1. Advantages & Disadvantages of DC-DC Conversion Schemes
Katja Klein
1. Physikalisches Institut B
RWTH Aachen University
Power Task Force Summary Meeting
January 30th, 2009

2. Advantages & Disadvantages of DC-DC Conversion
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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

3. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

4. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

5. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

6. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

7. 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.
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

8. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

9. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

10. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

11. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

12. 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)
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

13. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

14. Advantages & Disadvantages of DC-DC Conversion
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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

15. 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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

16. 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
Mother-
boards
Analog Opto-Hybrids
Kapton circuits
FE-hybrids
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

17. Material Budget for DC-DC Conversion
ICB electronics: -38.1%
Multi Service cables: -39.2%
Gain for TEC was evaluated
Assumptions
1 converter per module
located close to module
r = 1/8
Cross sections of conductors for V and 2.5V scaled with 1/8
Motherboards “designed“ for a maximal voltage drop of 1V (converters can regulate)
Electronics & cables: -16.6%
TEC total MB: -4.4%
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

18. Material Budget for Serial Powering
ICB electronics: -48.7%
Multi Service cables: -61.6%
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
Electronics & cables: -30.7%
TEC total MB: -7.8%
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

19. Advantages & Disadvantages of DC-DC Conversion
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 fs
Pos. 6.4
 No converter
 Type L
 Type S
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

20. Advantages & Disadvantages of DC-DC Conversion
Disadvantages: Noise
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
Aachen system tests
 No converter
 Solenoid
 Wire toroid
 Strip toroid
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

21. Advantages & Disadvantages of DC-DC Conversion
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
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

22. Advantages & Disadvantages of DC-DC Conversion
Back-up Slides
Katja Klein
Advantages & Disadvantages of DC-DC Conversion

23. MB of Whole Tracker for DC-DC Conversion
Katja Klein
Advantages & Disadvantages of DC-DC Conversion