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

Katja Klein 2Advantages & Disadvantages of DC-DC Conversion

• 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)


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)


Step-down layout: capacitors charged in series & discharged in parallel

n = number of parallel capacitors

Iout = nIin

r = 1 / n

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 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


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


• 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


• 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


• 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


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


• 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


MB of Whole Tracker for DC-DC Conversion


advantages & disadvantages of dc-dc conversion...

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