1. System Test Measurements with DC-DC Converters
Katja Klein
1. Physikalisches Institut B
RWTH Aachen University
Tracker Upgrade Power Task Force
November 24th, 2008

2. System Test Measurements with DC-DC Converters
Caveat
Results are obtained with the APV Other front-end chips might behave differently.
Not all effects are fully understood.
This is work in progress.
Katja Klein
System Test Measurements with DC-DC Converters

3. System Test Measurements with DC-DC Converters
The APV25
f = 1/(250nsec) = 3.2MHz
Katja Klein
System Test Measurements with DC-DC Converters

4. System Test Measurements with DC-DC Converters
The APV25
1.25V
2.5V *
* is connected to 2.5V since about 2000
Katja Klein
System Test Measurements with DC-DC Converters

5. System Test Measurements with DC-DC Converters
The APV25
1.25V
2.5V
Katja Klein
System Test Measurements with DC-DC Converters

6. On-Chip Common Mode Subtraction
128 APV inverter stages powered from 2.5V via common resistor (historical reasons)  mean common mode (CM) of all 128 channels is effectively subtracted on-chip
Works fine for regular channels which see mean CM
CM appears on open channels which see less CM than regular channels
CM imperfectly subtracted for channels with increased noise, i.e. edge channels
pre-amplifier
inverter
V250
V250
R (external)
V125
vCM
strip
vIN+vCM
vOUT = -vIN
VSS
Node is common to all 128 inverters in chip
Katja Klein
System Test Measurements with DC-DC Converters

7. System Test Measurements with DC-DC Converters
System Test Setup
TEC petal with InterConnect Board
Four ring-6 modules powered & read out
Petal housed in grounded metall box
Optical readout
Optical control communication
Thermally stabilized at +15°C
Final components (spares)
Official DAQ software
6.4
6.3
6.2
6.1
Katja Klein
System Test Measurements with DC-DC Converters

8. Definitions and Analysis
Raw (or total) noise = RMS of fluctuation around pedestal value
Common mode (CM) = common fluctuation of subset of strips, calculated per APV
The raw noise includes the common mode contribution
One ring-6 module has four APVs a 128 strips = 512 strips
Most tests performed in peak mode
Modules are biased with 250V
A typical noise distribution {
1 APV
Higher noise on edge channels
(e.g. from bias ring)
Open channels
Katja Klein
System Test Measurements with DC-DC Converters

9. System Test with DC-DC Converters
Measurements with commercial buck converters with internal ferrite-core inductor
Katja Klein
System Test Measurements with DC-DC Converters

10. Commercial Buck Converter EN5312QI
Enpirion EN5312QI:
Small footprint: 5mm x 4mm x 1.1mm
High switching frequency: fs  4 MHz
Unfortunately modest conversion ratio (r): Vin = 2.4V – 5.5V (rec.) / 7.0V (max.)
Most measurements done with Vin = 5.5V  r(2.5V) = 0.45; r(1.25V) = 0.23
Sufficient current: Iout = 1A
Integrated planar inductor
Internal inductor in MEMS technology
Katja Klein
System Test Measurements with DC-DC Converters

11. Integration into TEC System
4-layer adapter PCB (W. Karpinski)
Plugged between Tracker End Cap (TEC) motherboard and FE-hybrid
2 converters provide 1.25V and 2.5V for FE-hybrid
Input and output filter capacitors on-board
standard caps; low ESR caps tested later, no improvement
Input power external or via TEC motherboard
Katja Klein
System Test Measurements with DC-DC Converters

12. Integration into TEC System
L-type:
Larger flat PCB,1 piece
TEC motherboard
(InterConnect Board, ICB)
Front-end hybrid
S-type:
Smaller inclined PCB, 2 pieces
Katja Klein
System Test Measurements with DC-DC Converters

13. Voltage Ripple (L Type)
Without load:
With load (I = 0.7A):
Vin = 6V
Vin = 6V
High frequency
ringing
50mV
V2.50
V2.50
10mV
Ripple with
switching
frequency
V1.25
V1.25
10mV
200ns
 100mVpp high frequency ringing on input
 10mVpp ripple with switching frequency on output
Katja Klein
System Test Measurements with DC-DC Converters

14. System Test Measurements with DC-DC Converters
Frequency Spectrum
Standardized EMC set-up to measure Differential & Common Mode noise spectra (similar to set-up at CERN) PS
Line impedance
stabilization network
Load
Load
Spectrum analyzer
Spectrum Analyzer
Copper ground
plane
Current probe
Enpirion
2.5V at load
Common mode
Enpirion
2.5V at load
Differential Mode fs
Katja Klein
System Test Measurements with DC-DC Converters

15. Raw Noise with DC-DC Converter
 No converter
 Type L
 Type S
Pos. 6.4
 No converter
 Type L
 Type S
Pos. 6.4
Pos. 6.4
 No converter
 Type L
 Type S
Raw noise increases by up to 10%
Large impact of PCB design and connectorization
Broader common mode distribution
Most optimization studies performed with L
Katja Klein
System Test Measurements with DC-DC Converters

16. Raw Noise on Different Positions
 No converter
 Type L
 Type S
Pos. 6.2
On position 6.1 type L does not fit
Preliminary
 No converter
 Type L
 Type S
 No converter
 Type L
 Type S
Pos. 6.3
Pos. 6.4
Preliminary
Preliminary
Katja Klein
System Test Measurements with DC-DC Converters

17. PCB Layout & Connectorization
---- No converter
---- L type
---- S type
Pos. 6.4
 Some variation within types, but L and S are clearly different
---- No converter
---- L type with S-connector
---- Worst S type
---- Best S type
Mean = Mean =
 =  =
Pos. 6.4
 Indication that difference comes from impedance of “bridge“ connector
Katja Klein
System Test Measurements with DC-DC Converters

18. Edge Channels with DC-DC Converters
On-chip CM subtraction: “real“ CM, i.e. before subtraction, visible on open and edge channels
Huge increase of noise at module edges and open strips with DC-DC converter
This indicates the real CM
Pos. 6.4
 No converter
 Type L
 Type S
Katja Klein
System Test Measurements with DC-DC Converters

19. System Test Measurements with DC-DC Converters
Module Edge Strips
V125
V250
VSS=GND
APV25 pre-amplifier
[Mark Raymond]
strip
bias ring
connect to V125
Edge strips are capacitively coupled to bias ring
Bias ring is AC coupled to ground
If V125 is noisy, pre-amp reference voltage fluctuates against input
Bias ring AC-coupled to V125  strip and pre-amp reference fluctuate together, lower noise
[Hybrid]
Katja Klein
System Test Measurements with DC-DC Converters

20. Comparison between Supply Voltages
 No converter
L10
L11
L10, 1.25V from converter
L11, 2.5V from converter
zoom on edge channels
Either 2.5V or 1.25V line powered via converter; other line powered normally
Overall noise increases when converter powers 2.5V
1.25V powers ~only pre-amp; noise on 1.25V subtracted by CM subtraction
2.5V used also after inverter stage; this contribution cannot be subtracted
Edge strip noise increases more if converter powers 1.25V
Katja Klein
System Test Measurements with DC-DC Converters

21. System Test Measurements with DC-DC Converters
Cross-Talk
Adjacent modules powered with EN5312QI converters  study correlations between pairs of strips i, j (r = raw data):
corrij = (<rirj> - <ri><rj>) / (ij)
Without converters
With converters on all positions
Pos. 6.4
Pos. 6.4
Strip number
Strip number
Pos .6.3
Pos .6.3
Correlation coefficient
Correlation coefficient
Pos .6.2
Pos .6.2
Pos .6.1
Pos. 6.1
Strip number
Strip number
High correlations only within single modules (= common mode)
No cross-talk between neighbouring modules
Katja Klein
System Test Measurements with DC-DC Converters

22. Noise vs. Conversion Ratio (Vout/Vin)
Vout fixed to 1.25V & 2.5V  change of Vin leads to change of conv. ratio r = Vout / Vin
Output ripple Vout depends on duty cycle D (D = r for buck converter):
Mean noise per module
Pos. 6.4
Type L
Typical Vin for system test
Mean noise increases for decreasing conversion ratio
Katja Klein
System Test Measurements with DC-DC Converters

23. Combination with Low DropOut Regulator
Low DropOut Regulator (LDO) connected to output of EN5312QI DC-DC converter
Linear technology VLDO regulator LTC3026
LDO reduces voltage ripple and thus noise significantly  Noise is mainly conductive and differential mode
Would require a rad.-hard LDO with very low dropout
 No converter
 Type L without LDO
 Type L with LDO, dropout = 50mV
Katja Klein
System Test Measurements with DC-DC Converters

24. Other Commercial Converters: MIC3385
Are we looking at a particularly noisy commercial device?  Micrel MIC fs = 8 MHz Vin = 2.7–5.5 V footprint 3 x 3.5 mm2
 No converter
 Enpirion L-type
 Micrel L-type
 No evidence that EN5312QI is exceptionally noisy
Katja Klein
System Test Measurements with DC-DC Converters

25. System Test with DC-DC Converters
Measurements with commercial buck converters with external air-core inductor
Katja Klein
System Test Measurements with DC-DC Converters

26. External Air-Core Inductor
Enpirion EN5382D (similar to EN5312QI) operated with external inductor:
Air-core inductor Coilcraft SMJLB; L = 538nH
Ferrite-core inductor Murata LQH32CN1R0M23; L = 1H
From data sheet
Air-core inductor
Ferrite-core inductor
10.55mm
6.60mm
5.97mm
Katja Klein
System Test Measurements with DC-DC Converters

27. External Air-Core Inductor
 No converter
 Internal inductor
 External ferrite inductor
External air-core inductor
Ext. air-core inductor + LDO
 No converter
 Internal inductor
 External ferrite inductor
 External air-core inductor
Pos. 6.2
radiative part
conductive part
 No converter
 Internal ind.
 Ext. ferrite ind.
 Ext. air-core ind.
Pos. 6.3
(Conv. is on 6.2)
Huge wing-shaped noise induced by air-core inductor, even on neighbour-module
Conductive part increases as well
Both shapes not yet fully understood
LDO leads only to marginal improvement
Katja Klein
System Test Measurements with DC-DC Converters

28. System Test Measurements with DC-DC Converters
Correlations
corrij = (<rirj> - <ri><rj>) / (ij)
APV 4
Per APV:
two halfs of 64 strips
strips within each half are highly correlated (90%)
but two halfs are strongly anti-correlated (-80%)  linear, see-saw like CM
50% correlations also on module 6.3 (no converter)
Pos. 6.4 (with conv.)
Pos. 6.3 (no conv.)
Katja Klein
System Test Measurements with DC-DC Converters

29. Radiative Noise from Air-Core Coil
Module “irradiated“ with detached converter board
Mean noise of 6.4
[ADC counts]
Reference without converter
Increase of mean noise even if converter is not plugged  noise is radiated
Noise pick-up not in sensor but close to APVs
Katja Klein
System Test Measurements with DC-DC Converters

30. External Air-Core Toroids
 No converter
 Solenoid
 Wire toroid
 Strip toroid
Pos. 6.4
Preliminary
Toroid wound from copper wire
Toroid wound from copper strip
As expected, toroid coils radiate less than solenoids
Significant improvement already observed with simple self-made toroid coil
Katja Klein
System Test Measurements with DC-DC Converters

31. Shielding the DC-DC Converter
Enpirion with air-core solenoid wrapped in copper or aluminium foil
Skin depth at 4MHz: Cu = 33m; Al = 42m
No further improvement for > 30m, thinner foils should be tried
No diff. betw. floating / grounded shield  magnetic coupling
Shielding increases the material budget
Contribution of 3x3x3cm3 box of 30m Al for one TEC: 1.5kg (= 2 per mille of a TEC)
 No converter  No shielding
 30 m aluminium
 35 m copper
No converter  Copper shield
 Aluminium shield
Katja Klein
System Test Measurements with DC-DC Converters

32. System Test Measurements with DC-DC Converters
Distance to FE-Hybrid
 No converter
 Type L
 Type S‘
Type S‘ + 1cm
Type S‘ + 4cm
The distance between converter and FE-hybrid has been varied using a cable between PCB and connector
Sensitivity to distance is very high
Conductive noise is decreased as well due to filtering in the connector/cable
Place converter at substructure edge?
Pos. 6.4
Type L with Solenoid
Type S‘ with Solenoid
Type S‘ 4cm further away
Katja Klein
System Test Measurements with DC-DC Converters

33. Combination of Measures
zoom on edge channels
Can get as good as without converter when toroidal coils are shielded and combined with an LDO.
Katja Klein
System Test Measurements with DC-DC Converters

34. System Test Measurements with DC-DC Converters
Summary and Worries
With DC-DC converters powering the APV25, noise is an issue:
noise ripple on power lines (conductive)
rapidly varying magnetic near field (radiative)
Possible countermeasures:
Filtering: low pass, LDO, CM filter
Shielding
Distance
A clever combination of these countermeasures will probably work
These countermeasures add complexity, cost, mass to the system
Noise-tolerant design required for FE-chip, hybrids, PCBs; DC-DC conversion does NOT factorize from remaining system
Noise performance should be robust and should scale with system size
Katja Klein
System Test Measurements with DC-DC Converters

35. System Test Measurements with DC-DC Converters
Back-up Slides
Katja Klein
System Test Measurements with DC-DC Converters

36. System Test with DC-DC Converters
Measurements with custom DC-DC converters
Katja Klein
System Test Measurements with DC-DC Converters

37. The CERN SWREG2 Buck Converter
Single-phase buck PWM Controller with Int. MOSFET dev. by CERN (F. Faccio et al.)
HV compatible AMI Semiconductor I3T80 technology based on 0.35 m CMOS
Many external components for flexibility
PCB designed by RWTH Aachen University
Vin = 3.3 – 20V
Vout = 1.5 – 3.0V
Iout = 1 – 2A
fs = 250kHz – 3MHz
S. Michelis (CERN) et al., Inductor based switching DC-DC converter for low voltage power distribution in SLHC, TWEPP 2007; and poster 24 at TWEPP 08.
Katja Klein
System Test Measurements with DC-DC Converters

38. The CERN SWREG2 Buck Converter
PCB located far away from module  only conductive noise measured
SWREG2 provides only 2.5V to FE-hybrid
Noise increases by about 20%
8-strip ripple structure understood to be artefact of strip order during multiplexing; converter affects readout stages of APV
No converter 2.5V via S type
0.60 MHz
0.75 MHz
1.00 MHz
1.25 MHz
Vin = 5.5V
Order in APV after MUX
Physical order
Katja Klein
System Test Measurements with DC-DC Converters

39. System Test Measurements with DC-DC Converters
The LBNL Charge Pump
Simple capacitor-based step-down converter: capacitors charged in series and discharged in parallel → Iout = nIin, with n = number of parallel capacitors
At LBNL (M. Garcia-Sciveres et al.) a n = 4 prototype IC in 0.35 m CMOS process (H35) with external 1F flying capacitors has been developed (0.5A, 0.5MHz)
P. Denes, R. Ely and M. Garcia-Sciveres (LBNL), A Capacitor Charge Pump DC-DC Converter for Physics Instrumentation, submitted to IEEE Transactions on Nuclear Science, 2008.
Katja Klein
System Test Measurements with DC-DC Converters

40. System Test Measurements with DC-DC Converters
The LBNL Charge Pump
Two converters connected in parallel: tandem-converter
fs = 0.5MHz (per converter)
In-phase and alternating-phase versions
Tandem-converter connected either to 2.5V or 1.25V
Noise increases by about 20% for alternating phase
 No converter
 In-phase on 2.5V
 Alt-phase on 2.5V
Katja Klein
System Test Measurements with DC-DC Converters

41. 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:
D = T1 / Ts
Convertion ratio r < 1:
r = Vout / Vin = D
Can provide relatively large currents (several Amps)
Technology must work in a 4T magnetic field  since ferrites saturate at lower fields, air-core inductors must be used
Need to find compromise between high efficiency (low fs) and low noise (high fs)
Discrete components (coil, capacitors, ...) need space and contribute to material
Katja Klein
System Test Measurements with DC-DC Converters

42. 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 In →
Inductorless technology
Everything but capacitors is integrated on-chip  low mass, low space
Provides relatively low currents (~ 1A)
Fixed “quantized“ conversion ratio (e.g. r = ½)
Output voltage cannot be controlled by feedback loop
Switching noise
Katja Klein
System Test Measurements with DC-DC Converters