N30-454

2008 IEEE Nuclear Science Symposium Conference Record

A control and monitor system
for serial powering of silicon strip detectors at SLHCY. Ikegami, K. Hanawa, K. Hara, T. Kohriki, K. Matsukuma, S. Terada and Y. Unno
Abstract- A serial powering scheme is a promising solution toreduce the total power, number of cables, passive detectormaterial and overall the cost for large-scale silicon tracking
detectors. We present a control and monitor system for a serial
powering scheme where the low-voltage lines of detector modules
are chained in series and the power is supplied by a single
current source. The voltage for each module is regulated using a
local shunt regulator. We propose to implement MOS FETs tomonitor and control to optimize the current passing through theshunt regulator. They also function as a bypass in case where themodule in a chain needs to be excluded.

current
sourceI. INTRODUCTION
serial powering scheme is considered as a solution toA reduce
the total power dissipation, number ofcables~

passive detector material and the overall cost for large-scale
silicon tracking detectors~ such as a silicon semiconductor
tracker (SCT [1]) at the super-LHC (SLHC). In such a scheme~
the low voltage lines of multiple modules are chained in series
and the power is supplied by a single current source. The
voltage for individual module is provided by a local shunt
regulator. Intensive studies are being carried out~ withemphasis on noise performance [2~3]. The modules areoperational by serial powering with no indication of any extra
noise increase in comparison by conventional independent
powering.
We present a control and monitor system for a serial
powering scheme~ where low power operation is aimed at~thus reducing the heat generation. The power supply current istuned to minimize the current passing through the shunt
regulators by monitoring the shunt current. We propose tointroduce MOS FETs for this purpose. They can also serve a
flexible configuration for module calibration to increase thenecessary current. They can function to bypass the current in
the case where the modules in a chain become un-operational.

Fig. I. Schematic diagram of serial powering.

Fig. 2 shows a simplified diagram of the module voltage~current~ and shunt current~ current passing through the shunt
regulator~ as a function of the source current. The blue dashedline indicates the critical operating point~ below which themodule voltage~ for example~ is not stabilized and DAQ
communication error occurs. Above the orange dashed line
region, the shunt regulators waste the power, generatingexcessive heat. Keeping the source current at the middle, e.g.
on the green dashed line ensures a stable operation at aminimum power consumption. In the actual systems~ theoptimized operation should be dependent on the processingfrequency (for example on the particle occupancy). We
therefore propose to implement a monitoring system of theshut current to realize the optimum operation.

II. SERIAL POWERING-Module Voltage- - Module Current- - Shunt Current

A serial powering we consider is shown schematically inFig. 1. N modules are chained in series where a single current
source is used for providing the power. The individual module
voltage is regulated using a local shunt regulator.

Source Current

Manuscript received November 14, 2008.
Y. Ikegami (e-mail: ikegami@post.kek.jp). T. Kohriki, S. Terada and Y.
Unno are with the lPNS, High Energy Accelerator Research organization~
KEK, Tsukuba, Ibaraki 305-080 I, Japan.
K. Hanawa , K. Hara and K. Matsukuma are with the Institute of Pure andApplied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan..

978-1-4244-2715-4/08/$25.00 ©2008 IEEEFig. 2. Source current dependence of module and shunt current.

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We have manufactured a test board to evaluate thefunctionality of the proposed system.
A. Shunt Regulator

A circuit diagram of the shunt regulator is shown in Fig. 3.
It consists of a programmable shunt regulator IC (TL43IA)
and a PMOS FEr (2SJ494) as a current booster.

required. The module voltage will not exceed Vo in both cases.
Therefore, the system can deliver the power to other modules,
even if stuck modules exist in the chain, as far as the system isallowed to deliver large enough current.

Module Voltage- - Module Current- - Shunt Current

Module ConductanceFig. 4. Module conductance dependence of module and shunt current.

A scheme of bypassing stuck modules, shown in Fig. 5, isefficient in reducing the un-necessary power consumption.
The switches are the PMOS FETs introduced as current
boosters, shown in Fig. 3. The bypassing was achieved bykeeping the gate voltage Vg minus against the drain voltage.
In this condition the PMOS FET works as a low Ron switch.
The gate resistance Rg works as a current limiter in bypassing
operation.
Fig. 3. Shunt regulator circuit. The voltages at three lines with arrow aremonitored.
Shunt RegUlator

B. Shunt Current Monitor

Since the most of the current passing through the circuit is acontribution from the current booster FET (2SJ494), themeasurement of the drain current of the FET is sufficient forthe shunt current monitoring. The drain current of the FET
could be obtained from measuring the gate (Vg), drain (Vd)and source (Vs) voltages, using a lookup table which wasmeasured beforehand. The voltage difference between thedrain and source corresponds to the module voltage. The
module voltage and the shunt current can be measured ingeneral using three wires per module (as shown in Fig. 3). In
this system, the voltage between the i-th drain and (i-I )-thsource can be estimated, since there are only pure Ohmiccomponents. We reduce the number of monitoring wires from
3n to 2n+ I for n modules.

C. Bypass Scheme
In the serial powering system, the influence of stuck
modules needs to be resolved. Fig. 4 shows a simplified
diagram for such a case as a function of module conductance.
We assume that modules work normally between the blue andthe green lines. Below the blue line, modules are in "openfailure mode", where the shunt regulator could manage thevoltage but wasting the power a lot. On the other hand, above
the green line, modules are in "short failure mode", where theregulator can provide the module voltage (=Vo) less thancurrent
sourceFig. S. Schematic diagram of bypass scheme for serial powering.

D. AC Coupling

In the serial powering, the ground levels are different in theAC coupled LVDS are used formodules in chain.
communication in order to isolate the module grounds fromthe general ground of the DAQ system. Fig. 6 shows a circuit
diagram of AC coupled LVDS signaling for data andclock/command communications.

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Data

ModuleDAQ
Clock &CommandFig. 7. SCT module connected to serial powering PC board.

Fig. 6. AC coupled LVDS circuit.

III. ENC MEASUREMENTS
The noise performance with the proposed system is
evaluated using ATLAS SCT modules [1]. Fig. 7 shows anSCT module (green box) and a serial powering PC board (redbox). Each SCT module is equipped with 12 readout front-endchips, each with 128 readout channels. The silicon microstrip
electrode, 12 cm long, is AC coupled to the amplifier. The
equivalent noise charge (ENC) measured in the SCT systemusing the standard SCTLV module was about 1,500e.
The SCT module requires an analog (+3.5V) and a digital(+4V) power. In the proposed system, the digital power isregulated by the shunt regulator directly and the analog poweris doubly regulated through a low voltage drop voltage
regulator (TPS71 01). Ambient temperature of modules andshunt regulators was kept at 15°C. Temperature of readout
hybrids was about 32°C, which was measured by thermistors
attached on the hybrids. High voltages to the sensors, served
by four isolated high voltage power supplies, were fixed to200V.

A. Source Current Dependence
Fig. 8 compares the chip averaged ENC values in the serial
powering (SP) and the conventional individual powering (lP).
In this measurement single module was operated. The current
in the individual powering scheme was about 1.5 A. In theserial powering scheme, we measured ENC data withchanging the source current. The ENC was observed to change
by a few percent during the measurement period of several
hours, due to accumulation of surface charge on the SCTsensor. We evaluated this effect as the difference in two ENC
measurements at I = 1.6 A. No significant difference wasfound between the two powering schemes. Note that thepreamp gain was about 5% lower at I = 1.4 A, reaching thecritical point, and communication error occurred below I = 1.3
A.
1600
1550
I

1500 r - - . . . _ - - - - - - - - - - - - - - - - - - - 'SP 1=1.4A_ SP 1=1.5A-~ 1450 f-------- .---._-------t~-------j • SP 1=1.6A
~

1400

=

I

SP 1=1.8A- SP 1=2.0A

r--------------=--------~

1350 . , - . . . . - . . . - - - - - - - - - - - - - - - - - 4
1300~_--'--o__ _ _ _ J._ _ _ l __6-IP_ _ L_ _ _L___ _10

12

Chip ID

Fig. 8. ENC values for different source current values in the serial
powering (SP) in comparison with individual powering (IP).

An example of shut current monitoring is shown in Fig. 9
where Vsg is measured as a function of the power supply
current. Also plotted is the shut current, which was measured
beforehand as a function of Vsg. The optimum power supply
current of 1.6 A is located above the threshold in terms of theshunt current. In the monitoring scheme, we propose tomeasure Vsg and feedback to the power supply to keep theshunt current at the optimum.

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...

1600
1550

1.6

0.8 en
c:

~

0.6 ;

zw:::3'"~ 1.2bOII)> 0.8/

0.4

o,/~/~

~

1400

1350

0.2~

1300

l- Bypass M1&4
• Bypass M1&3

-IP

_ _...L-_----L.-_---1.._ _6..L--_~_10___J12Chip ID

2.52

Bypass M3&4

I Ioo

1.5- Bypass M3• Bypass M4(,) 1450

c:

0.4

- Bypass M1

1500

Power Supply Current [A]Fig. 11. ENC values of the module at SP2 for different configurations
where one or two modules were bypassed.

Fig. 9. Shunt current Monitoring.

B. Dependence on the Module Location

IV.

Fig. 10 shows the ENC data in the serial powering scheme
where four SCT modules (SP1-4) were operated at a power
supply current of 1.6A. To eliminate the individual module
dependence, we measured the ENC for a same module,
replacing it at different positions in the serial powering
configuration. In the "SP 1" configuration, the target module
was placed nearest to the current source supply ground. In the"SP4" configuration, the module was operated with thehighest voltage levels. No significant difference was found inthese configurations.
1600
1550
- SPl V=VO xl_ SP2 V=VO x2~ 1500(,) 1450

~

1400

1350
1300

1-.
....

•~

•

1------------------"1EFFECTS OF PIN HOLE FAILURE

A couple of hybrids will be installed in an SCT short strip
module for the SLHC application. A serial powering operation
may be applied among these hybrids, providing different
ground potentials in one module. This will not cause any
problem as far as the AC coupling is preserved between theamplifier and the strip. However, pinholes in the oxide layer
cannot be excluded completely, tying the corresponding strips
to their local grounds. We investigated the influence of suchchannels tied to the DC offset in the serial powering
environment. Fig. 12 shows an equivalent circuit diagram for areadout channel DC or AC coupled. The left side corresponds
to the sensor and the right a simplified bipolar preamp part.
An external voltage of Vex simulates the DC offset between
the sensor and preamp grounds. The current from Vex was
limited by Rbias, such as lex = Vex / Rbias.

• SP3 V=VO x3
SP4 v=vo x4
L------=I'----P_ _---'_ _L - _ - - I . . - _ - - L _ - - - - - - l_ _-'------_---'

o61012Chip ID

VeeFig. 10. ENC values shown for four module positions in the SP.

C. Dependence on Bypassed Modules
The ENC data were taken in configurations where one or
two modules were bypassed. The ENC results are shown in
Fig.II. The module to be measured was set in the "SP2"
configuration. "Bypass M1" means that the module with thelowest voltage levels was bypassed. "Bypass MI & M4"means that both modules at the lowest and highest voltage
levels were bypassed. No significant difference was foundbetween the six bypass configurations we tested. The voltage
between the drain and source of the PMOS FET which wasbypassed was found to be about 50 mV at a -20V gate
voltage.

VexFig. 12. Equivalent circuit for DC or AC coupled readout channel in serial
powering at the ground level Vex.

A. Effects ofDC Coupled Strip
Fig. 13 shows the ENC data for a simulated DC coupled
strip with changing the DC offset Vex. The results for AC
coupling case for the same strip are also plotted for2609

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comparison. Typical uncertainty size is attached to the AC
coupled data, which was derived from repeated ENC
measurements. The plotted ENC data are the same within
uncertainty. The data could not be obtained for Vex < -1 V orVex> 4 V due to fitting errors in the ENC calculation. In fact
the bipolar transistor at the first preamp stage could not set thebias in working region. In Vex < -1 V, the base-emitter
voltage of transistor could not activate the transistor. In Vex>
4 V, or lex> 3 flA, the base current of the transistor exceededa normal base current of about 3 flA. The results conclude thatthe strips DC coupled to the local ground at higher level will
not function properly.

by measuring the shunt current of the regulator. We detailed
the method of monitoring the shunt current and effectivenessof a bypass scheme for stuck modules. In a test system with
serial powering, the SCT modules were operated stably
without introducing any extra noise. We found small
dependence of the noise level on the source current, groundvoltage levels, number and locations of bypassed modules.
The noise is degraded on and around the DC coupled strips if
a higher ground voltage is applied to the DC coupled strip.
The influence is, however, limited and in most configurations
the degradation is not significant.
REFERENCES
[1]
[2]
[3]

1600
1550

A. Abdesselarn et aI., Nucl. Instr. and Meth. A568 (2006) 642.
D.B. Ta, et aI., Nucl. Instr. and Meth. A 557 (2006) 445.
M. Weber, et aI., NucI. Instr. and Meth. A 579 (2007) 844.

,....., 1500
~o 1450zw
1400

1350
1300-2-101

24Vex [V]Fig. 13. ENC values for AC or DC coupled strip, which is tied to thepotential of Vex.

B. Influence to Neighbor StripFig. 14 shows the ENC data for the strips neighboring to theDC coupled strip. The DC offset Vex was varied. The locationof the DC coupled strip is at Ch.O. The neighboring strips did
not show significant increase for the DC offsets between -25Vand 25V. For the larger offset, several strips at neighborshowed increased ENC values but the other strips are notinfluenced.
2000
1900
1800
1700
::!: 1600..... Vex=-30Voas 1500
1400 ~~iJq~D~:J!rI~:ialitJ~~
1300
1200
1100
1000

..... Vex=-25V• Vex=-20V
Vex=OV--- Vex=+20V..... Vex=+25V..... Vex=+30V..... Standard-10-5o10ChFig. 14. ENC values around the DC coupled strip, of which the local
ground was varied by Vex.

V. SUMMARY
A control and monitor system is presented which canminimize the power consumption in a serial powering system

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