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Test of the STAR Time-of-Flight START Detector MOSFET PMT Bases.
by
Vahe Ghazikhanian
UCLA Department of Physics and Astronomy
(Tests Performed on November 9, 2005)
This report presents the results of the HV tests performed on new PMT bases for the
STAR TOF START detectors. The bare printed circuit boards (no components mounted
on the boards) were tested at up to a maximum of 6 KV. The voltages and associated
currents applied to the board were measured concurrently. There were no abrupt changes
in current and no corona was observed at up to 6000 volts. The current monotonically
increased to about 2.5 nA +/- 0.18 nA at 6 KV. Two Bertan HV supplies were used to
supply a maximum of 3KV each. Red RG58 cables with SHV connectors were used to
transmit the HV to the test setup. In what follows a brief description of the test procedure
is described.
Test Conditions:
The printed circuit board had no components mounted. The 4-layer 1.6” diameter FR4
printed circuit board prototype (same as final fabrication design) was coated with 0.005”0.010” layer of KonForm (produced by Chemtronics) HV conformal coating (dielectric
breakdown voltage for KonForm is 1100 V/0.001”). After about 2 hours of curing the
printed circuit board under test was connected to the test setup. Figure 1 represents the
schematic of the test setup.
Figure 1 schematics of the test setup.
Humidity 55%
Temperature 70.5 degrees F
Testing period 2 hours at maximum voltage (6KV).
The PC board was connected to +/- HV using 1 G ohm high voltage current limiting
resistors in series, and on positive side an additional 100 K ohm current sensing resistor
was added in series to the 1 G ohm limiting resistor.
A Fluke model 89 IV multimeter was used to measure the voltage drop across the 100 K
ohm resistor (the meter’s voltage measurement resolution is 1 uV and accuracy of better
than 0.1%).
Two Bertan model 313A high voltage power supplies were used to supply positive and
negative voltages to the board. This was done since we had no HV supplies that could
supply more than 3KV. The HV was transmitted to a special plastic panel which had
SHV connectors and banana receptacles (to allow one to connect the voltmeter to the
current sensing resistor) mounted on. All resistors and solder joints in the system were
covered with two layers of heat shrink tubing. Areas which could not be covered in this
manner were sealed with RTV sealant. The HV was connected to the PC board via 5 KV
rated silicone wires. One wire was further covered with heat shrink tubing.
The area where the wires were soldered to the PC board was also covered with conformal
coating, and nothing else. The design requires that this area be further covered with
silicone sealant (as in the prototype unit); however, in order to give clear access to these
joints (which are most vulnerable to electrical discharge) this additional protective layer
was not applied to the test board.
High voltage was initially applied after about 2 hours of application of the conformal
coating (full curing time for the conformal coating is 24-48 hours) . The observed
voltages ranged from a maximum of 0.35 mV (= 3.5 nA) which drop over a 10 minute
period to 0.25 mV (2.5 nA) at an applied HV of 6 KV. During the 2 hour test no smoke,
discharges or corona were observed. Figures 2 and 3 show photographs of the test setup
and its components.
.
Figure 2. A photograph of the test setup showing the plastic panel with SHV connectors,
the voltmeter, red RG 58, cables, and the PC board under test held by a isolated vice.
Figure 3. A photograph of the setup showing the power supplies, the voltmeter, humidity
and temperature monitor and part of the vice holding the PC board.
Printed Circuit Board Leackage Current
3
Current (nA)
Current (nA)
2.5
Linear (Current (nA))
2
I = 0.0005V - 0.6105
1.5
1
0.5
0
0
1000
2000
3000
4000
5000
6000
Applied Voltage (Volts)
Figure 4. A plot of the printed circuit board leakage current vs. applied voltage. Note that
voltage drop in the limiting resistors have been ignored.
7000
Figure 4 shows the result of the measurements performed after 72 hours (allowing 72
hours for the conformal coating to fully cure). Please note that the linear fit superimposed
on the data is only valid for the range where the data was taken (i.e., 1400 – 6000 volts).
For the purpose of this analysis we have ignored the voltage drop in the current limiting
resistors in the above plot (~5 volts at 6 KV).
In order to assess the limit of accuracy of these measurements one needs to consider the
thermal noise contribution of the 2 G ohm limiting resistors given by:
Thermal
VRMS
 4KTRf
where 4KT = 1.63 x 10-20 W/Hz (at 70.5 degrees F), R = resistance in ohms, and  f is
frequency bandwidth in Hz. Using 2 G ohm total resistance, and a bandwidth of 10 Hz
we obtain a voltage noise contribution of ~18 uV which would translate into a current
noise of 0.18 nA across the sense resistor. To this one needs to add external noises.
Therefore, this number is to be used as a figure of merit of what is the accuracy of this
setup. The leakage current presented in figure 4 also includes the leaks in the SHV
connectors, RG59 cables and other components of the setup. The uncertainty introduced
by the additional leakage currents due to the components of the setup could have been
reduced if a single power supply was used for these measurements.
As seen from the plot in figure 4, there are no discontinuities in the current (eg current
spikes) as a function of voltage. This rules out electrical discharge and corona effects.
In fact the fitted line represents a constant effective resistance of 2 x 1012 ohms which is
consistent with surface resistivity of FR4 (~2 x 1011 to 1013 ohms depending on the
production process).
In order to make sure that the system could observe discharge or corona, the conformal
coating where the cables were attached to the PC board were scraped off slightly to
expose one HV conductor. The HV was slowly increased from 1400 volts to 6000 volts.
At approximately 4400 volts visible electrical discharges started in the vicinity of the area
where the cable was exposed (no other place on the board exhibited this behavior). As the
HV increased to 6 KV the rate of discharge increased from a few per minute to several
Hz at 6 KV. The current increased by a factor of 100-200 and the variations in the actual
current were estimated to be faster than the sampling rate of the voltmeter rendering the
current measurements unreliable.
Conclusions:
Based on results presented above the 4-layer printed circuit boards (including the
conformal coating) are adequately protected against HV discharge and corona up to the
safety limit of (2 x maximum operational voltage + 1000 volts = ) 5400 volts as required
by the RHIC safety committee.