![](media/image1.wmf){width="5.607638888888889in" height="0.85in"}

**SVX II Silicon Detector Upgrade Project**

**Readout Electronics**

**High-Voltage Bias Protection Modules**

**\--PRELIMINARY\--**

**20 March 2002**

**F. Vince Pavlicek, Stewart Bledsoe**

**Document \# ESE-SVX-011211**

# GENERAL INFORMATION 

## System Introduction 

The Silicon Vertex tracking detector in the CDF experiment uses bulk
silicon strips to detect the passage of high-energy particles, which
leave a charge trail in the silicon material. The silicon is reverse
biased to enhance charge collection. The Caen high-voltage power
supplies providing this bias voltage experienced a high rate of failure
after installation. The failing part, the output regulator, shorted and
provided the maximum possible voltage to the output connections. The
sensor silicon can be damaged by excess bias so protection circuitry was
needed to protect against this failure. Space constrains dictated a
module external to the power supplies.

## Description Of Component & How It Fits Into The System

The module will be inserted in series with the bias supply cable at the
back of the Caen power supplies. All five channels of the bias voltages
from one supply will pass entirely through one module. The module will
be a plastic case that completely encloses the electronic circuitry.
Electrical connectors at either end of the module match the connectors
on the cable and Caen power supply.

## List Of Component Requirements

The important characteristics of the needed circuits are 1) limiting the
maximum voltage that can be applied to the silicon sensors, 2) rapid
operation, 3) reliable operation with minimal impact on the bias
voltage, due to leakage, during normal operation.

# THEORY OF OPERATION AND OPERATING MODES

## Basic Features & Operation Analysis

Three circuits that satisfy the above conditions can be labeled by their
effect on the bias voltage when they operate. The crowbar circuit will
short the bias output when the bias voltage exceeds the threshold value.
The clamp circuits (2 styles) try to hold the bias voltage at the
threshold value. If a fuse is added between the power supply and the
circuit then it can isolate the silicon detector (and the crowbar) from
the failed power supply.

## Analysis of candidate circuits

The "standard" power supply crowbar circuit as shown in Figure 1 is a
good fit here. The required working voltages and currents can be handled
by commonly available parts and allow for physically small devices. The
devices can be selected for low leakage currents. The fault operation of
the circuit removes all of the voltage, and therefore the stress, from
the SVX silicon sensor.

![](media/image2.wmf){width="5.45625in" height="7.2in"}

Figure 1 Crowbar Circuit

The clamp circuit shown in Figure 2 can also be optimized for low
leakage current and use physically small devices. However, there are a
higher number of parts to the circuit, which affects cost and might
affect reliability. Also undesirable is the feature that if the fuse
does not open, the clamp voltage remains on the sensor until an operator
takes action in response to the fault. The power dissipated across the
MOSFET device is of concern if the fuse does not open.

![](media/image3.wmf)

Figure 2 Clamp 1 Circuit - Regulator

The transient voltage suppressor shown in Figure 3 is more suited
protection from lightening strikes and large voltage spikes. The voltage
threshold is "soft" with the device only beginning to draw current at
the specified threshold voltage. a 160-volt device was tested and the
device threshold, determined by a noticeable increase in current above
the leakage current, was approximately 162 volts. However, the device
was not drawing 5 milliamps until 170 volts. This circuit also does not
remove the voltage from the sensor but clamps it at the threshold
voltage. In this case, however, the voltage was not even firmly clamped.

![](media/image4.wmf){width="5.23125in" height="3.8361111111111112in"}

Figure 3 Clamp 2 Circuit - Transient Voltage Suppressor

### Leakage Analysis

The protection circuit will have leakage currents that will add to the
existing leakage of the silicon sensors. It is important to minimize
these currents in the protection circuit because sensor leakage will be
monitored as an indicator of radiation damage. There are four types of
active devices used in these circuits that can contribute to the leakage
currents. They are Zener Diodes, Silicon Controlled Rectifiers, Metal
Oxide Semiconductor Field Effect Transistors, and Transient Voltage
Suppressors. The table below shows example leakage currents for these
devices.

  ---------------- -------------- ----------- ----------------- ----------
                   Zener Diode    SCR         MOSFET            TVS

  Max Leakage (ua) \<0.1          2           1                 1
  ---------------- -------------- ----------- ----------------- ----------

Using selected parts and their data sheet specifications, the three
circuits, crowbar, clamp1 and clamp2, add to the load on the power
supply by amounts shown in the table below.

  ----------------- ----------------- ----------------- -----------------
                    Crowbar           Clamp1            Clamp2

  Max Leakage (ua)  2.1               1.2               1
  ----------------- ----------------- ----------------- -----------------

With a fuse in line the circuit must draw enough current to melt the
fuse wire. The power supply output is rated at 5 milliamps but testing
has determined that a failed channel is capable of approximately 20
milliamps. The normal operating current is less than 500 micro amps. The
smallest fuse value that is easily commercially available is 2
milliamps. This puts the fault current at least 2.5 times the fuse
rating. The manufacturer specifications rate the fuse opening time at 5
seconds (maximum) at twice the fuse rating. The crowbar circuit has an
advantage in that it will minimize the voltage on the sensor during the
time it takes to melt the fuse. The two clamp circuits will hold the
sensor voltage at the threshold voltage during this time.

### Conclusion

The crowbar circuit is the first choice and will be implemented with an
in line fuse. The zener diodes will be selected for the different power
supply and ladder configurations but can be limited to three or four
specific voltages. The circuit will have component positions for three
zeners to allow flexibility in setting the voltage. A single circuit
board with five crowbars can be contained within a plastic case similar
to the existing "scrambling" adapter used on some of the Caen supplies.
Thus it will be a stand-alone module with connectors at each end that
can be put in line with the cable to the detector. The fuse will be
removable but for safety they will not be accessible without opening the
case.

# INTERFACE SPECIFICATIONS

The crowbar circuit should be usable on all types of CAEN power supplies
used in the CDF silicon systems, Types A509 -- SVX, A509H -- Layer 00,
and A510 -- ISL. The pin-outs are different between some of these
modules but are matched to the single detector cable pin-out by the use
of scrambling adapters.

The crowbar module mounts in line between the high voltage power supply
and the silicon bias cable into the detector. The Layer 00 module is
assembled in a plastic case with the DB-25 connectors accessible at
either end. The connectors are polarized (male and female) so the module
cannot be inserted backwards, although there is no electrical
directionality to that module.

## Connector Pin Configurations

The following pin out (from the web documentation "CDF Silicon Vertex
Detector Power Supply and FIB Cables - 2/15/00") supersedes the pin out
in the technical drawing.

  ------ ------- ----------- --------------- ------------- -----------------------
  PIN @  PIN @   Conductor   Description     CAEN          Comment
  CAEN   JuncB   Number                      Designation   

  1      \-      \-          ENABLE          EN_0          Connected to PIN 14 in
                                                           the Connector at CAEN

  14     \-      \-          ENABLE          EN_1          See Pin 1

  2      1       1           +VBIAS layer 0  VBPISL0       
                             pos.                          

  15     2       2           VBIAS layer 0   VBNISL0       
                             neg.                          

  3      3       3           H.V.Ref layer 0 VBFISL0       

  16     n/c     n/c         n/c                           

  4      4       4           +VBIAS layer 1  VBPISL0       
                             pos.                          

  17     5       5           -VBIAS layer 1  VBNISL0       
                             neg.                          

  5      6       6           H.V.Ref layer 1 VBFISL1       

  18     n/c     n/c         n/c                           

  6      7       7           +VBIAS layer 2  VBPISL1       
                             pos.                          

  19     8       8           -VBIAS layer 2  VBNISL1       
                             neg.                          

  7      9       9           H.V.Ref layer 2 VBFISL2       

  20     n/c     n/c         n/c                           

  8      10      10          +VBIAS layer 3  VBPISL3       
                             pos.                          

  21     11      11          -VBIAS layer 3  VBNISL3       
                             neg.                          

  9      12      12          H.V.Ref layer 3 VBFISL3       

  22     n/c     n/c         n/c                           

  10     13      13          +VBIAS layer 4  VBPISL4       
                             pos.                          

  21     14      14          -VBIAS layer 4  VBNISL4       
                             neg.                          

  11     15      15          H.V.Ref layer 4 VBFISL4       

  23     n/c     n/c         n/c                           

  12     \-      \-          GROUND          EARTH         n/c

  25     \-      \-          SHIELD          SHIELD        The shield pin on the
                                                           CAEN supply should be
                                                           attached to the shield
                                                           of the cable. The
                                                           shield will attach to
                                                           the junction board as
                                                           discussed above.

  13     \-      \-          GROUND          EARTH         n/c
  ------ ------- ----------- --------------- ------------- -----------------------

## Signal Descriptions

The two ENABLE and the SHIELD signals are passed through the module.

Each of the five Layer 00 bias voltages for the silicon layers is
carried on three connections. The total voltage is between the +VBIAS
and --VBIAS connections. The HV Ref connection is carried through the
crowbar module but the sensors make no connection to this voltage.
Essentially this wire is a no connect and is ignored by the module.

# ELECTRICAL & MECHANICAL SPECIFICATIONS

## Packaging & Physical Size

The module is package in a PakTech CNL-0303 plastic case. The case
secures the printed circuit card and has openings at each end to allow
access to the PCB electrical connectors. The whole module is just less
than 2 ¼ inches wide by 4 inches long by 1 inch thick.

## PC Board Construction

The printed circuit board is standard technology FR4 fiberglass
construction with high voltage (500 volt) design rules applied to the
trace and part spacing. The board is composed four layers totaling 0.032
inches thick. The layers are evenly distributed in thickness to After
the PC board is assembled and tested, Krylon 1302 Crystal Clear
protective spray coating was applied as additional protection against
the possibility of arcing even in a humid environment. The coating has a
dielectric strength of over 1000 volts per mil of thickness and three
coats are applied to ensure complete coverage. A single test coat
measured XX mills.

## Power Requirements and Cooling Requirements

The module is entirely powered by the bias voltages passing through it.
There are no cooling issues with this module.

# SAFETY FEATURES and Failure Analysis

(Details of all component safety features such as module fusing as well
as methods to assure high quality of this component such as
environmental testing should be included in this section. Note that this
is not a request but is a requirement of Fermilab that modules are built
in a safe manner. We will not be able to install and use any equipment
that is not built to operate safely. In fact, this system and all its
components will be thoroughly reviewed by official lab safety personnel
to certify its safety)

## Printed Circuit Board Safety 

For purposes of this design and the following specification, references
to \"high voltage\" are defined as 512 DC volts under worst-case
conditions. Only one of the existing power supplies (A509H) is capable
of this voltage.

High voltage traces will be run on internal layers whenever possible to
minimize electrical leakage due to high humidity or condensation.

High voltage traces will be run on as few internal layers as possible,
and a minimum dielectric thickness/spacing of 0.28mm (IPC-2221 Table
6-1) around any layer (z-axis) carrying high voltage will be specified
on the master drawing. All layers carrying high voltage will be
designated on the master drawing as well.

The minimum dielectric strength of the printed circuit core material and
bonding material shall be specified on the master drawing as 750
volts/mil. The PC board fabricator will be specifically instructed to
choose prepreg resins and adhesives with similar dielectric strength
characteristics to that of the of the dielectric core material being
bonded.

The circuit board will utilize a conformal coating (soldermask) on all
external conductor layers. Via holes carrying high voltage will be given
a conformal coating over their external pads.

Adhering to IPC guidelines (IPC-2221 Table 6-1), the following minimum
high voltage trace spacing will be adhered to:

Internal HV conductors: 0.28mm (11.02 mils)

External HV conductors with conformal coating: 0.83mm (32.68 mils)

External HV component lead termination (uncoated): 1.53mm (60.24 mils)

External HV via hole pads with conformal coating: 0.83mm (32.68 mils)

External HV via hole pads without conformal coating: not permitted

The minimum spacing defined here will also apply between conductive
patterns, layer-to-layer conductive materials (z-axis) and between
conductive materials such as mounting hardware.

Minimum and maximum high voltage trace widths will be dependant upon the
current being delivered, the amount of acceptable temperature rise in
the circuit traces and the copper weight of the layers carrying the high
voltage traces. A safety factor of 100% in current capacity will be
implemented.

In normal operation the circuit will have up to 180 volts across it. In
the event of a power supply failure that applies the maximum possible
voltage to the output, the crowbar will operate at the threshold voltage
and reduce the voltage on the modules output connector to less than 2
volts. If the fuse operates normally and melts under the over current
condition, it will disconnect the crowbar from the power supply output.
If the fuse does not open, the voltage across the crowbar will be held
below 2 volts until an operator turns off the power supply.

The devices chosen for the crowbar have these specifications:

+---------+---------+-----------------------+----------+-------------+
| Device  | Type    | Max fwd or Rev        | Fwd I    | Rev I -     |
|         |         | Voltage               |          | leakage     |
+---------+---------+-----------------------+----------+-------------+
| TCR22-8 | SCR     | 600 Vdc               | 1.5 A    | 2 ua        |
+---------+---------+-----------------------+----------+-------------+
| 1N52vv  | Zener   | 75 Vdc, 3 places      | 6 ma     | 0.1 ua      |
|         |         |                       | I~z~     |             |
+---------+---------+-----------------------+----------+-------------+
| 272.002 | Fuse    | 125 V working,        | 2 ma     | NA          |
|         |         |                       |          |             |
|         |         | \>500 V open          |          |             |
+---------+---------+-----------------------+----------+-------------+
| Rn      | R       | 400 V                 | NA       | NA          |
|         | esistor |                       |          |             |
+---------+---------+-----------------------+----------+-------------+

NA = not applicable or not measureable.

## Materials safety

The plastic enclosure used for the Layer 00 module is a CN-0303 model
from PacTec Enclosures ([www.pactecenclosures.com)]{.underline}. The
case material is a Polylac ABS plastic and it holds a UL94HB flame
class rating. The printed circuit board material is standard FR4
fiberglass that is used throughout the industry. The components are
common types and packages used throughout the industry and HEP systems.

## Failure analysis

Failure modes of the crowbar devices after the power supply has failed:

-   If the SCR fails shorted, the crowbar will not allow the voltage to
    rise above a low voltage.

-   If the SCR fails open, the crowbar will not protect the sensor. The
    zener diodes and the resistors will have the full power supply
    voltage across them. The zeners will conduct as much current as the
    resistors will allow.

V ~resistors~ = V ~output~ -- V ~zeners~.

I ~resistors~ = I ~zeners~ = V ~resistors~ / R ~resistors~

The zeners for this design are expected to be in the range of 75 to 180
volts. The resistors are approximately 130 K ohms. The current should be
less than 3.3 ma. The lowest zener value (75v) will allow the voltage
across the resistor to exceed the specification of the resistor by about
10%.

-   If the zeners or upper resistors fail open the crowbar will not
    protect the sensor. The crowbar circuit will survive the 512 volts
    across it.

-   If the lower resistor fails open the crowbar will not allow the
    voltage to rise above a low voltage.

-   If one of the zeners fail shorted the crowbar will operate at a
    significantly lower voltage.

-   If upper resistor fails shorted the SCR gate current will be higher
    than needed until the fuse operates of the operator turns off the
    supply. The maximum current will be 1/200^th^ the maximum allowed
    for this device.

-   If the lower resistor fails shorted, the crowbar will clamp the
    output voltage at the zener threshold voltage plus a few volts due
    to the voltage developed across the conducting zeners and the upper
    resistor.

# APPENDICES

## Component Documentation

The files for this project reside in the ESE department. The most
commonly needed information can be reached through the department web
server at

[http://www-ese.fnal.gov/eseproj/svx/Adapters/Caen_crowbar/crowbar_index.html]{.underline}

The schematic and layout were done with Orcad and those files are
available on request. Some of those files follow.

## Schematics

![](media/image5.wmf)

## Parts List

![](media/image6.wmf)

## Additional Appendices

![](media/image7.wmf)