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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 = rnI0 • Power drop on cables: Pdrop = RI2 = RI02n2r2 Katja Klein Advantages & Disadvantages of DC-DC Conversion 2 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 3 The Charge Pump (Capacitor-Based) Step-down layout: capacitors charged in series & discharged in parallel n = number of parallel capacitors Iout = nIin r=1/n Katja Klein Advantages & Disadvantages of DC-DC Conversion 4 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 5 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 6 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 7 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 8 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 9 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 10 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 11 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 (0.80.8 = 0.64) Katja Klein Advantages & Disadvantages of DC-DC Conversion 12 Disadvantages: Currents in Cables • 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 Katja Klein Advantages & Disadvantages of DC-DC Conversion 13 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 14 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 = I0nr; 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 15 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 Katja Klein Advantages & Disadvantages of DC-DC Conversion 16 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) Katja Klein Advantages & Disadvantages of DC-DC Conversion 17 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 Katja Klein Advantages & Disadvantages of DC-DC Conversion 18 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 Katja Klein Advantages & Disadvantages of DC-DC Conversion 19 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 Katja Klein Advantages & Disadvantages of DC-DC Conversion 20 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 21 Back-up Slides Katja Klein Advantages & Disadvantages of DC-DC Conversion 22 MB of Whole Tracker for DC-DC Conversion Katja Klein Advantages & Disadvantages of DC-DC Conversion 23