Advantages & Disadvantages of DC-DC Conversion...
Transcript of Advantages & Disadvantages of DC-DC Conversion...
Advantages & Disadvantages
of DC-DC Conversion Schemes
Power Task Force Summary Meeting
January 30th, 2009
Katja Klein
1. Physikalisches Institut B
RWTH Aachen University
Introduction
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• Parallel powering of n modules with DC-DC conversion
• Conversion ratio r = Vout / Vin << 1
• Total supply current: I = rnI0
• Power drop on cables: Pdrop = RI2 = RI02n2r2
The Buck Converter (Inductor-Based)
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The “buck converter“ is the simplest inductor-based step-down converter:
Switching frequency fs:fs = 1 / Ts
Convertion ratio r < 1:
r = Vout / Vin = D
Duty cycle D = g:
D = T1 / Ts
The Charge Pump (Capacitor-Based)
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Step-down layout: capacitors charged in series & discharged in parallel
n = number of parallel capacitors
Iout = nIin
r = 1 / n
Advantages: Grounding
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• 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 modulesNote: 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
Advantages: Communication
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• 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
Advantages: Start-Up & Selective Powering
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• 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.
Advantages: Different Voltages
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• 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
Advantages: Flexibility
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• 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
Advantages: Changing Loads
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• 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
Disadvantages: Chip Technology
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• 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
Disadvantages: Converter Efficiency
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• 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 (0.80.8 = 0.64)
Disadvantages: Currents in Cables
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• Here DC-DC conversion cannot compete with Serial Powering
Currents in power group with DC-DC conversion = I0nr
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
Disadvantages: Risks
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• 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
Disadvantages: Material & Space
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• 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 = I0nr;
PCB traces can be narrow due to regulation capability of buck converters
Achen system test PCB
~ 3cm
Disadvantages: Material Budget
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Components simulated in CMSSW: Kapton substrate with 4 copper layers Copper wire toroid Resistors & capacitors Chip
FE-hybrids
Kapton
circuits
Analog
Opto-
Hybrids
Mother-
boards
TEC
1 buck conv. / module
J. Merz, Aachen
Material Budget for DC-DC Conversion
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• Gain for TEC was evaluated
• 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)
ICB electronics: -38.1% Multi Service cables: -39.2%
TEC total MB: -4.4%Electronics & cables: -16.6%
Material Budget for Serial Powering
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• Additional componets: 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
ICB electronics: -48.7% Multi Service cables: -61.6%
Electronics & cables: -30.7% TEC total MB: -7.8%
Disadvantages: Noise
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• 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
Enpirion
2.5V at load
Common mode
fs
Pos. 6.4
No converter
Type L
Type S
E.g. Aachen system tests on commercial DC-DC converters:
Disadvantages: Noise
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Radiated noise
From inductor near field via inductive
(and capacitive?) coupling
From cables
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
No converter
Solenoid
Wire toroid
Strip toroid
Aachen system tests
Summary
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• 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
Back-up Slides
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MB of Whole Tracker for DC-DC Conversion
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