Electrical

1
Section 15 Electrical
. . . Bruce Zink EO-1 Electrical Lead, Swales
Aerospace, Inc.
2
Contents

• System Block Diagram
• Box IT Flow
• Remaining Work to be Performed
• Critical RFAs
• Status of Open Actions
• Residual Risk
• Launch Readiness

3
System Block Diagram
4
Power System Block Diagram
5
Verification Matrix
6
Accumulated Power On Time for EO-1 S/C
Components
7
Box Level IT Process Flow(SAI-PLAN-130)
8
Remaining Work to be Done

• Harness Closeouts
• Remaining work related to re-installation of WARP
  and bay closeouts for flight.
• WARP Re-Integration
• Repeat of previous successful integration
  following repair
• Hyperion Re-Integration
• Repeat of previous successful integration
  following removal
• ALI, Hyperion, and LAC to WARP Re-Integration
• Repeat of previous successful integration
  following WARP removal

9
Remaining Work to be Done (continued)

• S/A Integration
• Repeat of previous successful integration for
  Vibration Acoustic Test
• Calorimeter Integration
• Two thermistors. S/C side already verified.
• IRU Re-Integration
• Following removal of IRU due to threat of Helium
  exposure
• 1773 Bus Characterization (following all 1773
  integrations)
• Required due to WARP, AST and Hyperion de-mate
• Repeat of previous successful integration
• Trickle Charge Diode installation and test
  (WOA 905 PR40-2)
• Replacing fuse to enhance reliability and reduce
  susceptibility to failure

10
Critical RFAs(not dealt with in other topics)

• CDR RFA 4.04 S/A deployment by hardware driven
  circuitry
• HKRSN has both HW and SW deployment capability
  driven by separation switches
• CDR RFA 4.10 Perform worst case analysis on all
  circuits
• Heritage (MAP), design techniques, and testing
  provide adequate solution
• CDR RFA 4.15 Grounding/Shielding philosophy
• System Level Electrical Requirements
  AM149-0020(155) specifies requirements
• System level EMI/EMC testing validated design and
  implementation

11
Status of Open Actions

• 1773 Bus Fault During MOC Sim (WOA 876 PR20-1,
  20-3)
• Latest MV SW Build eliminated problem
• Solar Array Deployment Test
• All testing to date successful
• Planned for launch site
• Primary side with flight fuse plug

12
Residual Risk

• One Time Events
• Chassis current Transient (WOA 770-20-3,
  PR20-3)
• One event of 300mA, also seen on stripchart
  recorder
• EMC testing was taking place with probes
  connected to spacecraft power bus
• No recurrence since event on 11/22/99
• Chassis current event (WOA 572-30-10)
• Prolonged chassis current event reaching a peak
  of 925 mA
• Solar Array Simulator cable (EGSE) found to have
  split insulation on multiple wires within
  backshell
• Following inspection a new Solar Array Simulator
  cable was fabricated
• No recurrence ( with exception of 770-20-3 which
  was a brief event )
• Deployables (See Special Topic)
• All testing to date successful (see Special
  Topic)
• Full system test with SA scheduled for launch
  site
• Nadir Deck Heater Short (See Special Topic)
• WOA 676-20-8, PR20-8
• Re-wiring to bypass stressed components in place
  and tested
• S/A Cell Bonding Issue (See Special Topic)
• WOA 765-30-1, PR30-1
• Inspection and test are sufficient to resolve
  concerns

15 - 12
13
Launch Readiness

• Complete system level testing (CPT, S/A
  deployment)
• Previous success suggests low risk
• Complete remaining activities
• See previous section on Remaining Work to be Done
• Install and test new trickle charge protection
  diode
• Low risk S/C closeouts and repeats of previous
  integrations

14
Special Topic MAP Heritage Issues
. . . Mark Perry Swales Aerospace, Inc.
15
Review of MAP PRs for Relevance to EO-1

• Special topic 15a, response to RFA 14.11 and 1.19
• All MAP PRs were reviewed assessed for
  relevance to EO-1. Only two PRs were added to the
  list of 14 that was presented at the PSR.
• Verified that there is no risk to EO-1 mission
  success. For some PRs, additional action was
  required.
• Statistics for the 16 PRs (EO-1 used multiple
  approaches for some PRs, so three PRs have two
  categories)
• 7 PRs categorized as SAME EO-1 incorporated the
  same corrections as MAP
• 2 PRs categorized as OTHER Fixed with a
  different method than used for MAP
• 6 PRs categorized as NOT SEEN These problems
  depend on MAP-specific workmanship, part lot, or
  design
• 4 PRs categorized as NO RISK Not fixed on EO-1,
  but determined that they are not a mission risk.

16
Residual Risk Effects Due to MAP PRs

• Problem 5 (from the next page) has a very low
  probability of causing a very slight attitude
  disturbance. The maximum disturbance should not
  cause the ACS to exceed its stability
  requirement, and should be undetectable in an
  image even if the ACE resets during an image.
• Problem 7b causes slight errors for some narrow
  regions of temperature data. Since none of this
  data is used on board for S/C control, there is
  no risk to the mission, but it may complicate
  some data analysis. (see related Special Topic
  A/D Conversion Errors)
• Problem 11 has a very low probability of causing
  an MV watchdog reset in the ACDS or WARP. If the
  WARP MV resets, stored data may be lost. If the
  ACDS MV resets, the only effect is reduced
  operational efficiency following the event (the
  spacecraft will enter safemode, and images are
  cancelled until restored by ground operations).
  There is no risk to mission safety.
• See table on next page for additional details

17
MAP PRs Relating to EO-1
18
MAP PRs Relating to EO-1 (2)
19
MAP PRs Relating to EO-1 (3)
20
Special Topic Nadir Deck Heater Short
. . . Bruce Zink EO-1 Electrical Lead, Swales
Aerospace, Inc.
21
Nadir Deck Short Circuit

• Summary of Event (WOA 676-20-8)
• Two LVPC Over Current Trips experienced in PSE
  LVPC during TVAC. No indication of chassis
  current coincident with the events. Strip chart
  data shows high current being sourced by the
  battery in both incidents.
• Following events, two LVPC services failed to
  respond (no current flow) for the remaining tests
  in TVAC.
• PSE LVPC service 9 provides power to nadir deck
  primary, and panel 6 secondary heater services
• CDH LVPC service 2 provides power to nadir deck
  secondary, and panel 2 primary heater services

22
Nadir Deck Short Circuit

• Cause of Event / Problem in S/C
• Nadir Deck heater wiring (both primary and
  secondary) had wiring error that caused a
  short-circuit with the closure of certain
  thermostats in the wiring network. Identical
  error present in both primary and secondary.
  Telemetry data is consistent with a short circuit
  in the nadir deck primary heaters for the first
  event.
• Ramifications to mission
• Two LVPC switched services have been removed from
  operational use.
• LVPC over-current protection not as robust as
  assumed.
• Selective fusing incorporated. See Special Topic
  15c
• Nadir deck and Panel 2 will only have primary
  heater services, no secondary in either
  location.

23
Nadir Deck Short Circuit

• Testing Performed Following Event
• Nadir deck heaters subjected to full battery of
  continuity testing.
• Wiring errors found in both circuits which can
  cause a short-circuit under certain combinations
  of thermostat closures.
• LVPC services in question subjected to testing to
  evaluate functionality/state of health
• Both services functioning since testing began
  following TVAC
• LVPC Service 9 shows leakage and increased
  on-resistance (WOA-723-20-14 Problem Report)
• CDH Service 2 functioning normally
• Spacecraft harness subjected to passive and
  powered testing
• All wiring found to be correct and functioning
  properly
• Panel 2 secondary heater wiring
• Exercised using power while chilling thermostats.
  Found to function properly

24
Nadir Deck Short Circuit

• Conclusions resulting from Testing
• PSE LVPC Service 9 was subjected to a short
  circuit emanating from the miswired nadir deck
  heater services. This resulted in damage to the
  switch.
• CDH LVPC Service 2 appears to be healthy, as
  does all wiring and circuitry for Panel 2s
  thermal harnessing. No direct evidence that CDH
  Service 2 experienced a short circuit.
• No collateral damage, or potential future
  collateral damage
• Analysis and testing on a breadboard support
  conclusions
• Corrective Actions
• Nadir Deck Heater circuits abandoned by
  disconnecting and grounding to structure

25
Pre-Thermal Balance Heater Layout
26
Post-Thermal Balance Heater Layout
Service 6
Service 6
CDH LVPC
Active
Panel 6
P
S
Reinstate
Service 9
PSE LVPC
Abandoned
NADIR DECK
P
Ground
S
Abandoned
Service 2
CDH LVPC
Abandoned
Thermal analysis shows sufficient margin without
Panel 2 Primary Heater Circuit
Panel 2
P
Ground
S
Active
Service 4
CDH LVPC
Thermostat Setpoints Primary Close _at_8, Open
_at_13C Secondary Close _at_4, Open _at_9C
27
Nadir Deck Short Circuit

• PSE LVPC Service 9 abandoned
• Wires removed from LVPC connector and terminated
  to ground
• Damaged switch not to be used in flight
• CDH LVPC Service 2 abandoned
• Wires removed from LVPC connector and terminated
  to ground
• Suspect switch not be used in flight
• New nadir deck heaters implemented
• All new wiring, heaters, and thermostats
• Serviced by remaining spare
• Panel 6 heaters re-routed to different service
• Serviced by remaining spare

28
Nadir Deck Short Circuit

• Residual Risk / Redbook Candidates
• Changes verified (jumpered thermostats to
  simulate closure where possible)
• Not a Redbook Candidate

29
Special Topic Fusing / Overcurrent Review
. . . Bob Vernot Swales Aerospace
30
EO-1 Power Distribution Block Diagram
31
EO-1 Power Services

• Unswitched
• Mission Critical Loads, no fault protection
• Fault Protection
• SSPC (PSE Output Module)
• Used for most mission critical loads plus
  instruments
• Breaker trips at 16-22 A, no accommodation for
  faults below these levels
• Over-current is sensed on power side of interface
• Critical load SSPCs are reset by flight software
• LVPC
• Used for most non-mission critical and
  contingency loads
• Selective fusing for fault protection of
  individual services
• Ground ops programmable over-current set point
  for summed services
• Removes power to all LVPC loads when over-current
  trips
• Resets 5 times or continuously (for essential
  services ) until fault clears itself
• Over-current sensed in power return only

32
SSPC Overcurrent Protection

• EO-1 SSPCs are rated for 15 Amps steady state
  (represented by 100 on curve)
• Steady state trip levels at 110 (16.5 A) to 145
  (21.75 A)
• All Load I/F analyzed to demonstrate that hard
  short will draw enough current to trip SSPC

May trip
Always trip

Never trip
33
Fusing Considerations

• Existing fault protection
• SSPC vs. LVPC
• Ease of fusing implementation
• S/C access required
• Design modifications required
• Reliability and induced failure modes
• Designing to survive launch environment
• Testing to survive space environment
• Risk vs. benefit
• Fuse non-critical loads to prevent fault
  propagation to mission critical functions.

34
Fusing Decision Process Matrix
35
Summary Actions

• Not necessary to fuse PSE Output Module Services
  as SSPCs provide protection against a hard short
• Fuse non-essential PSE, ACE and CDH LVPC
  Services in order to
• Provide individual service protection
• Protect against chassis ground short
• Provide an approach that is verifiable
• Provide benefits that outweigh the risk
• For all fused loads, loss of a single service
  results in no appreciable degradation of critical
  mission functions

36
Fuse Plug Design

• Standard 50 derating criterion used, including
  protection against smart shorts.
• Minimum allowable fuse rating is 2A
• Redundant fusing used selectively for more
  essential loads
• Redundant fusing meets derating criterion via
  steering resistors.

37
Fuse Plug Construction

• Flight proven implementation using back-to-back
  connectors
• All splices are solder splices
• Custom brackets to stiffen assembly
• Fuse plugs fully potted

38
LVPC Fuse Plug Verification Summary

• Fuse Plug Verification during Assembly
• Full continuity and isolation performed as
  follows
• After termination and before potting.
• After Potting.
• Visual inspection at key points in assembly
  process
• X-ray after Potting
• Assembly Level Verification
• All fuse plugs tested in T/V environment using
  worst case current plus 20 (min) for steady
  state duration (including transitions and 8
  cycles, per GEVS).
• Current through all fuses and bus bars monitored
  continuously
• High fidelity Continuity and Isolation
  verification before and after environmental test.
• All fuse plugs subjected to protoflight random
  vibration levels in three axes (unpowered).
• Harness mass loads simulated for test.
• High fidelity Continuity and Isolation
  verification before and after environmental test.
• S/C Level Verification
• All fuse plugs verified at system level via
  safe-to-mate open circuit voltage measurements,
  loaded voltage and current balance measurements.
  Dummy loads used for thermostatically controlled
  circuits.

39
Summary

• EO-1 is protected against overcurrent faults for
  all non-essential loads by selective fusing of
  LVPC services
• Essential, non-redundant loads serviced by an
  LVPC are not fused to provide opportunity for
  fault to clear
• Mission Critical loads services by SSPC are not
  fused, but S/C is protected against hard shorts
  by SSPC overcurrent protection
• All fuse plugs successfully tested and installed
  on S/C
• Complete Design drawings, Certification Logs and
  Verification documentation package delivered to
  GSFC customer

40
Special Topic Solar Array Adhesion
. . . Mike Cully Swales Aerospace, Inc.
41
Contents

• Overview of Problem
• Inboard Panel Inspection Results by TECSTAR
• Impact to On-Orbit performance

42
Overview of Problem

• Based on TIMED Program Solar Array panel
  problems, an inspection was performed to evaluate
  cell bonding integrity of cells on outboard
  panel. At the time only outboard panel was
  accessible since array was in stowed
  configuration. Both TIMED, MAP EO-1 solar
  arrays cells were manufactured at TECSTAR in
  around the same time frame (includes cell
  laydown).
• Note that TIMED panel has significantly different
  cells, bond area requirements and substrate
  characteristics
• Inspection technique used by APL/TECTSAR in
  response to TIMED problem is a newly developed
  inspection technique developed by TECSTAR for
  TIMED. Standard inspection consists of cleaning
  each cell and assessing any motion of the cell.
  Verifying final bonding area percentage is never
  performed.
• All of the EO-1 panels passed this inspection
  without issue
• Solar Array was fully inspected cleaned at
  Swales by TECSTAR prior to environmental testing
  at GSFC. Inspection revealed no issues with Cell
  Bonding using standard inspection technique.
• Post Environmental Inspection by TECSTAR of
  Outboard panel revealed one crack in the cover
  glass

43
Initial Inspection Results

• Initial inspection by APL of EO-1 Outboard
  personnel indicated several silicon cells had
  bonded areas less than 65 which is the
  specification for EO-1 (Landsat 7, GPS,...).
  TIMED requirement is 85. Typical programs use a
  bond area of between 65 70 (VCL, ORBVIEW3-4,
  Marie Curie Orbcomm)
• Swales performed a further detailed inspection
  mapping of cells (100 grid) in question and
  confirmed APL results for silicon cells (GaAS/GE
  all seem to be within spec.). Following this
  TECSTAR was called in to perform more detail
  mapping (12/15-12/99) and confirmed results.
• Based on outboard panel inspection TECSTAR
  personnel were called in a second time to perform
  detailed inspection of inboard panels. This was
  to be performed after the array was deployed off
  the spacecraft prior to LAPSS testing.

44
Inboard Panel Inspections Results by TECSTAR

• Mid Inboard Panel
• Deflection test performed on 200 300 cells
• No unacceptable deflection failures identified.
  Met requirements of 65 bonded.
• 100 inspection performed for cracked cells
• No cell cracks found. Cover glass crack
  identified touched up with Epoxy 9350 per Std
  repair procedures. (see map)
• 100 inspection of interconnects
• No out of spec conditions found
• Inboard Panel
• Deflection test performed on 200 300 cells (see
  map)
• No unacceptable deflection failures identified.
  Met requirements of gt65 bonded.
• 100 inspection performed for cracked cells
• No cell cracks found
• 100 inspection of interconnects
• No out of spec conditions found

45
Outboard Panel Inspection Results by TECSTAR

• Outboard Panel (12/15/00 12/16/00 findings)
• Deflection test performed on 100 of cells
• Approximately 50 of the Si cells exhibited
  deflection ranging from 5 to 58 of the cell
  area
• Cascade cells (Hyperion Mod), bonded in a
  different time period, did not exhibit out of
  spec conditions
• 100 inspection performed for cracked cells
• No cell cracks found. Cover glass crack
  identified and touched up with Epoxy 9350 per
  standard repair procedures.
• 100 inspection of interconnects

46
TECSTAR Process Assessment

• In an effort to troubleshoot cause of outboard
  panel non-conformance TECSTAR reviewed all
  paperwork associated with process of EO-1 panels
  (including Hyperion). Evaluation of process
• All panels were within limits of the process
  however, the outboard panel, during initial
  curing of adhesive, was at the lower limit of
  Shore A hardness (coupon). This is significant
  since, at this stage of the process, the tape
  which is used to secure the cells to the mylar
  template is removed. When this occurs the cells
  have a small normal tension load applied.
  Insufficient hardness of the cell adhesive can
  cause the pre-existing voids (65 bonded) to grow
  in size. It is believed that this was the cause
  of the non-conformance of the outboard panel
• TECSTAR has implemented process improvements
  independent of EO-1 to assure that the adhesive
  hardness is adequate prior to tape removal

47
TECSTAR Conclusion

• The number of out of spec conditions on the
  outboard panel is minimal. Studies by TECSTAR
  working with APL to understand the TIMED void
  anomaly concluded that any area of the cell that
  remains bonded after bonding adhesive full cure
  through environmental test, does not degrade
  further even after additional thermal vacuum
  testing.
• Panel tests data indicate very high pull
  strengths were demonstrated with this bonding
  recipe for all panels
• Inboard and center panels show no adhesion out of
  spec conditions
• Based on inspection results, traveler review
  experiments TECSTAR recommends no further action
  be taken with the outboard panel
• Swales Performed Further Assessment
• Mechanical Analysis
• Thermal Analysis
• Power Performance

48
Mechanical Assessment

• Margin of Safety for 100 G static load for a 35
  bonded area, using a factor of safety 2.6
• Maximum EO-1 adhesive stress with 100 G static
  load and 35 bond area is .38 psi
• MS gt 150
• Array has seen full Protoflight Acoustic and been
  exposed to S/C level sine environment
• Margin of Safety for rapid depressurization for a
  35 bonded area using a factor of safety of 2.6
• MS 2.738
• All panels have been exposed to TV with outboard
  panel being exposed twice for Hyperion
  Modification

49
Thermal Assessment

• Summary
• Thermal analysis indicates that a 65 percent
  bond requirement (i.e., 35 disbonding) does not
  have a significant effect on the cell
  temperature. It is not until you reach a 10-15
  bond that the cell temperature increases
  significantly. This can be attributed to the
  conductivity of the Silicon adhesive (CV 2568 by
  Nusil, Inc.) and the conductivity of the cells.
• Assumptions
• Inboard Panel with 651 Silicon Cells
• Solar Absorptivity 0.75, Emissivity 0.88
• 8 mil coverglass and 8 mil Silicon Cells
• Cell Adhesive is CV 2568
• Solar Constant 450 Btu/hr-sq.ft.-F
• Modeling results are consistent with TIMED
  analysis

50
Assess On-Orbit Performance Value AssumingWorst
Case Bonded Value (35)

• The primary assumption is that all cells exhibit
  adhesive voids of 65 (35 bonded). This
  extremely conservative since only a small
  percentage of cells on outboard panel exhibit
  this.
• EOL current for baseline Solar Array running at
  an average cell temperature of 72C
• Current 18.876A at 33.5 Volts
• EOL current for Solar Array running at an average
  cell temperature of 76C due to reduced bond area
  of 35
• Current 18.273A at 33.5 Volts
• Reduction in current for worst-case bonding (35
  bonded) is less than 3.1. Minimal impact to
  on-orbit EOL performance.

51
Conclusions

• All required performance will be achieved
• Removal of cells that are out of specification
  would result in risk to array
• Array has been exposed to all flight environments
  without any problems
• Disposition is Use As Is

52
Special Topic Contingency Plan for Partially
Deployed Solar Array Including Initial Power
Consumption if Solar Array Does Not Deploy (RFA
16.20)
. . . Bob Vernot EO-1 Systems Engineer
53
List of Assumptions

• EO-1 goes to internal power prior to launch with
  battery at 100 SOC at T-10 minutes
• Consistent with launch countdown and narrow
  launch window
• Battery has nameplate capacity of 50 AH
• Battery has actual capacity of between 50 and 55
  AH
• SA collects zero power prior to attempted Sun
  Acquisition
• Outer panel is likely to collect some power
  during coast and after tip-off. Power collection
  not quantified.
• Following contingency Sun Acquisition, SA
  collects 192 W of power when lit
• Consistent with IV curve predicts for outer panel
  at 100º C, 28 VDC and sun intensity of 0.97

54
List of Assumptions

• EO-1 orbit average load of 130 Watts until
  heaters are required, then 198 Watts with heaters
• Consistent with Thermal Balance Test data power
  predicts
• Current drain on Battery assumes above loads at
  26.5 VDC
• Reflects bus voltage with battery at minimum SOC.
  Actual voltage will be higher
• S/C requires heater power six hours after launch
• Consistent with Thermal predicts for thermostatic
  control to 8 C
• S-band transmitter on for 10 minutes per orbit
  following heater power on
• One contact per orbit
• Contingency sun pointing to within 10 degrees of
  sun line
• ACS should provide better than 1 degree accuracy
  using AST

55
Contingency Operations

• Assumes that all reasonable efforts to deploy SA
  have been exhausted
• All subsequent subsystem activation events to be
  aborted
• Operations to activate AST
• ACS to use AST to control attitude for undeployed
  SA sun pointing (procedure under development)
• Transmitter duty cycle minimized
• See Additional Considerations to follow

56
Battery SOC through Sun Acquisition
57
Worst Case Battery SOC for Undeployed SA
58
Additional Considerations

• Additional power may be available.
• Possible 3-5 AH during coast phase of ascent
  (6-10 SOC)
• Possible 0-14 AH during despin, dependent upon
  spin axis (not predictable before separation)
• Contingency Sun Acquisition may occur prior to
  worst case predict for Sun Acquisition
• Ground management of heaters could greatly extend
  life
• Selectively allow areas of S/C to get cold
• Thermostatic control is set for 8 C, survival
  temperatures are at -10 C
• Many areas of S/C will settle to steady state
  temperatures above survival limits, without
  heaters. Further analysis is required.
• Will report on this topic at the Red Team
  Follow-up Review

59
Bounded SOC for Undeployed SA
60
Summary

• For nominal heater operation SOC is reduced by
  10 per orbit
• Under worst case conditions EO-1 can survive 10
  orbits
• With all heaters disabled SOC is reduced by
  1.25 per orbit
• Under worst case conditions EO-1 can survive 60
  orbits(4 days)
• Reality lies somewhere between these conditions
  but is skewed toward the latter case if S/C is
  allowed to go to survival temperatures

61
Special Topic Oscillatory Starts on HSSR 7110
Switches
. . . Dave Speer Litton Amecom, ACS Lead
Designer
62
Oscillatory Starts on HSSR 7110 Switches

• Background Description
• LVPC switched 28V power services can exhibit
  3-amp current spikes, negative-going voltage
  oscillations, or even lack of turn on for
  sufficiently capacitive loads. Two known
  examples are Autonomous Star Tracker (AST) and
  Magnetic Torquer Bar (MTB) loads on ACE LVPC.
• Problem was first recognized during MAP testing
  with their AST power interface. The EO-1 MTB
  power interface was tested for same problem due
  to electrically similar load circuitry.
• At high input voltage, there is an LVPC output
  oscillation into the AST and MTB (refer to PR
  346-20-4 and PR 483-20-1) reactive loads
• At low to nominal bus voltage (22V to 28V), these
  service-to-capacitive-load circuits turn on
  normally.
• At intermediate to high bus voltages (30V to
  35V), these service-to-load circuits will
  oscillate momentarily but then turn on after a
  short delay (msecs).

63
Oscillatory Starts on HSSR 7110 Switches

• Key Issue
• Whether the spiking/oscillating condition
  damaged/overstressed components on either the
  LVPC side of 28V power interface or on the
  AST/MTB load side of power interface.
• Analysis Test Results
• Voltage and current waveforms that have been
  directly observed at the power-to-load interfaces
  (on a scope) do NOT represent an overstress
  condition. Max observed ?Vout was 56V, less
  than 90V rating. Current spikes are 3A for 2
  usec, less than 10A for 10 msec (one-shot)
  rating.
• Circuit simulations that drove current spiking
  waveform into the MTB load circuitry did NOT
  indicate an electronic component overstress
  condition at the load side. Lab testing found
  that continuous spikes/oscillations are not a
  thermal overstress condition for 7110 switch
• Action Taken
• Transient Suppression Assembly (TSA) box was
  integrated in-line between AST 28V power service
  and AST load, which eliminated spikes/oscillations
  prior to first electrical integration and
  operation of AST. So no overstress on load side
  of AST power interface.
• NO TSA box integrated in-line between MTB power
  service and MTB load, and this interface still
  experiences spikes/oscillations when turned on at
  high (29V to 35V) bus voltages.
• MTB service is turned ON at launch. Command
  restrictions have been implemented such that the
  MTB can only be turned on at VB ? 28V

64
Context Diagram
65
Special Topic Torquer Bar Potential Overstress
. . . Dave Speer Litton Amecom, ACS Lead
Designer
66
Torquer Bar 15V Output Variations

• Background Description
• During normal Magnetic Torquer Bar (MTB)
  operations, the dedicated MTB 15V output supply
  voltage telemetry exceeded limits based on
  manufacturers specification for balanced load
  output regulation. Refer to PR 677-20-2 and PR
  681-20-12.
• Problem was first recognized when Interpoint
  MTR2815D converter was being operated with very
  light (less than 40 mA) load on 15V output, and
  range (-110 to -190 mA) of load on -15V output.
• Problem was repeated when this range of output
  load conditions was entered on four separate
  occasions during normal testing. When the
  torquer bars were operated in this range, the
  15V output varied by about 1, and the -15V
  output varied by about 2.
• Status
• Special MTB output characterization test was run
  (WOA-915) to verify that
• With less than 40 mA on the 15V output, the
  behavior was independent of which individual bar
  was operating in the -110 mA to -190 mA range.
• With less than 40 mA on the 15V output, the
  behavior was independent of whether the 3-bar
  total current was in the -110 mA to -190 mA
  range.
• Amplitude of the 15V output variations was much
  smaller as load on 15V output was increased
  above 40 mA, and the output variation behavior
  disappeared when the load on the 15V output was
  above 70 mA.

67
Torquer Bar 15V Output Variations

• Closure
• Four sets of Excel data files and output voltage
  variation plots were sent to Interpoint for their
  review and analysis. They were then able to
  duplicate the same behavior that we saw, in their
  laboratory set-up, under nearly the same type and
  range of MTR output load conditions. They were
  also able to confirm that the 15V output voltage
  variations disappear when slightly more load
  (above the 40 mA level) is added to the 15V
  output.
• Apparently, there is an output inductor inside
  the MTR2815D that goes into a discontinuous
  mode over a range of -15V loads when the 15V
  load is small. The discontinuous mode of
  operation affects the output voltage control loop
  and causes small variations in the 15V output
  voltages. If the 15V load is increased, or if
  the -15V load is above or below the sensitive
  range, then the output inductor goes back into
  continuous mode, and the output variations stop.
• Small MTB converter output voltage variations do
  NOT represent a damaged condition, will NOT
  affect the operation of the MTB drivers, and are
  acceptable now that we understand them. Telemetry
  limits will be expanded slightly to allow for the
  observed behavior under a certain range of normal
  operating conditions.

68
Context Diagram