Power distribution for SLHC trackers: Challenges and solutions

ARTICLE IN PRESS

0168-9002/$ - se

doi:10.1016/j.ni

�Tel.: +44 12

E-mail addr

Nuclear Instruments and Methods in Physics Research A 592 (2008) 44–55

www.elsevier.com/locate/nima

Review

Power distribution for SLHC trackers: Challenges and solutions

Marc Weber�

Rutherford Appleton Laboratory, STFC, Harwell Science and Innovation Campus, Chilton Didcot OX11 0QX, UK

Received 21 November 2007; received in revised form 18 February 2008; accepted 21 March 2008

Available online 3 April 2008

Abstract

The inner tracking detectors of the Large Hadron Collider (LHC) detectors consume tens of kilowatts of power. Getting power from

the rack supply to the detectors has proved to be challenging, inefficient and cumbersome. At Super-LHC (SLHC), the traditional

approach of delivering power for each detector module independently will fail. Solving the power distribution problem at SLHC has been

recognized as a critical challenge, and significant R&D effort is being devoted to solving it. There are a number of promising concepts,

including serial powering and parallel powering combined with local DC–DC conversion, which will be discussed in this introduction

and overview article.

r 2008 Elsevier B.V. All rights reserved.

Keywords: Power distribution; Semiconductor detectors; Particle physics

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2. Power distribution for the LHC trackers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3. The SLHC power distribution challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.1. Cable volume and material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.2. Module power consumption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.3. Power efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4. Alternative power distribution concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.1. Serial powering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2. DC–DC conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.2.1. Process selection and radiation hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.2.2. Test with commercial converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.2.3. Custom converter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.3. Piezoelectric transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5. Specifications and constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.1. Output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.2. Output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3. Dynamic output impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.4. Operation in magnetic fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.5. Radiation-tolerance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.6. Real-estate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.7. Inefficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.8. High reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.9. Minimum electromagnetic interference (EMI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

e front matter r 2008 Elsevier B.V. All rights reserved.

ma.2008.03.107

35 44 6061.

ess: [email protected]

www.elsevier.com/locate/nima

dx.doi.org/10.1016/j.nima.2008.03.107

mailto:[email protected]

ARTICLE IN PRESSM. Weber / Nuclear Instruments and Methods in Physics Research A 592 (2008) 44–55 45

6. System considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1. Risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.2. Protection features (over-current and over-voltage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.3. Slow control and monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.4. Power supply development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

1. Introduction

After 7 years of operation, the Large Hadron Collider(LHC) is expected to be upgraded to the Super-LHC(SLHC) to deliver a 10-fold increased luminosity [1]. Bythen the inner silicon pixel and strip detectors of the LHCexperiments will have suffered severe radiation damage andwill need to be replaced. The new detectors will faceunprecedented particle densities and radiation levels.Detector granularity and the number of electronic channelswill increase by factors of 2–10 depending on radius,angular region (pseudorapidity) and sensor technology(silicon pixels, short strips, long strips). Minimizing thepower consumption of the front-end electronics anddistributing power to the detectors efficiently and with aminimum volume of cables are key challenges, which mustbe met to make tracking at SLHC possible.

The electrical characteristics of the power distributionsystem have a significant impact on other detectorcomponents, e.g. the readout electronics, hybrids, modules,supermodules and the cooling system. The mass and therouting of services influence the material distribution of thedetector and thus detector performance. Power distributionis thus of relevance for the overall detector layout as well,in particular for the transition between the central (barrel)and forward (end cap) region.

The importance and urgency of solving the powerdistribution problem at SLHC and elsewhere has beenrecognized in the particle physics community and anintense R&D effort is devoted to it.

1A parallel power distribution scheme without DC–DC converters or

other means of power conversion as chosen for the CMS tracker is

equivalent to independent powering in terms of total conductor mass and

power efficiency.

2. Power distribution for the LHC trackers

While the design of LHC power supplies and powerdistribution systems has been described as ‘‘unglamorous’’,it is certainly of great importance and has proved to bechallenging. The reasons are manifold: the LHC trackers arerather power hungry with a rack power of tens to hundredsof kilowatts; power supplies and cables are expensive;conductive interference through the power lines is frequentlya dominant noise source; the intense radiation with particlefluences between 1015 and 109 n/cm2 requires placingcommercial power supplies at a large distance of the loador otherwise to develop custom supplies and qualify themcarefully for radiation and magnetic field tolerance.

Traditionally the front-end electronics of silicon detec-tors are powered by an independent power line for eachmodule (independent powering) or group of modules(parallel powering). For the ATLAS silicon tracker, thisset comprises digital power; analog power; sensor biasvoltage and power for the optical data, clock and controllinks plus the corresponding return lines and sense wires.1

This scheme leads to an impressive number and volume ofcables since thousands of modules need to be served. Fig. 1shows a representative picture of the ATLAS detectorillustrating the severity of the ‘‘cable pollution’’. Due to thesize of the LHC detectors, power cables are long (wellabove 100m for the ATLAS silicon trackers). Cable lengthand the need to minimize material lead to cable resistancesof several ohms, which in turn cause unwanted resistiveheating (I2R losses). Fifty to eighty percent of the rackpower supply output is lost in the power cables.The power distribution systems of the LHC trackers

have been described in detail in [2,3]. An overview of theirfeatures is compiled in Table 1. A power system has manycomponents including the rack power supplies; the slowcontrol and monitoring system; the power distributionsystem including cables and possibly on-detector regula-tors, converters and transformers. This paper focuses onthe power distribution scheme and on-detector powerdevices.

3. The SLHC power distribution challenge

Independent powering fails for the SLHC trackers for anumber of reasons including:

�
lack of space to feed the power cables to the trackers; � increased material; � reduced power efficiency.
3.1. Cable volume and material

A substantial increase in the volume and number ofcables and current, driven by the larger number ofchannels and modules at SLHC, would be prohibitive.

ARTICLE IN PRESS

Fig. 1. End view of the ATLAS barrel detector. The SCT is placed in the centre of the detector at a radius of 25–55 cm from the beam line. The (red) cable

strands extending from the centre to larger radii at 2, 4, 8 and 10 O’clock are the SCT power cables. (The same number of cables leaves the other end of the

detector.)

Table 1

Parameters of the power distribution systems for the LHC ATLAS and CMS silicon trackers

ATLAS pixels CMS pixels ATLAS strips CMS strips

Number of modules 1744 1440 4088 15148

Total number of channels (M) 80 66 6.2 10

Total rack power incl. optical links and cable losses (kW) 30 7 45 67

ROIC name and technology FE-I3 PSI46 ABCD APV25

0.25mm CMOS 0.25mm CMOS 0.8mm bi-CMOS 0.25mm CMOS

ROIC analog (digital) voltage (V) 1.6(2.0) 1.5(2.5) 3.5(4) 1.25/2.5(2.5)

ROIC power consumption/channel 84 mW 40mW 3.6mW 2.9mW

Total ROIC current (kA) 3.8 1.5 6 15

Cable length (one way)/resistance (round trip) �110m �50m �110m/4.5O 34–62m

Power efficiency (%) �20 �42 �50 52

Power distribution schemes IPa PPb IP PP

Local regulators (near/on-detector) Yes Yes No Yes

Some of the numbers are approximate only due to the lack of operational experience, depending on settings and the effects of irradiation. Power efficiency

is defined as the power consumed on the detector (including ROICs and optical links, but excluding sensor bias) divided by the power delivered by the

closest rack DC power supply.aIP ¼ independent powering.bPP ¼ parallel powering.

M. Weber / Nuclear Instruments and Methods in Physics Research A 592 (2008) 44–5546

Firstly, the space allocated for the services through theLHC detectors turned out to be barely sufficient for thecurrent trackers. There is simply no room to route more orthicker cables. Secondly, the SLHC trackers must not bemore massive than the current trackers, which alreadyexceed their target specifications. The aluminium powertapes of ATLAS Semiconductor Tracker (SCT) [4] forexample correspond to 0.6% of a radiation length for oneSCT barrel layer. Simple scaling of cable mass by a factorfive, as suggested by the anticipated increase in channeldensity in this detector region, would drive the cablematerial budget to 3% of a radiation length, which isunacceptable.

3.2. Module power consumption

The target technology for the readout electronics of theSLHC trackers is 0.13 mm CMOS or below. The operationvoltage of the readout integrated circuits (ROICs) will scaledown with reduced feature size to be 1.2–1.5V. While it isimportant to reduce the power consumption per channel,reducing ‘‘current consumption’’ is harder. This is due tothe increase of subthreshold leakage currents, addedfunctionality and higher readout speed. It is too early toestimate SLHC tracker power consumption reliably. Giventhe anticipated increase in the number of channels and thedifficulty of reducing the current per channel, it is however


prudent to assume the total current to at least double ortriple compared with the LHC trackers.

3.3. Power efficiency

The efficiency of the power distribution is given by1/(1+IR/V), where I is module current, R is cableresistance and V is the front-end electronic operationvoltage. Power efficiency, being already poor at LHC,could easily be as low as 10% at SLHC. (This assumes R tobe 4.5O, a typical value for the ATLAS trackers, a modulecurrent of 4A and an operation voltage of 1.5V.) Theincreased thermal losses present an additional load to thecooling system and thus add to its mass.

All this implies that tracking at the SLHC is not possiblewithout a novel approach to power distribution. Fortunately,there are several promising concepts which offer a solution.

4. Alternative power distribution concepts

The increased module current is the cause of the powerdistribution challenge. Delivering power to modules atreduced current provides the solution. There are twofundamentally different approaches to do just that:

(a)
Serial powering of modules (b) Power transmission at high DC voltage and low
currents combined with local DC–DC conversion

2This neglects the losses on the much shorter cables between adjacent

modules.

A detailed description of these approaches is givenelsewhere [5–12]. In this paper, I will only describe theirbasic features and challenges. Each requires a localregulator or converter on the module. Fig. 2 illustratesthe differences between the various power distributionschemes. For independent powering, analog and digitalpower are typically provided separately. For the alter-natives, serial powering or DC–DC conversion schemes,only one power line is needed, since analog power will bederived by series regulators on the modules. The presenceof these regulators reduces the number of power cables by afactor of two and has some other advantages, but does notdecrease total (analog and digital) current or conductormass. It does however add significantly to the total powerdissipated in the detector volume.

4.1. Serial powering

Serial powering is arguably the most unorthodoxapproach and has not yet been implemented in particlephysics experiments. In serial powering a constant currentsource provides current to a series of detector modules.Local shunt regulator and shunt transistor circuitry oneach module provide module voltage. The voltage acrossthe chain of modules is n times the module voltage and the‘‘ground’’ potential of the modules differ. The differentground levels require AC coupling or optical decoupling of

digital control and data signals. The main features of serialpowering are illustrated in Figs. 2 and 3.In serial powering the module current is ‘‘re-cycled’’ and

only one long power cable is required between the rackpower supply and the first module. Thermal losses aregiven by I2R, with I being module current and R cableresistance. For independent powering with n sets of cableswith, for the sake of the argument, each the same resistanceR, thermal losses are n times larger and so is the number ofcables.2 About 10 to 20 modules would be chained in seriesin typical applications of serial powering. The advantagesof this scheme are so great that much fewer power cableswould be required at SLHC than at LHC, despite theincrease in channel number!Prototype serial powering systems have been run for

several years with pixel and strip detectors and substantialexperience has been gained. The R&D has focussed onunderstanding noise performance and stability of serialpowering systems, design of custom regulators and under-standing of system properties. The electrical performanceof serially powered models is excellent.Interference between modules is suppressed in serial

powering systems since the supply current is constant bydesign and module voltage is derived locally. A represen-tative comparison of the noise performance with ATLASSCT barrel modules [13] powered independently and inseries in a bench test is shown in Fig. 4. Similar results havebeen obtained with pixel systems. More effort is requiredbut currently serial powering looks very promising.For a more in depth discussion of experimental resultssee Refs. [5–7].The design of custom regulators optimized for the

respective readout chips is the next crucial step to take.There are three plausible but complementary implementa-tions of serial powering, sketched in Fig. 5. The natural andconceptually simplest configuration has a single shuntregulator and shunt transistor external to the ROICs. Itrequires, however, a dedicated power chip with thecapability to sink a significant current through the shunttransistor in case of a failure condition. A dedicated powerchip can be avoided if each ROIC carries its own shuntregulator and transistor all connected in parallel. Here, thechallenge is matching and the switch-on behaviour of theshunt transistor. A situation where one transistor carriesmost of the total shunt current and could possibly bedamaged must be prevented. Finally, a configuration witha single external regulator and distributed shunt transistorsis conceivable. While this avoids putting many regulatorsin parallel, the inductance and resistance of bond wires andhybrid power traces/planes could impact the performanceat high frequencies.Within the next year, we hope to see serial powering

prototypes with up to 30 prototype silicon strip modules ina dense package. This assembly would still use commercial

ARTICLE IN PRESS

Fig. 2. Illustration of different powering schemes. In this specific example each sensor has four columns of strips. The hybrids shown carry a double row of

eight ROICs and some auxiliary electronics. There are two hybrids per sensor. Sense wires are not shown. (Top) Independent powering with digital and

analog power lines. (Middle) DC–DC conversion with independent and parallel powering. (Bottom) Serial powering.

Fig. 3. Illustration of serial powering with constant current source, shunt

regulator and transistor and AC-coupling of clock and control signals.

Only one module is shown.


electronics. In addition, several design projects of customserial powering ICs are in the pipeline and hands-onexperience will be gained with each of the configurationsintroduced above. In a year from now, our understandingof realistic output impedances, noise performance in asystem, power inefficiencies and real-estate will haveimproved greatly.

4.2. DC–DC conversion

Using local DC–DC step-down conversion is a naturaland conventional approach, which would have hugebenefits over independent powering without conversion.DC–DC conversion can be implemented with parallel orindependent powering as shown in Fig. 2.

ARTICLE IN PRESS

1350

1400

1450

1500

1550

1600

755 663 159 628 662 006Module #

<EN

C>

Independent powering Serial Powering

1000

2000

3000

4000

5000

6000

7000

0 500 1000 1500Channel #

ENC

DC-DC conversion: no shield 13 micrometer aluminium

Fig. 4. (Left) Equivalent noise charge (ENC) averaged over all channels of six SCT modules powered independently (full line) or in series (dashed line).

(Right) ENC versus channel number for a commercial air-coil DC–DC regulator providing analog voltage to a single SCT barrel module (see Section 4.2

and Ref. [8] for more details). The bold line (blue) corresponds to a DC–DC converter placed at �10mm distance above the silicon sensor without

shielding. The light line (pink) corresponds to a similar configuration but with a grounded 13-mm-thick aluminium shield between converter and sensor.

Note that the vertical axes are zero suppressed.

3The EN5360 from Enpirion.

M. Weber / Nuclear Instruments and Methods in Physics Research A 592 (2008) 44–55 49

In the later case, DC–DC conversion reduces cablethermal losses from I2R to (I/g)2R, where I and R aredefined as above and g is the converter gain: the ratio ofinput to output voltage. The number of cables is notreduced. The DC–DC conversion with parallel poweringand a single long cable of resistance R, reduces thermallosses to n(I/g)2R. If n equals g, the reduction is the same asin serial powering. When designing a detector system, cableresistance R would be chosen to represent the bestcompromise between cable mass and power efficiency.

A popular DC–DC converter is the buck converter,which consists of an inductor as an energy storage unit,switching transistors, a switch control unit and a filteringcapacitor. A possible schematic is shown in Fig. 6. Thisarchitecture offers high efficiency and is able to deliver highcurrents. An alternative is the charge-pump where anumber of capacitors are being charged up at high voltagein the first cycle and are then being discharged at lowvoltage but increased current (see Fig. 7). Gain isdetermined by the duty cycle for the buck converter andby the number and arrangement of switching capacitors forthe charge pump. Powering with DC–DC conversion is amuch more conventional approach than serial powering,and there is a wealth of experience with the design and useof DC–DC converters. For application with SLHCtrackers, the main challenges are to miniaturize DC–DCconverters, make them withstand an extreme radiationenvironment and (for buck-converters) suitable for opera-tion in a strong magnetic field. In addition, techniques tominimize and shield electromagnetic radiation (EMR)caused by switching activity must be applied and/ordeveloped. In a first investigation of possible switchingnoise a commercial air-coil buck converter operating at5MHz was placed at a short distance from an ATLAS SCTbarrel module [8]. Noise performance was measured fordifferent geometrical and shielding configurations. Fig. 4

shows that converters could easily be a dominant noisesource, but also that simple shielding measures can be veryeffective.Understanding possible interference mechanisms in de-

tail is relevant for both buck converters and charge pumpsand is one of the most urgent tasks. A complete list ofspecifications for power devices is given in Section 5.Currently several complementary R&D directions re-

lated to DC–DC conversion are pursued in parallel [8–12]:

�
Custom design of buck converters � Custom design of switching capacitor converters � Market survey and commercial prototypes � System design.
4.2.1. Process selection and radiation hardness

The DC–DC converters suitable for SLHC rely onradiation-hard switches, which operate at voltages wellbeyond those of the readout-chips. Conventional submi-cron CMOS processes with a thin gate oxide have provedto be radiation-hard to the level required at LHC, providedcertain design rules are followed [14]. These processes arehowever limited to drain-source voltages (VDS) of severalvolts. Fortunately several dedicated high-voltage technol-ogies offering maximum VDS of several tens of volts havebeen identified and NMOS and PMOS transistors indifferent layouts have been irradiated with encouragingresults [9,11]. Optimization and characterization of radia-tion-hard power transistors with low on-resistance(to increase efficiency) will remain an important focus forsome time. A commercial buck regulator3 was exposed toan ionizing dose of 100Mrad in a Co60 source in a first testand showed (maybe surprisingly) no significant change inperformance [8].

ARTICLE IN PRESS

Fig. 5. Sketch of alternative serial powering implementations in a two-module configuration with three ROICs. (Top) A single shunt regulator and shunt

transistor external to the ROICs. (Middle) Parallel shunt regulators and shunt transistors, one each in each ROIC. (Bottom) A single external shunt

regulator combined with parallel shunt transistors, one in each ROIC.


4.2.2. Test with commercial converters

The properties of a number of commercial buckconverters of interest for SLHC tracking are discussed inRefs. [8,10]. It is too early to judge the prospects ofcommercial devices for SLHC trackers, but they serve as abench mark and possible upper limit in performance(ripple, efficiency), provide insight to system design andhelp accelerate DC–DC converter R&D in general. Firstinvestigations of EMR effects and radiation-hardness with

commercial converters have been mentioned above. Toconfirm the magnetic field operation, a commercial air-coilbuck converter was placed in a 7T field. No relevantchange in performance was observed. A miniaturized PCBwith a commercial air-coil buck converter is beingconstructed to power silicon module read out by ABCDchips. This and custom devices will enable us to investigatethe severity of EMR and if fringe-fields leaking out of theair coil could be picked up by the loop formed of silicon

ARTICLE IN PRESS

Fig. 6. (Top) Simplified schematic of a buck converter powering a module. (Bottom left) ‘‘Charging up’’ phase. (Bottom right) ‘‘Discharging phase’’. The

switch control logic (pulse width modulator) is not shown.

Fig. 7. (Top) Simplified schematic of a divide-by-four charge pump powering a module. (Middle) ‘‘Charging up’’ phase. (Bottom) ‘‘Discharging phase’’.

M. Weber / Nuclear Instruments and Methods in Physics Research A 592 (2008) 44–55 51

strips [15] although first estimates suggest that this isunlikely.

4.2.3. Custom converter design

A custom switching capacitor IC was designed at LBNL[9] and became available recently. It will be a most usefultool to build prototype pixel and strip detector modulesand investigate the prospects of DC–DC conversionfor tracking detectors. Custom design with air-coilinductors has started at CERN [11]. Given a radiation-hard high-voltage process, the main challenges are devel-

oping layouts that suppress EMR and allow delivery ofhigh-currents.Within the next year, several commercial and custom

DC–DC converters would have been characterized indetail, and the basic features of DC–DC conversion forsilicon tracking should be understood quantitatively.

4.3. Piezoelectric transformers

The piezoelectric transformer was invented in the 1950s[16] but has become of significant commercial interest only

ARTICLE IN PRESS

Table 2

Specification for SLHC power regulators, converters or transformers

Required range Desirable range

Output voltage

(V)

1.2–1.8 1.2–2.8

Output current 42A 44A

Dynamic output

impedance

10O at o10MHz 0.1O at o100 kHz

0.5O at o10MHz

Magnetic field

operation

44T 44T

Radiation-

tolerance

1015 n/cm2 1016 n/cm2

100Mrad 500Mrad

Size – 100–250mm2

Inefficiency o20% �5%

Minimum EMI EMI susceptibility is detector specific. Limits for

radiative EMI (e.g. from inductor coils) are not yet

understood. For conducted EMI, 40 dbmA of

common-mode should not be exceeded in the

frequency range of 100 kHz to 30MHz.

High reliability System dependent. Targeted module power failure

rate o1% per module over 5 years of operation

See the text for explanations.


recently. A four-terminal piezo transformer (PT), like amagnetic transformer, provides ground isolation betweenprimary and secondary electrodes. A PT used for DC–DCconversion consists of four elements: a voltage-controlledoscillator and driver circuit to create the size-wave drivevoltage at the primary (input); the piezoelectric ceramic,which converts the electrical energy at the primary intomechanical energy and couples it mechanically into thesecondary, where it generates an output voltage of differentamplitude; a rectifying and filtering circuit at the output;and finally feed-back circuitry, which sets the drivingfrequency and stabilizes the output voltage (see Fig. 8). Thestrengths of the PT are its high energy storage capacity,leading to small devices; high energy efficiency; lowelectromagnetic noise (the drive wave can be sinoidal andat relatively low ultrasonic frequencies of tens to hundredsof kHz); and large gain. All this is quite relevant andattractive for detector instrumentation. A high-voltage PT-based power supply system was successfully developed forthe ATLAS thin gap muon chambers [17].

Application of PTs to power silicon tracker front-endelectronics in particle physics experiments has only beendiscussed recently, and it is too early for a full assessmentof their prospects. Some critical features of PT for silicontrackers, which are not yet well understood in the particlephysics community, have to do with the size, packagingand assembly on a hybrid, radiation length and radiationhardness of PTs. Clarification of these issues should bereasonably straightforward and is urgent. In addition,custom electronic circuitry to operate the PT transformerat high-radiation levels and in a magnetic field must bedeveloped. While the latter task is not without challenges,it is clearly feasible. Piezoelectric low-voltage DC–DCconverters are of great interest for low-voltage powerconversion and supplies in particle physics, even if theywould turn out to be impractical to mount on silicondetector modules.

5. Specifications and constraints

The compilation of Table 2 is an attempt to definespecifications for the shunt regulators, DC–DC convertersor piezo transformers for the SLHC power distributionsystems. These specifications are meant as a basis for

Fig. 8. . Sketch of a four-terminal piezoelectric transformer with voltage-con

powering a module.

discussion and guidance for designers. The challenge is notachieving ultimate performance at minimum costs, typicalfor commercial devices, but rather building devices thatoperate reliably in an extreme environment.While the constraints of magnetic field operation or

radiation-hardness are firm, some of the listed specifica-tions can only be indicative and will mature with time. Therequirements depend strongly on the number of electronicchannels of the future trackers, which will be betterunderstood when LHC background rates are measuredand the SLHC machine parameters are known. Powerdistribution R&D cannot proceed in isolation and theproperties of power devices have to match those of thereadout ICs, hybrids and other system components, whichare not yet available.

5.1. Output voltage

The recommended output voltage for the next genera-tion of power devices is 1.2–2.8 V. The lower valuecorresponds to the operation voltage of ROICs in0.13 mm CMOS technology. Output voltages of �2.8V

trolled oscillator, drive circuits and rectifier used as a DC–DC converter


are required to operate 0.25 mm CMOS electronics. This isimportant for the next few years detector prototyping.

5.2. Output current

The increased detector granularity will lead to detectormodules with more electronic channels and higher currentconsumption. Power devices capable of delivering currentslarger than 4A should, however, not lead to undueconstraints on module design. Lower currents are likelyto limit the number of ROICs that can be placed on ahybrid, possibly increase the number of hybrids permodule, which is undesirable for a number of reasons.Devices delivering less than 2A will thus be less useful.

5.3. Dynamic output impedance

The dynamic output impedance can only be specifiedrigorously knowing the power supply rejection ratio (PSRR)of the ROIC and the specifications of the remote powersupply, etc. Guided by our experience with ATLAS SCTmodules and elsewhere that a dynamic output impedance of0.1O for o100 kHz, �0.5O foro10MHz would bedesirable. The cited high-frequency value is conservative.

5.4. Operation in magnetic fields

The trackers of ATLAS and CMS are placed in asolenoidal magnetic field of 2 and 4T respectively. Themagnetic field constraint excludes the use of inductors withferromagnetic materials that saturate too early. While the2T magnetic field of the ATLAS solenoid might just be lowenough to allow the ferrites to be useful, fields of 4Tdemand air-coil inductors, which could lead to a perfor-mance and size penalty. It should be noted that there aresignificant stray fields at the end of the trackers and itwould not be advisable to rely on a design that requires afixed magnetic field direction.

5.5. Radiation-tolerance

The extreme radiation levels at SLHC are far too high formany commercial electronic devices and require use ofradiation-tolerant design rules in custom ICs. Particle fluencesof 1015 to 1016n/cm2 and ionizing doses of 100–500Mrad areexpected. The radiation levels vary strongly with distancefrom the beam axis and will be well known in several years.Devices that can only stand 1015n/cm2 could not be used on apixel sensor hybrid close to the centre of the detector.Depending on the details of regulator/converter design,robustness against Single Event Upsets could become anadditional radiation-related constraint.

5.6. Real-estate

This is a relevant but soft specification and relates to theintegration of regulators, converters or transformers on the

hybrid. An increase of 10% hybrid real estate due to theaddition of a local power supply should be acceptable. Thistranslates into a floor area of 100–250mm2 for powerdevices and auxiliary passive components. A hybrid with20 ROICs could accommodate a larger local power supplythan a 10 ROIC hybrid. The maximum acceptable heightof power regulation circuitry would for typical detectorlayer arrangements range between 5 and at most 10mm.For an unpackaged die, height is not critical.

5.7. Inefficiency

Any local power supply will hugely improve theefficiency of the overall power system. The cost is a smalllocal increase in power consumption on the module.The anticipated inefficiencies of regulators, converters or

transformers range between 5% and 20%. Given the hugeoverall gain in power efficiency this is deemed acceptable. Theadded load on the cooling system and possible damage to thelocal power supplies due to hot spots define the constraints.

5.8. High reliability

There will be no access to the inner trackers at SLHCuntil the end of data taking. Failure of the local powersupply could imply the loss of a full detector module.Minimum failure probability for a period of 5–10 years ishighly desirable. During this time the duty cycle of thepower devices is typically 30%.

5.9. Minimum electromagnetic interference (EMI)

This is a crucial requirement since the input signals ofsilicon detectors are low (possibly less than 2 fC afterirradiation at SLHC) and silicon strip sensors are prone to‘‘pick-up’’. The EMI requirements cannot be quantified inisolation. The effects of EMI depend on strip length,preamplifier specifications, shielding and geometrical or-ientation. The sources of EMI have to be identified andtheir impact on detector electric performance be evaluatedquantitatively. This is, for example, relevant for the effectsof high-frequency switching of DC–DC converters.

6. System considerations

The alternative power concepts presented in this paperare highly innovative. While the development of regulatorsor DC–DC converters is a significant challenge, much moreis required to build up a working system, in particular ifthis comprises serial or parallel powering. Some selectedsystem aspects are discussed below. A preliminary compi-lation of power system features is given in Table 3.

6.1. Risk assessment

Powering several modules in series or in parallel createsthe risk of loosing many modules simultaneously in case

ARTICLE IN PRESS

Table 3

Features of independent powering (IP), serial powering (SP) and parallel powering with DC–DC conversion (DC–DC) for SLHC trackers

IP SP DC–DC PP Comment

Power efficiency 10–20% 60–80% 60–80% Varies with I, n (SP); gain

(DC–DC)

Local regulator inefficiency N/A �10% o20% This does not include linear

regulator for analog voltage

Number of power cables 4 per hybrid

(+sense wires)

Reduction by factor 2n Reduction by factor

2n

n ¼ number of hybrids

Voltage control over individual

hybrids

Yes On/Off;

fine-adjustment

‘‘Stand-by mode’’: 2.5/1.5V

�4�0.7V; Limited fine-

adjustment

Yes On/Off; limited

fine-adjustment

New schemes have regulators;

no fine- adjustment should be

needed

Hybrid current info Yes Yes (sensing current through

power device)

Yes –

Hybrid voltage info Yes (need sense

wires)

Yes (with slow-control

circuitry on hybrid)

Yes (with slow-

control circuitry on

hybrid)

–

Floating hybrid power supplies Yes No, voltage chain No –

Protection features Separate set of

cables for each

hybrid

Local over-current protection;

redundant regulators

Not yet known Protect against open (SP) and

short (DC–DC)


of an open in the constant current loop (serial powering) orin case of a short (parallel powering). It is important to tryand assess the failure probability and the risk quantitatively.Common practice defines risk as the product of probabilityand consequence of an adverse advent. In our context theadverse advent is the occurrence of a short or an openquantified by a failure probability and the consequence isquantified by the number of lost modules. A first attempt toestimate the risk related to broken connection is discussed inRef. [6]. For both serial and parallel powering, the numberof connections which could possibly break is significantlyreduced compared with independent powering. Whenapplying a scientific risk definition, the new poweringsystems turn out to be much less risky that naivelyanticipated. In addition, the huge real-estate savings ofserial and parallel powering can be exploited for robustengineering of connections, wire-bonds, etc.

6.2. Protection features (over-current and over-voltage)

Broken connections are a relatively trivial thoughimportant cause of failure. More sophisticated failuremodes relate to the regulator and converter circuitrythemselves. These failure modes have to be identified andeliminated. One difficulty arises from the conflictingrequirements of high current (several amperes) andminimum hybrid, IC and thus ultimately transistordimensions. This can best be illustrated using serialpowering as an example, where the shunt transistor is apossible weakness of the serial powering circuitry. Instandard operation the current through the shunt transistoris small, possibly only 5% of the module current. In somefailure conditions, the shunt transistor current couldbecome as high as the total module current.

Once this failure condition has been identified, it is easyto protect against it. Either by generous dimensioning of

the shunt transistor, by placing parallel shunt transistors inthe readout ICs, or by sensing the current through theshunt transistor and reducing the voltage across it in caseof an over-current condition.

6.3. Slow control and monitoring

For independent powering schemes, voltage and currentinformation can easily be monitored at the remote powersupply through sense wires. It is desirable to avoid anexcessive number of sense wires at SLHC and for serial andparallel powering the current through individual modulescannot be sensed at the common remote power supply.Slow-control circuitry will thus be placed on silicondetector modules at the SLHC and there are many waysto implement this. The circuitry would typically consist of anumber of analog-to-digital converters (ADCs). It could beimplemented in a dedicated slow-control IC or as a blockin a data multiplexer chip. In the latter case slow-controldata would be sent along the data path of the read-out ICs.Apart from monitoring voltages, currents and tempera-tures, the slow-control commands could be used to setvoltages and to switch the power devices to stand-by.

6.4. Power supply development

The local power devices need to be powered by remotepower supplies. The specification of the latter will changecompared with the LHC. Thanks to the power distributiontechniques described here, fewer supplies will be neededand significant rack space will be saved. Specification anddevelopment of the rack supplies is not yet required forDC–DC conversion systems, which might well use com-mercial solution and devices. The situation is somewhatdifferent for serial powering since the availability ofcommercial constant current sources is limited. It would


be prudent to specify the requirements and investigate theneed for custom designs well in time.

7. Summary

Modern particle detectors are cramped with bulky andmassive cables, which constrain detector layout severelyand affect performance. Radically new approaches topower distribution are required at SLHC to make trackingpossible. Very promising concepts have been identified andare vigorously pursued in a world-wide effort. Theseinclude serial powering, parallel powering with DC–DCconversion and development of piezoelectric transformers.Most attention is devoted to developing devices like shuntregulators and DC–DC converters, which can deliver highmodule current at low voltages, while withstanding severeparticle radiation and operating in strong magnetic fields.More effort is required to understand the electricalperformance of these devices in a detector system andeventually in the design of power supplies and slow-controlsystems.

If this R&D is successful, the familiar picture of particledetectors congested by power cables will belong to the past.The trend to detector with hundreds of millions of channelsis driven both by physics requirements and the advances ofthe microelectronics industry. It is not limited to the SLHCor to tracking detectors. Our solutions are likely to findwide application in detector instrumentation for theInternational Linear Collider (ILC), at future light sources(e.g. X-FEL), in space science and elsewhere.

Acknowledgements

I would like to thank Wladek Dabrowski, SatishDhawan, Federico Faccio, Philippe Farthouat, MauriceGarcia-Sciveres, Richard Holt, Dave Lynn, StefanoMichelis, Simone Paoletti and Giulio Villani for theirinput to this article and for many helpful discussions. Iappreciate the support of the ATLAS SCT Group, whichprovided six SCT barrel modules for R&D on powerdistribution.

References

[1] The CERN Council Strategy Group, The European strategy for

particle physics, approved by CERN Council on July 2006, Lisbon,

/http://council-strategygroup.web.cern.ch/council-strategygroup/

Strategy_Brochure.pdfS, published 24 July 2006.

[2] S. Paoletti, The implementation of the power supply system of the

CMS silicon strip tracker, in: Proceedings of the Topical Workshop

on Electronics for Particle Physics (TWEPP), /http://indico.cern.ch/

event/11994S, CERN-2007-007, ISBN 978-92-9083-304-8, 2007,

pp. 377–381.

[3] P. Phillips, The ATLAS SCT power supply system, in: Proceedings

of the Topical Workshop on Electronics for Particle Physics

(TWEPP), /http://indico.cern.ch/event/11994S, CERN-2007-007,

ISBN 978-92-9083-304-8, 2007, pp. 365–368.

[4] ATLAS Collaboration, Inner Detector Technical Design Report,

CERN/LHCC/97-16 and CERN/LHCC/97-17, 1997.;

ATLAS Collaboration, The ATLAS Experiment at the CERN Large

Hadron Collider, ATL-COM-PHYS-2007-087, J. Instrument.

(submitted).

[5] D.B. Ta, T. Stockmanns, F. Hugging, P. Fischer, J. Grosse-Knetter,

O. Runolfsson, N. Wermes, Nucl. Instr. and Meth. A 557 (2006) 445.

[6] M. Weber, G. Villani, M. Tyndel, R. Apsimon, Nucl. Instr. and

Meth. A 579 (2007) 844.

[7] G. Villani, M. Weber, M. Tyndel, R. Apsimon, Serial powering of

silicon sensors, in: Proceedings of the Topical Workshop on

Electronics for Particle Physics (TWEPP), /http://indico.cern.ch/

event/11994S, CERN-2007-007, ISBN 978-92-9083-304-8, 2007,

pp.382–386.

[8] S. Dhawan, O. Baker, P. Tipton, J. Kierstead, D. Lynn, S. Rescia, M.

Weber, High-radiation resistant DC-DC converter regulators for

use in magnetic fields for LHC high-luminosity silicon trackers,

in: Proceedings of the Topical Workshop on Electronics for

Particle Physics (TWEPP), /http://indico.cern.ch/event/11994S,

CERN-2007-007, ISBN 978-92-9083-304-8, 2007 , pp. 394–398.

[9] Robert Ely and Maurice Garcia-Sciveres, DC to DC Power

Conversion, in: Proceedings of the 12th Workshop on Electronics

for LHC and Future Experiments (LECC 2006), CERN-2007-001,

ISBN 978-92-9083-288-1, 2007, pp. 89–92.

[10] S. Dhawan, D. Lynn, H. Neal, R. Sumner, M. Weber, R. Weber,

Ideas on DC-DC converters for delivery of low voltage and high

currents for the SLHC/ ILC detector electronics in magnetic field and

radiation environments, in: Proceedings of the 12th Workshop on

Electronics for LHC and Future Experiments (LECC 2006), CERN-

2007-001, ISBN 978-92-9083-288-1, 2007, pp. 442–446.

[11] S. Michelis, Inductor based switching DC-DC converter for low

voltage power distribution in SLHC, in: Proceedings of the Topical

Workshop on Electronics for Particle Physics (TWEPP), /http://

indico.cern.ch/event/11994S, CERN-2007-007, ISBN 978-92-9083-

304-8, 2007, pp. 399–403.

[12] M. Imori, Low voltage power supply incorporating ceramic

transformer, in: Proceedings of the Topical Workshop on Electronics

for Particle Physics (TWEPP), /http://indico.cern.ch/event/11994S,

CERN-2007-007, ISBN 978-92-9083-304-8, 2007, pp. 389–393.

[13] J. Carter, et al., Nucl. Instr. and Meth. A 568 (2006) 642.

[14] A. Marchioro, Deep submicron technologies for HEP, in: Proceed-

ings of the Fourth Workshop on Electronics for LHC Experiments

(LEB 98), CERN-LHCC-98-36, 1998.;

W. Snoeys, et al., Nucl. Instr. and Meth. A 439 (2000) 349;

G. Anelli, IEEE Trans. Nucl. Sci. NS-46 (6) (1999) 156.

[15] Robert P. Ely, Marc Weber, Sergio Zimmermann, Rong-Shyang Lu,

Paul J. Lujan, IEEE Trans. Nucl. Sci. NS-52 (5) (2005) 1892.

[16] C.A. Rosen, Ceramic transformers and filters, in: Proceedings of the

Electric Component Symposium, 1959, pp. 205–211.

[17] M. Imori, N. Matsui, M. Ishino, T. Kimura, T. Mieno, S. Imada, M.

Katsuno, a high voltage system with 60 high voltage power supply

channels in 2U Height EURO Crate, in: Proceedings of the 11th

Workshop on Electronics for LHC and Future Experiments (LECC

2005), CERN-LHCC-2005-038, ISBN 92-9083-262-2, 2005, pp. 336–340.

http://council-strategygroup.web.cern.ch/council-strategygroup/Strategy_Brochure.pdf

http://council-strategygroup.web.cern.ch/council-strategygroup/Strategy_Brochure.pdf

http://indico.cern.ch/event/11994









Documents

Power distribution for SLHC trackers: Challenges and solutions