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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

Tracker Upgrade Power WG Meeting

October 7th, 2008

Katja Klein

1. Physikalisches Institut B

RWTH Aachen University


  • 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

The basic ideas
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

The buck converter
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

The charge pump
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

Implementation examples
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

What conversion ratio do we need
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

Power losses in cables
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


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

Local efficiency
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

Compatibility with lic cables
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

Power supplies
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

Bias voltage
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

Safety i
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

Safety ii
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

Slow control
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

Start up selective powering
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


  • 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


  • 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

Potential to provide different voltages
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

Process considerations radiation hardness
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

Interplay with fe chip
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

Readout controls
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


  • 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

Material budget
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


  • 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

Test systems for construction phase
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

Work on powering within cms tracker
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


  • 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