potential through HV return - sense through AC-coupled data-out termination - sense from bypass transistor gate • PP with DC-DC conversion • Slow control IC or block on hybrid • For on-chip charge pump: would be useful to have SC information from individual chips • Could be used to set converter output voltage and switch on/off converters Serial Powering vs. DC-DC Conversion

  16. Start-up & Selective Powering • Serial powering • If controls powered from separate line, it can be switched on first • Devices in chain switched on together (both module controller and FE-chips) • Can take out modules only by closing bypass transistor from outside • PP with DC-DC conversion • If converter output can be switched on/off, then easy and flexible: - controls can be switched on first - bad modules (chips?) can be switched off - groups of chips/modules can be switched on/off for tests • This should be a requirement! Serial Powering vs. DC-DC Conversion

  17. Scalability • Consequences if more modules are powered per chain or in parallel? E.g. barrel vs. end caps: different # of modules per substructure • Serial powering • Current is independent on # of modules • Number of modules reflected in maximal voltage within chain; relevant for • capacitors for AC-coupling • constant current source • bypass / shunt transistors • PP with DC-DC conversion • If one converter per module: perfect scalability • PP of several mod. by one converter: current depends on # of modules, must be able to power largest group • Should specify soon what we need • current per chip • # of chips per module • # of modules per substructure • Otherwise we will be constraint by currents that devices can provide Serial Powering vs. DC-DC Conversion

  18. Flexibility • Flexibility with respect to combination of devices with different currents • E.g. trigger vs. standard module (or 4 / 6-chips) • PP with DC-DC conversion • If one converter per module: very flexible, do not care! • If PP of several modules by one converter: distribution between modules arbitrary • Serial powering • Current of chain is equal to highest current needed by any member chains with mixed current requirements are inefficient! Serial Powering vs. DC-DC Conversion

  19. Potential to Provide Different Voltages • Chip supply voltage(es): ~ 1.2V (Atlas: 0.9V for digital part to save power) • Opto-electronics supply voltage: 2.5 – 3V • Serial powering • Needed voltage created by regulators • ~1.2V by shunt regulator • Lower voltage derived from this via linear regulator  efficiency loss • Technically could power opto-electronics and controls via own regulators, but inefficient to chain devices with different current consumption • Decouple from chain (Atlas: plan to power separately from dedicated cables) • PP with DC-DC conversion • With charge pumps, only integer conversion ratios are possible • With inductor-based designs, arbitrary Vout < Vin can be configured (but feedback circuit optimized for a certain range) • Only hard requirement: Vin >= Vopto • Analogue and digital voltage can be supplied independently no efficiency loss Serial Powering vs. DC-DC Conversion

  20. Process Considerations & Radiation Hardness • Serial powering • Regulators must be rad.-hard • Standard CMOS process can be used; but... • HV tolerant components (up to nU0): - capacitors for AC-coupling - bypass transistor • Shunt transistors must stand high currents (~2A) if one per module • PP with DC-DC conversion • Commercial devices are not rad.-hard • Apparent exception: Enpirion EN5360 (S. Dhawan, TWEPP2008) • Standard 130nm CMOS: 3.3V maximal • For high conversion ratio transistors must tolerate high Vin , e.g. 12V • Several “high voltage“ processes exist • Rad.-hard HV process not yet identified • This is a potential show stopper • For r = ½ (e.g. charge pump) can use 3.3V transistors - radiation hardness? Serial Powering vs. DC-DC Conversion

  21. Interplay with FE-Chip • PP with DC-DC conversion • Ideally fully decoupled • Not true anymore in two-step approach with on-chip charge pump • Next Atlas strip FE-chip (ABCnext): - linear regulator to filter switching noise • Next Atlas pixel chip (FE-I4): - LDO - Charge pump (r = ½) • No influence on protocol • Serial powering • Several options for shunt - Regulator and transistor on-chip - Only shunt transistor on-chip - Both external • Linear regulators typically on-chip • Next Atlas strip FE-chip (ABCnext): - linear regulator - shunt regulator circuit - shunt transistor circuit • Next Atlas pixel chip (FE-I4): - Shunt regulator - LDO • DC-balanced protocol Serial Powering vs. DC-DC Conversion

  22. Readout & Controls • Serial powering • Modules are on different potentials AC-coupling to off-module electronics needed • Decoupling either on the hybrid (needs space for chips & capacitors) or at the end of the rod (Atlas strips, P. Phillips, TWEPP08) • Needs DC-balanced protocol increase of data volume • PP with DC-DC conversion • Nothing special: electrical transmission of data and communication signals to control ICs • No DC-balanced protocol needed Atlas pixels, NIM A557 Serial Powering vs. DC-DC Conversion

  23. Noise • Serial powering • Intrinsically clean - current is kept constant - voltages generated locally • Main concerns: - pick-up from external source - pick-up from noisy module in chain • Tests by Atlas pixels (digital) and strips (binary) revealed no serious problems - noise injection - modules left unbiased - decreased detection thresholds - external switchable load in parallel to one module (changes potential for all modules): some effect (Atlas pixels, NIM A557) • PP with DC-DC conversion • Switching noise couples conductively into FE • Radiated noise (actually magnetic near-field) is picked up by modules • Details depend on FE, distances, filtering, coil type & design, switching frequency, conversion ratio, ... • Shielding helps against radiated noise, but adds material, work and cost • LDO helps against conductive noise, but reduces efficiency • Surprises might come with bigger systems • Not good to start already with shielding and system-specific fine-tuning Serial Powering vs. DC-DC Conversion

  24. Material Budget • Serial powering • Regulators ~ one add. chip per hybrid • Components for AC-coupling - HV-safe capacitors (might be big!) - LVDS chip • Flex for discrete components • Cable cross-section from PP1 to detector (rest stays) scales with current - One cable must carry I0 - Total mass depends on modules / cable • Motherboard/-cable: power planes can be narrow, small currents & voltages created locally • PP with DC-DC conversion • Converter chip(s) • Discrete components - air-core inductor (D = 1-2cm!) - output filter capacitor(s) • Flex for discrete components • One cable must carry I0nr  total mass depends only on conv. ratio • Motherboard/-cable - buck converter can tolerate certain voltage drop since input voltage must not be exact  low mass - charge pumps have no output regulation: need exact Vin • Shielding? Serial Powering vs. DC-DC Conversion

  25. Space • Serial powering • Different options are discussed, but regulators + shunt transistors are either in readout chip or in a separate chip ~ one additional chip per hybrid • Components for AC-coupling - LVDS buffers - HV-safe capacitors (might be big) • Bypass transistor? • PP with DC-DC conversion • Charge pump in readout chip or in a separate chip • Buck converter: - controller chip - discrete air-core inductor (D = 1-2cm!) - discrete output filter capacitor(s) - more? very unlikely to be ever fully on-chip • In all other inductor-based topologies more components (inductors!) needed Serial Powering vs. DC-DC Conversion

  26. Test Systems for Construction Phase • Serial powering • If AC-coupling at end of stave, a decoupling board is necessary to read out single modules • Adapter PCB needed anyway for electrical readout • PP with DC-DC conversion • Electrical readout of single modules possible with adapter PCB Serial Powering vs. DC-DC Conversion

  27. Work on Powering within CMS Tracker • RWTH Aachen (L. Feld)– proposal accepted • System test measurements with commercial and custom DC-DC (buck) converters • Simulation of material budget of powering schemes • Rad.-hard magnetic-field tolerant buck converter in collaboration with CERN group • Bristol university (C. Hill)– proposal accepted • Development of PCB air-core toroid • DC-DC converter designs with air-core transformer • PSI (R. Horisberger)– no proposal, but private communication • Development of on-chip CMOS step-down converter (charge pump) • IEKP Karlsruhe (W. de Boer) – proposal under review • Powering via cooling pipes • Fermilab / Iowa / Mississippi (S. Kwan) – proposal under review • System test measurements focused on pixel modules (DC-DC conversion & SP) • Power distribution simulation software Serial Powering vs. DC-DC Conversion

  28. Summary • Both schemes have their pros and cons – how to weigh them? • SP is complicated, but I do not see a real show stopper • DC-DC conversion is straightforward, but two potential show stoppers • noise, radiation-hardness of HV-tolerant process • Need to understand SP better • In particular safety, slow controls • Up to now, we focus on DC-DC conversion – should we start on SP? Who? • Both Atlas pixels and strips integrate power circuitry in their new FE-chips: shunt regulators, charge pump, LDO • Seems to be a good approach - can we do the same? Serial Powering vs. DC-DC Conversion

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