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Advantages & Disadvantages of DC-DC Conversion Schemes

Advantages & Disadvantages of DC-DC Conversion Schemes. Katja Klein 1. Physikalisches Institut B RWTH Aachen University. Power Task Force Summary Meeting January 30 th , 2009. Introduction. Parallel powering of n modules with DC-DC conversion Conversion ratio r = V out / V in << 1

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Advantages & Disadvantages of DC-DC Conversion Schemes

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  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. 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 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 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 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 controlsNote: sensing voltages is not straightforward with Serial Powering 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 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/offNote: 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 & 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 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 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 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 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) 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 Advantages & Disadvantages of DC-DC Conversion

  14. 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 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 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 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 1.25V 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% 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% Advantages & Disadvantages of DC-DC Conversion

  19. Disadvantages: Noise  No converter  Type L  Type S • 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 Pos. 6.4 E.g. Aachen system tests on commercial DC-DC converters: Enpirion 2.5V at load Common mode fs Advantages & Disadvantages of DC-DC Conversion

  20. 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 Advantages & Disadvantages of DC-DC Conversion

  21. 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 Advantages & Disadvantages of DC-DC Conversion

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

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

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