HV-Supply for the ALICE-TPC Readout
Chambers: Test of the ISEG EHQ F020p High
Voltage Module
R. Renfordt1), H.R. Schmidt2)
1)
I
IKF, University of Frankfurt, 2) GSI Darmstadt
Introduction
This paper reports on test results of an ISEG EHQ F020p high voltage module.
This unit has been chosen for evaluation as the outcome of a tendering procedure
for ALICE-TPC readout chamber HV supplies carried out at GSI in spring 2000.
The specification required in the tendering and the nominal specs of the EHQ
F020p module are listed in Table 1.
requested
EHQ F020p
Vmax [V]
2000
2500
HV set resolution [V]
<1
0.04
HV output accuracy [V]
<1
±0.44
HV ripple [mV]
<50
< 20
Temperature coefficient
<12
1
<5
Imax []
200
200
current resolution [nA]
<20
4
programmable current trip
√
√
Ethernet interface
√
(√)2
CAN bus interface
√
√
Table 1
Comparison of requested and given specifications
1
For a detailed discussion of the measured temperature coefficient refer to Sec. III.4
2
Third party product
The purpose of the tests described below was to verify the HV accuracy in
terms of ripple, relative set resolution, reproducibility and the dependence on
temperature as far as these values are relevant for the proper operation for the
TPC readout chambers. It was not intended to provide a comprehensive test of
the ISEG specifications.
II
Test Setup
II.1
Hardware
The test setup is sketched in Figure 1. It consists of one 16-channel EHQ F020p
HV module, a custom made 16-channel voltage divider box, a CAMAC LeCroy
8252 scanning ADC in a CAMAC crate with a HYTEC™ CAMAC controller, a
PEAK™ CAN bus controller with parallel port interface and a PC running
Windows NT 4.0 equipped with a HYTEC™ PCI interface card. The divider box
contains a resistor chain of 150 and 1 M yielding a factor 150 downscaling of
the HV output. To match the impedance of the box to the scanning ADC the
resistor chain is followed by a buffer amplifier with gain =1. This setup allows to
monitor simultaneously the voltage and current from the internal EHQ F020p
ADC’s and from the external measurement via divider box and CAMAC ADC.
The load is similar to the one expected under ALICE running conditions (≈ 10
A). In addition, the temperature of the module and the ambient temperature
was monitored with several PT100-elements and also recorded via the LeCroy
8252 scanning ADC.
Figure 1
PC - Windows NT 4.0
CAN-bus
CAMAC-controller
16
CAN-controller
ADC LeCroy 8252
ISEG EHQ F020p
divider 150:1
Schematic sketch of the measuring setup
2
II.2
Software
A custom control program, written in BORLAND C++, allows to record the
measured parameters to disk as well as to control all other features of the HV
module. The CANbus driver routines were provided by ISEG, the CAMAC
driver routines by H. Stelzer.
1401
1400
U
meas-ISEGN [V]
1400.5
1399.5
1399
0
10
20
30
40
50
60
70
80
time [min]
Figure 2
II.3
Example of the measurement cycle for one channel
Accuracy of the Measurement
The precision of the LeCroy 8252 ADC is 12 bit for a range of 10 V, folded with
the division factor of 150 from the resistor chain, an accuracy of 366 mV/bit is
reached. The precision of the ISEG internal measurement is 50 mV/bit and 4
nA/bit for the voltage and current measurements, respectively. It should be
noted that the precision of the external measurement is only relative, since no
special selection or calibration of the resistors in the divider box was done.
3
III
Test Results
Figure 2 shows one HV channel as example of the measurement cycles: the
HV is ramped up at a fixed speed (typically 0.1  Vset/sec), then the voltage is
kept at its nominal value for a certain period of time and thereafter ramped
down again. The question under investigation is whether after repetitive
switching on/off the nominal voltage is always restored with the specified
accuracy. For the actual measurement all 16 channels of the ISEG module were
switched simultaneously.
548 on/off switches, internal (ISEG) HV measurement
1250.2
U
HV Channel # 1
nominal
=1250 V
1250.15
1250.05
U
meas/ISEG
[V]
1250.1
1250
1249.95
1249.9
0
200
400
600
800
1000
relative time [min]
Figure 3
Voltage values after switching on/off cycles for one channel
measured internally by the ISEG module
III.1
Switching On/Off
Figure 3 gives, as representative sample, the voltage of channel #1 as function
of time. Each point corresponds to a measurement following a switching off/on.
Altogether each channel was switched on/off 548 times with about 2 minutes
time between successive cycles. As can be seen from the figure the stability is ±50
mV, corresponding to the fluctuation of one LSB. Figure 4 summarizes the
measurement of all 16 channels: plotted is the distribution of 16  548
4
measurements. The plot shows that the average over both channels and the time
is within =50 mV.
These results are independently checked by the external measurement. Figure
5 shows the measured voltages from the divider box (ADCchannel  150  10000/4096 mV). The measurement of the different channels
varies up to 30 volts due to the non-calibrated resistor chains or ADC’s.
However, the fluctuation in time of the individual channels is again of ±1 LSB,
indicating that the external measurement confirms the reproducibility of the HV
setting.
548 on/off switches, 16 HV channels,
internal (ISEG) HV measurement
4000
entries
3000
2000
1000
0
1249.75
1250
1250.25
U
[V]
1250.5
ISEG
Figure 4
16 channels
Distribution of voltage value for 548 switching cycles for all
5
548 on/off switches, external (CAMAC) HV measurement
1335
1330
CAMAC
[V]
1325
U
1320
1315
1310
1305
0
200
400
600
800
1000
relative time [min]
Figure 5
Voltage values after switching on/off cycles for all 16
channels measured by the external CAMAC ADC
1310.5
HV channel #1
HV channel #2-16
1309.5
U
meas/CAMAC
[V]
1310
1309
1308.5
0
200
400
600
800
1000
time [sec]
Figure 6
Voltage values of channel #1 (red, thin curve) while
switching channel #2-16 (blue, thick curve) simultaneously from 0 to V max
(measured via the external CAMAC ADC)
6
III.2
Mutual Influence
The mutual influence of channels was tested by setting one channel to the
nominal voltage (1250V) while switching all other channels periodically from 0
to Vmax. The behavior of HV can be seen from Figure 6 indicating no significant
change in voltage of the selected channel (#1) while switching all other channels.
III.3
Settling Time
Figure 7 and Figure 8 give views of the leading edge of one cycle as in Figure 2
to demonstrate the time needed until the voltage is stable. As can be seen the HV
ramps up until it reaches a voltage about 5 Volts below Vset and then settles
slowly within about 20 second to the nominal voltage without any overshoot.
1500
U meas/ISEG [V]
1000
500
0
0
20
40
60
80
100
time [s]
Figure 7
Leading edge of one cycle
7
1410
1400
U meas/ISEG [V]
1390
1380
1370
1360
1350
1340
0
20
40
60
80
100
time [s]
Figure 8
III.4
Enlarged view of the leading edge of one cycle
Temperature Dependence
The temperature dependence, again shown as representative sample, is
depicted for channel #16 in Figure 9, Figure 10 and Figure 11. A marked
day/night variation is visible in a 24 hrs measurement both for the voltage and
the current. Both values are anti-correlated with the also measured ambient
temperature, shown as the red curve. The variation in voltage is 
and =153 mV for the internal and external measurements, respectively. The
corresponding range in temperature is Tpeak-peak ≈ 7 °C. Figure 12 shows the
same parameters as those in Figure 11, however, for an extended period of 6
days. A correlation analysis of the {Ti, Vi} samples yields temperature
coefficients, cT:=1/V  dV/dT, of 12

externally and internally (as, e.g., in Figure 10) measured voltages, respectively.
Both numbers have to be interpreted carefully, the first one because it includes
the temperature dependence of the load resistors as well as the temperature
dependence of the external measurement circuit, the latter one because it
probably is in feedback with the generated voltage. In any case, however, the
numbers are comparable with the value quoted by ISEG for the temperature
coefficient (<5

8
8.8
30
6/12/2001 11:09
6/13/2001 9:16
28
26
T [°C]
Imeas/ISEG[A]
8.795
8.79
24
8.785
22
T [ °C]
Imeas/ISEG [A]
8.78
0
5
10
15
20
25
20
relative time [hrs]
Figure 9
Time dependence of the internally measured current (black
symbols) and the measured ambient temperature (red curve). The black
curve is a fit through the measured currents.
1250.2
30
6/12/2001 11:09
6/13/2001 9:16
1250.15
28
26
1250.05
T [°C]
U meas/ISEG[V]
1250.1
1250
24
1249.95
22
T [°C]
1249.9
Umeas/ISEG [V]
1249.85
0
5
10
15
20
25
20
relative time [hrs]
Figure 10
Time dependence of the internally measured voltages (black
symbols) and the measured ambient temperature (red curve). The black
curve is a fit through the measured voltages.
9
1314.5
30
6/12/2001 11:09
6/13/2001 9:16
28
[V]
1314
T [°C]
meas/CAMAC
26
U
24
1313.5
22
T [°C]
Umeas/CAMAC[V]
1313
0
5
10
15
20
25
20
relative time [hrs]
Figure 11
Time dependence of the externally measured voltage (black
symbols) and the measured ambient temperature (red curve). The black
curve is a fit through the measured voltages.
1314.4
28
1314.2
26
1314
22
1313.6
20
T [°C]
U CMAC [V]
24
1313.8
1313.4
18
1313.2
16
1313
14
1312.8
0
1000
2000
3000
4000
5000
6000
7000
8000
time [min]
Figure 12
Time dependence of the externally measured voltage (blue
symbols) and the measured ambient temperature (red curve) for an
extended period of 6 days.
10
III.5
Low Voltage Supply Failure
The effect of dripping LV supply voltages due to potential failures in the LV
power supplies has been investigated: the 24V or the 5V supply voltages was
turned off slowly while recording the output HV. In both cases the HV switched
off rapidly when the low voltage went below a certain level. Above this level the
HV output did not change with varying low voltage. The modules stayed
switched off when bringing one or the other voltage back up, i.e., the module
delivered HV again only after setting the voltage explicitly.
III.6
Ripple
Ripple (and noise) is specified to be < 20 mV at full load for voltages above 400
V and voltage differences between channels below 1400 V. In the present case the
ripple was measured employing an AC coupled probe. All channels were set at
1250 V and loaded with 8.8 
scope used (2 mV) no ripple was detected.
IV
Summary
In summary, this specific set of measurements confirms that the module meets
the specifications required for operating the ALICE TPC readout chambers in
terms temperature variations, ripple, as well as the reproducibility of Vset.
11