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

Katja KleinSerial Powering vs. DC-DC Conversion2 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

The Basic Ideas Katja KleinSerial Powering vs. DC-DC Conversion3 Conversion ratio r: r = V out / V in ! << 1 P drop = R  I 0 2  n 2  r 2 V drop = R  I 0 P drop = R  I 0 2 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

Katja KleinSerial Powering vs. DC-DC Conversion4 The Buck Converter Convertion ratio g > 1: g = V in / V out Switching frequency f s : f s = 1 / T s The “buck converter“ is simplest inductor-based step-down converter:

Katja KleinSerial Powering vs. DC-DC Conversion5 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

Implementation Examples Katja KleinSerial Powering vs. DC-DC Conversion6 PP with DC-DC conversion:Serial powering: 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

What Conversion Ratio do we need? Katja KleinSerial Powering vs. DC-DC Conversion7 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 I 2  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 Conversion ratio needed for parallel powering with DC-DC converters? 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!

Power Losses in Cables Katja KleinSerial Powering vs. DC-DC Conversion8 Consider system with n modules: P det = n  I 0  V 0 Voltage drop on cables & power loss P cable calculated within each scheme Efficiency = P det / P total = P det / (P det + P cable ) 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 SP DC-DC, r = 1/10 DC-DC, r = 1/5 Serial powering Eff. increases with n. Since modules can be chained, efficiency can be very high! 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

Local Efficiency Katja KleinSerial Powering vs. DC-DC Conversion9 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  R on 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

Compatibility with LIC Cables Katja KleinSerial Powering vs. DC-DC Conversion10 PP with DC-DC conversion 30V is largely enough For any reasonable segmentation and conv. factor currents should be lower e.g. 20 chips a 53mA per module  1.2A / module 20 modules per rod  24A /rod r = ¼  I = 6A  looks compatible Serial powering Current is small 30V allows for chains with more than 20 modules  looks compatible 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

Power Supplies Katja KleinSerial Powering vs. DC-DC Conversion11 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 Assume that power supplies will be exchanged after 10 years

Bias Voltage Katja KleinSerial Powering vs. DC-DC Conversion12 PP with DC-DC conversion Same options as for SP 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 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

Safety (I) Katja KleinSerial Powering vs. DC-DC Conversion13 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 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? Fermilab expressed interest to perform a systematic failure analysis

Safety (II) Katja KleinSerial Powering vs. DC-DC Conversion14 2.One shunt regulator + transistor per module + no matching issue - no redundany - needs high-current shunt transistor - must stand total voltage 3.One reg. per module + distributed transistors + no matching issue + some redundancy - feedback more challenging 1.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

Slow Control Katja KleinSerial Powering vs. DC-DC Conversion15 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 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 Module voltage(s) Module current(s)? Bias current

Start-up & Selective Powering Katja KleinSerial Powering vs. DC-DC Conversion16 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 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

Scalability Katja KleinSerial Powering vs. DC-DC Conversion17 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 Consequences if more modules are powered per chain or in parallel? E.g. barrel vs. end caps: different # of modules per substructure

Flexibility Katja KleinSerial Powering vs. DC-DC Conversion18 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! Flexibility with respect to combination of devices with different currents E.g. trigger vs. standard module (or 4 / 6-chips)

Potential to Provide Different Voltages Katja KleinSerial Powering vs. DC-DC Conversion19 PP with DC-DC conversion With charge pumps, only integer conversion ratios are possible With inductor-based designs, arbitrary V out < V in can be configured (but feedback circuit optimized for a certain range) Only hard requirement: V in >= V opto Analogue and digital voltage can be supplied independently  no efficiency loss 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) Chip supply voltage(es): ~ 1.2V (Atlas: 0.9V for digital part to save power) Opto-electronics supply voltage: 2.5 – 3V

Process Considerations & Radiation Hardness Katja KleinSerial Powering vs. DC-DC Conversion20 Serial powering Regulators must be rad.-hard Standard CMOS process can be used; but... HV tolerant components (up to n  U 0 ): - 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 V in, 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?

Interplay with FE-Chip Katja KleinSerial Powering vs. DC-DC Conversion21 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 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

Readout & Controls Katja KleinSerial Powering vs. DC-DC Conversion22 PP with DC-DC conversion Nothing special: electrical transmission of data and communication signals to control ICs No DC-balanced protocol needed 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 Atlas pixels, NIM A557

Noise Katja KleinSerial Powering vs. DC-DC Conversion23 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

Material Budget Katja KleinSerial Powering vs. DC-DC Conversion24 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 I 0 - 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 I 0 nr  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 V in Shielding?

Space Katja KleinSerial Powering vs. DC-DC Conversion25 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

Test Systems for Construction Phase Katja KleinSerial Powering vs. DC-DC Conversion26 PP with DC-DC conversion Electrical readout of single modules possible with adapter PCB 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

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 Katja KleinSerial Powering vs. DC-DC Conversion27 Work on Powering within CMS Tracker

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? Katja KleinSerial Powering vs. DC-DC Conversion28 Summary