1. Electromagnetic Compatibility of a Low Voltage Power Supply for the ATLAS Tile Calorimeter Front-End Electronics G. BLANCHOT CERN, CH-1211 Geneva 23, Switzerland
G. Blanchot1, U. Blumenschein4, I. Hruska1, I. Korolkov4, B. Palan1, J. Pontt3, A.Toro3, G. Usai2
1CERN, CH-1211 Geneva 23, Switzerland 2Enrico Fermi Institute, University of Chicago, USA 3UTFSM, Valparaiso, Chile 4IFAE, Universitat Autonoma de Barcelona, Spain
12th Workshop on Electronics for LHC and Future Experiments September 2006, Valencia, Spain
September 27th, 2006.

2. Tile Calorimeter Barrel Calorimeter Module Module Finger Drawer
Tilecal is organized of a central barrel and two endcaps. In total, it is built with modules that contain drawers with the front end electronics. The drawers are inserted inside the girder of each module. A mechanical assembly called finger closes the girders. Inside the finger is located the front end power supply discussed here.
Tilecal: one barrel (6m), 2 endcaps (3m) drawers with front end electronics power supplies, 300W each.
LECC 2006
G. Blanchot, CERN

3. Power distribution scheme
DCS
AUX
Auxiliary Power
200VDC Power
1:4
1:4
CANBus
200VDC Power
+3.3V 4.6A DIGI
DRAWER Up to 45 PMTs
+5V A DIGI
+5V 16.5A MB
Auxiliary Power
+15V 0.8A MB
Bulk power supplies that deliver 200VDC are located in the USA15 control room. Auxiliary power supplies and the DCS system are located there too. The 200VDC is brought to the supports of the calorimeter, where it is split in 4. Each split line is routed on the calorimeter surface up to the finger.
The drawer is not provided with regulators: accurate voltage levels are required for the proper operation of its eletcronic. For this reason, the power supply is located in the vicinity of the drawer.
The power supply is water cooled.
-5V A MB
Converters
+5V HV 0.3A
ELMB
+15V HV 0.5A
- 15V HV 2.7A
LECC 2006
G. Blanchot, CERN

4. Digitizers (up to 45 channels, 40Msps)
System Noise
PMT
Digitizers (up to 45 channels, 40Msps) … 45 1 n
Fast Readout DAQ
GLink
Mux and ADC (250kbps)
Slow Readout PC
CANBus
Shaper
Integrator
Must be able to resolve muons:
Must be able to resolve minimum bias events at low luminosity:
The tilecal frontend consists of two paths. Fast readout, that uses 40 Msps digitizers to sample the shapers output. The data is transmitted over GLink to a DAQ system. Slow readout that mutiplexes the output of slow integrators and digitizes them at 250 kbps. The data is sent over canbus to a dedicated monitoring system.
The fast readout must be able to resolve muons, for this the RMS of each channel pedestals shall not exceed 1.5 counts on the highest gain.
The slow readout must be able to resolve minimum bias events at low luminosity, for this the pedestals RMS from any of the gains shall not exceed 2 counts.
The system noise is contributed by the internal noise (can’t be corrected), by external couplings (limited) and by the power supply noise. This latter dominates. To reduce the system noise, the power supply noise must be minimized with adequate mitigation techniques.
Internal noise: limited by design.
External couplings: shielded by module steel, limited effect.
Power supply noise: dominant contribution. To reduce the system noise, the power supply noise must be understood in order to apply the most appropriate mitigation method.
LECC 2006
G. Blanchot, CERN

5. Focus on Fast Readout Noise
PMT
Digitizers (up to 45 channels, 40Msps) … 45 1 n
Fast Readout DAQ
GLink
Mux and ADC (250kbps)
Slow Readout PC
CANBus
Shaper
Integrator
We first look at the fast readout system. The shaped pulses are sampled at 40 Msps; 7 samples are required to estimate accurately the deposited charge. Therefore, the fats readout will be affected by noise starting at few MHz. Noise components blow the MHz are seen as pedestals shifts only.
The fast readout samples shaped PMT pulses at 40Msps. As a result of this, the sensitive bandwidth is expected to be above few MHz and up to the limited of conducted noise phenomenons, in the form of common mode currents.
LECC 2006
G. Blanchot, CERN

6. DC/DC Converters Common Mode Measurement on Single Outputs
Simplified diagram
Only 2 converters shown.
LF Common Mode Current
Calibrated current probe.
To measure the current that returns through other converters that share same ground.
LF current does not enter the drawer electronics: limited disturbance.
Current compared with reference ‘Temporary’ power supply.
DrawerElectronics
ICM
ICM/2
The common mode current is expected to be the dominant noise source for the system. It is measured first on each converter output with a calibrated current probe and an EMI receiver. The switching noise of each converter is sent into the outputs of the other converters that share the same return line inside the drawer. The converters are beating against each other, however this current can hardly reach the drawer electronics because it lacks a low impedance return path.
LECC 2006
G. Blanchot, CERN

7. DC/DC Converters Common Mode Measurement on All Outputs
HF Common Mode Current
To measure HF current that returns through stray capacitances to the steel.
Stray capacitances coupled to electronics: full disturbances due to CM current.
Current compared with reference ‘Temporary’ power supply.
Simplified diagram
Only 2 converters shown.
ICM
ICM
DrawerElectronics
At higher frequencies, the impedance of the stray capacitances between the calorimeter steel, the drawer electronics and the power supply, will become small enough to allow the CM current to return that way. In addition to this, the inductance of the return lines of the other converters increase. Therefore, the high frequency CM current enters the drawer and deteriorates the noise performance of the fast readout eletcronics.
ICM
ICM
ICM
LECC 2006
G. Blanchot, CERN

8. Common Mode Couplings LECC 2006 G. Blanchot, CERN 200VDC Power
Auxiliary Power
+3.3V 4.6A DIGI
Drawer
DC / DC
+5V A DIGI
CStray
INoise_HF
+5V 16.5A MB
+15V 0.8A MB
-5V A MB
INoise_LF
ELMB
+5V HV 0.3A
+15V HV 0.5A
The different paths are shown here, between the converters, the steel and the drawer.
- 15V HV 2.7A
CANBus
LECC 2006
G. Blanchot, CERN

9. DC/DC Converters Measurement on Single Outputs (LF)
Limit
- Both LVPS exhibit large switching noise up to 2 MHz
HF noise is 20 dB below the limit.
Temporary LVPS shows up to 40 dB less HF noise than FLVPS.
FLVPS
Temp
The measurement on individual outputs shows a switching noise current as large as 80 dBuA, that is 10mA amplitude, at 300 kHz for the FLVPS, and at 158kHz for the temporary LVPS. All harmonics up to 5 MHz are clearly visible. Above that frequency, the FLVPS exhibits clearly 40 dB more noise current than the temporary LVPS. Of course, the temporary LVPS is fitted with filters, while the FLVPS does not have them. It must be noted that both power supplies are “as bad” on the switching frequency domain, and that above few MHz, both are comfortably below the usual limit of 39 dBuA.
LECC 2006
G. Blanchot, CERN

10. DC/DC Converters Measurement on All Outputs (HF)
- Switch noise does not enter the drawer: peaks are largely reduced.
HF noise is almost within the limits, but it will be shown that the drawer is very susceptible in that frequency range: CM currents lower than 10uA (20dBuA) are in fact required!
Temporary LVPS shows up to 40 dB less HF noise than FLVPS.
Limit
FLVPS
Temp
The measurement of the CM current on the entire bundle corresponds to the CM current that enters the drawer and does not return through cables but through the steel. Below 1 MHz only a small fraction of the switch noise actually enters the drawer (30 dBuA versus 80 dBuA, that is less than 1%). However above 2 MHz, the FLVPS exhibited much larger noise than the temporrary LVPS, but not exceeding the limit yet.
LECC 2006
G. Blanchot, CERN

11. Pedestals RMS inside Drawer
Patch Panel
The disturbance is maximum close to the patch panel and to the LVPS.
The deeper into the drawer, the less CM current, as it couples capacitively to the steel.
At the end of the drawer, no CM current and no disturbance.
The measurements on pedestals show that close to the patch panel (close to the power supply), the RMS of pedestals is much degraded when compared with the same measurement made with the temporary LVPS. The disturbance is stronger next to the patch panel, where the CM current is at its maximum. At the end of the drawer, where almost all CM current was already coupled to the steel, the disturbance is small.
LECC 2006
G. Blanchot, CERN

12. Pedestals Distribution
FLVPS with HF noise degrades the Gaussian property of the pedestals distribution.
The Temporary LVPS allows to get a fully Gaussian distribution.
Distribution tails are tagged as LVPS induced noise.
The comparison of the pedestals distribution indicates that the gaussian property of the distribution is lost when using the FLVPS. This would have a strong impact on the ability to resolve muons (small signals).
LECC 2006
G. Blanchot, CERN

13. Filtering: Chokes
One CM choke on each converter output allows to reduce efficiently the common mode current on all the frequency range.
A 10A rated chokes is 40mm diameter. Non negligible space required, not available in FLVPS envelope.
The usual solution to block the CM current is to add a common mode choke on each output. However, the chokes must be sized to sustain the full differential current, and in low voltage, large current applications they tend to be large. Here chokes from Schaffner, rated for 10A and that were 40 mm diameter, were tried. The performance was much improved, the CM current is raised below the 20dBuA user limit on almost the entire range, the gaussian property is recovered. However, 8 chokes would be needed, they don’t fit in the tight envelope of the FLVPS.
LECC 2006
G. Blanchot, CERN

14. Filtering: Ferrites
The addition of ferrites on each converter output allows to reduce efficiently the common mode current on the high frequency range.
They are efficient to reduce the fast readout noise below 1.4 counts.
The switch noise is not attenuated efficiently, but it has no impact on the measured fast readout pedestals RMS anyhow.
A good alternative is to use selected ferrites. Those are not very effective against the switching noise because their impedance is usually too small in that frequency range. But it was understood that the switch noise does not enter the drawer. The ferrites are effective to raise the CM impedance above few MHz. The effect is seen on the EMI spectrum, where the CM current is brought below the user limit of 20 dBuA in the range of interest. The gaussian property is restored, and an RMS better than 1.4 counts is obtained.
The solution based on ferrites was retained and is currently implemented on all the drawers.
LECC 2006
G. Blanchot, CERN

15. Focus on Fast Readout Noise
The slow readout samples time multiplexed signals levels whose bandwidth is limited to 100Hz. The scan of 45 levels is performed in 10 msec. Low frequency disturbances, mainly in the form of voltage ripple, are expected to degrade the scan accuracy.
PMT
Digitizers (up to 45 channels, 40Msps) … 45 1 n
Fast Readout DAQ
GLink
Mux and ADC (250kbps)
Slow Readout PC
CANBus
Shaper
Integrator
Let us now look at the slow readout path. Here, the outputs of current integrators whose bandwidth is limited to 100 Hz is time multiplexed. Each output is therefore applied to the ADC for 200 usec for sampling and transmission over canbus. Therefore this system is susceptible to be disturbed by very low frequency noise (DC to few tens of kHz). In this range, only the differential noise (ripple) is able to enter the drawer and to cause observable disturbances.
LECC 2006
G. Blanchot, CERN

16. Low Frequency Noise
+3.3V 4.6A DIGI
+5V HV 0.3A
+15V HV 0.5A
- 15V HV 2.7A
+5V A DIGI
+5V 16.5A MB
+15V 0.8A MB
-5V A MB
Converters
ELMB
All converters are rated for 150W output power, but some are only slightly loaded.
Poorly loaded converters were operated in discontinuous mode, that is the load is too weak, and the converter is turned on and off at very low frequency.
This results in large ripple, that is suscpetible to disturb the low frequency circuits.
Large ripple voltages were recorded on the outputs of the converters that were less loaded. In fact, all the converters were rated for a maximum power of 100W. When running on small loads, the converter is turned off periodically to avoid overvoltages on the output. This is known as discontinuous mode and causes non negligible ripple, beyond specification. Amplitudes as large as 500 mVpp at 500 Hz were observed.
500 mVpp, 500 Hz ripple observed.
LECC 2006
G. Blanchot, CERN

17. 10 msec time multiplexed scan
Correlation Chart
The correlation factors chart for the time multiplexed scan allows to detect periodic low frequency disturbances.
These disturbances are attributed to the discontinuous mode of operation of some converters.
Channel
2 msec periodic noise, 500 Hz.
Channel
10 msec time multiplexed scan
The reduction of the power range of the converter puts it back in continuous mode of operation, that is the switching circuit is not turned off anymore.
This was achieved by reducing the switch frequency by a factor 2 in the lightly loaded converters.
The use of preload resistors are also effective, at expenses of efficiency and cooling.
The analysis of the pedestals data by means of correlation charts allowed to put in evidence the presence of a low frequency periodic disturbance on the slow integrator readout path. The simplest but effective solution consists to preload with resistors the outputs of the concerned converters. This however creates a thermal problem, that is to cool down the preload resistors. As an alternative, the power range of the affected converters was reduced: this was achieved reducing the main switching frequency by a factor 2.
LECC 2006
G. Blanchot, CERN

18. Conclusions
The fast readout system of the drawers was found to be very susceptible to HF common mode currents. A limit of only 10 uA between 2 MHz and 100 MHz was defined to achieve the required noise performance.
The use of ferrites is effective to reach this limit, despite the large switching noise produced by the converters.
The slow readout system of the drawers is susceptible to low frequency differential mode voltages (ripple).
The tuning of the converter such that they operate at least at 10% of their full load allowed operating them in continuous mode, with reduced ripple.
The presented method, that is the characterization of the converters properties and of the susceptibility ranges of the drawers allowed to tag the coupling mechanisms and to apply adequate filters and tunings, independently of the grounding configuration.
The fast readout system of the drawers was found to be very susceptible to HF common mode currents. A limit of only 10 uA between 2 MHz and 100 MHz was defined to achieve the required noise performance.
The use of ferrites is effective to reach this limit, despite the large switching noise produced by the converters.
The slow readout system of the drawers is susceptible to low frequency differential mode voltages (ripple).
The tuning of the converter such that the operate at at least 10% of their full load allowed operating them in continuous mode, with reduced ripple.
The presented method, that is the characterization of the converters properties and of the susceptibility ranges of the drawers allowed to tag the coupling mechanisms and to apply adequate filters and tunings, independently of the grounding configuration.
LECC 2006
G. Blanchot, CERN