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

Tracker Upgrade Power WG MeetingOctober 7th, 2008

Katja Klein1. Physikalisches Institut BRWTH Aachen University

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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

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Conversion ratio r:r = Vout / Vin ! << 1 Pdrop = RI0

2n2r2

Vdrop = RI0

Pdrop = RI02

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

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The Buck Converter

Convertion ratio g > 1:g = Vin / Vout

Switching frequency fs:fs = 1 / Ts

The “buck converter“ is simplest inductor-based step-down converter:

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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

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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?

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• 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

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

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• Consider system with n modules: Pdet = nI0V0

• Voltage drop on cables & power loss Pcable calculated within each scheme

• Efficiency = Pdet / Ptotal = Pdet / (Pdet + Pcable)

• 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 10-20

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

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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

Compatibility with LIC Cables

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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

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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

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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)

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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)

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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

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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

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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

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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

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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

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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• 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

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Serial powering• Regulators must be rad.-hard• Standard CMOS process can be

used; but...

• HV tolerant components (up to nU0): - 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?

Interplay with FE-Chip

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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

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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

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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

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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?

Space

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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

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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

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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?

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Summary