Advantages & Disadvantages
of DC-DC Conversion Schemes
Katja Klein
1. Physikalisches Institut B
RWTH Aachen University
Power Task Force Summary Meeting
January 30th, 2009
Introduction
• Parallel powering of n modules with DC-DC conversion
• Conversion ratio r = Vout / Vin << 1
• Total supply current: I = rnI0
• Power drop on cables: Pdrop = RI2 = RI02n2r2
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The Buck Converter (Inductor-Based)
The “buck converter“ is the simplest inductor-based step-down converter:
Switching frequency fs:
fs = 1 / Ts
Duty cycle D = g:
D = T1 / Ts
Convertion ratio r < 1:
r = Vout / Vin = D
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The Charge Pump (Capacitor-Based)
Step-down layout: capacitors charged in series & discharged in parallel
n = number of parallel capacitors
Iout = nIin
r=1/n
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Advantages: Grounding
• Standard grounding scheme
 Module ground potentials are all the same
 Common ground reference for bias, analogue and digital voltage for
whole substructure (rod, petal)
 Bias voltage ground reference is the same for all modules
Note: in Serial Powering (SP) bias is referenced to “local ground“, which can differ by
several tens of volts between first and last module on a substructure.
 Easier for slow controls
Note: sensing voltages is not straightforward with Serial Powering
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Advantages: Communication
• Readout and control scheme is very standard
 Standard DC-coupled communication (LVDS, data readout etc)
Note: with SP, modules must be AC-coupled to outside world due to missing ground
 Thus no need for DC-balanced protocols
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Advantages: Start-Up & Selective Powering
• Easy start-up
 If separate DC-DC converters are used for control chips, the controls
can be powered on first
 If one converter is employed per module, individual modules can be
powered on/off
 In a scenario with one charge pump employed per chip, individual
chips can be powered on/off
Note: with SP, the whole chain is powered on at once from a constant current source PS.
If a module needs to be bypassed, its current must be shunted and burned in regulators,
which leads to inefficiency.
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Advantages: Different Voltages
• Different voltages can be provided
 Needed because:
 Vopto > Vchip
 Vana ≠ Vdig ?
 Buck-type converters: the same converter chip can be configured for
different output voltages
 Via a resistive bridge
 Two conversion steps can be combined
 No efficiency loss
 Note: with SP linear regulaters must be used to decrease the operation voltage
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Advantages: Flexibility
• Great flexibility with respect to
 combination of modules with different load
 Different numbers of readout chips
 Trigger modules vs. standard modules
 power groups with different number of modules
 End cap vs. barrel
Note: with SP the current is fixed to highest current needed by any
chain member  chains must be uniform to avoid burning power in regulators
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Advantages: Changing Loads
• Compatibility with changing loads, relevant for
 pixel detector
 load is driven by occupancy
 trigger modules
Note: in SP the highest current potentially needed must always be provided  inefficiency
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Disadvantages: Chip Technology
• Need for a “high voltage“ tolerant process (> 10-12V)
 ... which is radiation hard!
 Good candidate identified, radiation hardness still to be fully proven
IHP (Frankfurt/Oder, Germany) SiGe BiCMOS process (SGB25VD)
 Strong dependency on foundry: support of process over years?
 Any changes in process must be followed closely and irradiation
tests be repeated
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Disadvantages: Converter Efficiency
• Converter efficiency will be around 80%
(ESR of passive components, Ron of transistors, switching losses)
 Local generation of heat
cooling of DC-DC converters needed
 Converter efficiency decreases with lower conversion factor (Uout/Uin)
 Local efficiency decreases with higher switching frequency
 In two-step schemes efficiencies multiply (0.80.8 = 0.64)
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Disadvantages: Currents in Cables
• Here DC-DC conversion cannot compete with Serial Powering
 Currents in power group with DC-DC conversion = I0nr
 I0 = current of a single module
 n = number of parallely powered modules in the power group
 r = conversion ratio = Uout/Uin
 Current in Serial Powering chain = I0, independent of n
 E.g. for 20 modules in power group need r = 20 to compensate
 Higher efficiency in SP (up to FE)  less cooling needed
 Cables inside tracker volume can be thinner with SP
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Disadvantages: Risks
• We have to stick with parallel powering
 Multiplicity (modules per cable) as today or higher
 Open connections (e.g. at PP1) lead to loss of power group
 Short on module leads to loss of power group
 Protection needed? Use DC-DC converter to switch off module?
 Converter can break: can imagine isolated failures (loss of
regulation...) and failures that lead to loss of power group (short)
 More risky if one converter powers several modules
 Do we need redundancy?
 This adds mass
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Disadvantages: Material & Space
• Material budget and space considerations
 Additional material:
PCB area, chip, air-core inductor, resistors, filter capacitors, maybe other filter
components, shielding?
 Material savings:
amount of copper in cables scales with current = I0nr;
PCB traces can be narrow due to regulation capability of buck converters
Achen system test PCB
~ 3cm
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Disadvantages: Material Budget
Components simulated in CMSSW:
 Kapton substrate with 4 copper layers
 Copper wire toroid
 Resistors & capacitors
 Chip
J. Merz, Aachen
TEC
1 buck conv. / module
Motherboards
Analog
OptoHybrids
Kapton
circuits
FE-hybrids
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Material Budget for DC-DC Conversion
• Gain for TEC was evaluated
ICB electronics: -38.1%
Multi Service cables: -39.2%
Electronics & cables: -16.6%
TEC total MB: -4.4%
• Assumptions
1 converter per module
 located close to module
 r = 1/8
• Cross sections of conductors for
1.25V and 2.5V scaled with 1/8
• Motherboards “designed“ for
a maximal voltage drop of 1V
(converters can regulate)
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Material Budget for Serial Powering
• Additional componets:
ICB electronics: -48.7%
Multi Service cables: -61.6%
Electronics & cables: -30.7%
TEC total MB: -7.8%
chip, Kapton/copper circuit,
caps for AC-coupling, resistors
for LVDS, bypass transistor
• Gain for TEC was evaluated
• Assumption: all modules on
a petal powered in series
• One cable per petal (4A)
• Motherboards “designed“ for
a maximal voltage drop of 1V
(regulators) and 4A
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Disadvantages: Noise
• DC-DC converters are noise sources by design
 Conductive noise through cables
 Ripple on output voltage: switching frequency (1-5MHz) + higher harmonics
are in the bandpath of the amplifier
 Switching leads to high frequency noise (tens of MHz, not so critical)
 Common Mode and Differential Mode contributions
E.g. Aachen system tests on commercial DC-DC converters:
Enpirion
2.5V at load
Common mode
 No converter
Pos. 6.4
 Type L
 Type S
fs
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Disadvantages: Noise
 Radiated noise
Aachen system tests
 From inductor near field via inductive
(and capacitive?) coupling
 From cables
 No converter
 Solenoid
 Wire toroid
 Strip toroid
 Has to be taken into account for all aspects of electronics system design:
readout chip, FE-hybrid, grounding & shielding, motherboard, layout ...
 Not clear what to prepare for: noise depends on implementation
 For same chip, noise emission can be rather different depending on PCB etc.
 Scalability from a lab system to the complete detector not obvious
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Summary
• DC-DC conversion powering schemes offer many advantages
 Modularity, flexibility, classical system design incl. grounding etc.
• Main issues to be adressed:
 Identification of HV-tolerant chip process with required radiation hardness
 Noise has to be brought under control
• Next natural steps:
 Development of chip(s) in final technology, optimization for high efficiency
 Realistic system tests with SLHC tracker hardware
 Optimization wrt material budget
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Back-up Slides
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MB of Whole Tracker for DC-DC Conversion
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