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

Introduction Katja Klein2Advantages & Disadvantages of DC-DC Conversion Parallel powering of n modules with DC-DC conversion Conversion ratio r = V out / V in << 1 Total supply current: I = r  n  I 0 Power drop on cables: P drop = R  I 2 = R  I 0 2  n 2  r 2

The Buck Converter (Inductor-Based) Katja Klein3Advantages & Disadvantages of DC-DC Conversion The “buck converter“ is the simplest inductor-based step-down converter: Switching frequency f s : f s = 1 / T s Convertion ratio r < 1: r = V out / V in = D Duty cycle D = g: D = T 1 / T s

The Charge Pump (Capacitor-Based) Katja Klein4Advantages & Disadvantages of DC-DC Conversion S tep-down layout: capacitors charged in series & discharged in parallel n = number of parallel capacitors I out = n  I in r = 1 / n

Advantages: Grounding Katja Klein5Advantages & Disadvantages of DC-DC Conversion 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

Advantages: Communication Katja Klein6Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein7Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein8Advantages & Disadvantages of DC-DC Conversion Different voltages can be provided  Needed because:  V opto > V chip  V ana ≠ V dig ?  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 Katja Klein9Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein10Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein11Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein12Advantages & Disadvantages of DC-DC Conversion Converter efficiency will be around 80% (ESR of passive components, R on of transistors, switching losses)  Local generation of heat  cooling of DC-DC converters needed  Converter efficiency decreases with lower conversion factor (U out /U in )  Local efficiency decreases with higher switching frequency  In two-step schemes efficiencies multiply (0.8  0.8 = 0.64)

Disadvantages: Currents in Cables Katja Klein13Advantages & Disadvantages of DC-DC Conversion Here DC-DC conversion cannot compete with Serial Powering  Currents in power group with DC-DC conversion = I 0  n  r  I 0 = current of a single module  n = number of parallely powered modules in the power group  r = conversion ratio = U out /U in  Current in Serial Powering chain = I 0, 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 Katja Klein14Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein15Advantages & Disadvantages of DC-DC Conversion 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 = I 0  n  r; PCB traces can be narrow due to regulation capability of buck converters Achen system test PCB ~ 3cm

Disadvantages: Material Budget Katja Klein16Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein17Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein18Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein19Advantages & Disadvantages of DC-DC Conversion 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 fsfs Pos. 6.4  No converter  Type L  Type S E.g. Aachen system tests on commercial DC-DC converters:

Disadvantages: Noise Katja Klein20Advantages & Disadvantages of DC-DC Conversion  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 Katja Klein21Advantages & Disadvantages of DC-DC Conversion 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 Katja Klein22Advantages & Disadvantages of DC-DC Conversion

MB of Whole Tracker for DC-DC Conversion Katja Klein23Advantages & Disadvantages of DC-DC Conversion