pre-ship iirt telecon efi 1 telecon, dec. 5, 2006 themis pre-ship iirt telecon efi preamp current...

Pre-Ship IIRT Telecon EFI 1 Telecon, Dec. 5, 2006

THEMISPre-Ship IIRT Telecon

EFI Preamp Current ConsumptionJohn Bonnell

EFI LeadUniversity of California - Berkeley


Outline Background. Repair Plan. Results of Repair. Diagnosis and Testing:

EFI Preamp PWB layout error. Polarity Check on all PWBs. Stress analysis of BEB and LVPS. Spin Balance and Magnetics cleanliness.

Updates on other EFI PFRs.

Probes 1- 5 at Rest in SAF


BackgroundEFI Preamp Current Consumption Issue:

Historic Current Variations in EFI Preamps 3-10 mA (PFRs 096, 181, Probe PER RFA #2).

PreShip Review Team Requested More Tests (bakeout, dry N2 purge, etc.)

Long-Duration Testing on FM6 hit 20mA (red limit). PWB layout inspection determined that the power supply bypass

caps on EFI Preamp PWB were reversed (marked polarity on silkscreen was reversed)

Manufacturer Allows 10-15% reverse bias intermittently; caps were run for hundreds of hours at 20% reverse bias (50-V caps run at 10-V supply voltage).

NASA studies (http://nepp.nasa.gov/eeelinks/February2002/Reverse_Bias_Behavior.pdf) show that lifetime severely limited in reversed polarity of this magnitude Caps were expected to fail prior to primary mission.

Replace all EFI Preamp Caps in proper orientation Verify No Other Caps Reversed

Probes 1- 5 at Rest in SAF

http://nepp.nasa.gov/eeelinks/February2002/Reverse_Bias_Behavior.pdf


Repair Plan

UCB-NASA Meetings to Gain Concurrence on Plan (9-17 Nov 2006) Build/Qualify New Probe Stands Remove Solar Arrays Remove SPBs and AXBs Transport SPBs and AXBs to UCB Remove Preamps Remove caps, replace with new from same lot in correct orientation. Preamp PWB Electrical Test Conformal Coat Workmanship TVAC (-135/+90 C; one cycle; unpowered). Reassemble SPBs and AXBs SPB and AXB Pre-Vibe Electrical Functional Tests (Boom CPTs) SPB and AXB Workmanship Vibration Tests SPB and AXB Post-Vibe Electrical Functional Tests (Boom LPTs) EFI Functional using FM1 SPBs and AXBs with FM6 IDPU Transport SPBs and AXBs to JPL. Re-Install AXBs and SPBs to Probes EFI Functional (Probe Level EFI CPT) Install Solar Arrays; Closeout Probes Probe Functional (Probe LPT) Process Complete, F1-F5: 29 Nov 2006 (30 units, 3 weeks).

Probe 1 on Rollover Fixture for ESA \


Performance Testing (1)

Replacement of bypass caps has reduced current consumption to nominal 3-mA level across the board:

Pre-Modification[min/median/max; mA]

Post-Modification[min/median/max; mA]

+10 V[mA]

2.8/3.0/5.5 2.8/3.0/3.3

-10 V[mA]

2.8/3.1/13.3 2.8/3.0/3.4

FGND[uA]

0.001/0.004/7.8 0.002/0.003/0.004


Current Consumption

THEM EFI PRE -- Current Consumption

0.001

0.010

0.100

1.000

10.000

100.000

0 5 10 15 20 25 30 35 40 45 50

Unit Index [arbitrary]

Dc

Cur

rent

[mA

]

PreMod, P10VFPreMod, M10VFPreMod, FGNDPostMod, P10VFPostMod, M10VFPostMod, FGND

SPBs AXBs

Leakage to FGND, 0.1 to 10 mA


Performance Testing (2)

The replacement of bypass caps had no effect on DC or AC performance of EFI:

No significant changes noted in DC or AC board-level and Probe level functional tests between pre- and post-rework values (< 2% across the board).


Source of Polarity ErrorLayout: The two power supply bypass caps (C1A, C1B) on the EFI Preamp PWB

were originally marked as generic non-polarized capacitors (ORCAD ‘DISCRETE/CAP-NP’ library part w/out footprint).

Subsequently, a polarized Tantalum capacitor was selected for C1A and C1B (P/N CWR06NH104KBB), and a polarized part footprint was created and specified for the caps in the design cache.

The polarization of the part footprint (+ to pin 1; - to pin 2) was not checked against the pin assignments and expected polarities for C1A and C1B, and the pin assignment and assigned polarity was exactly opposite to the desired polarity.

Layout and PWB fabrication followed the (erroneous) polarity assignment in the schematic, resulting in erroneous polarity markings on silkscreen layer.

No other aspects of layout were suspect (via clearances through ground plane, for example).

Assembly: 36 PWBs (30 Flight, 6 Flight Spare) were assembled according to

THM_EFI_PREAMP_PWB_AI. During rework, only one of the 72 caps was found to have been placed

incorrectly relative to the silkscreen markings.


EFI Preamp Schematic

Power Supply Bypass Caps


EFI Preamp PWB Layout

Power Supply Bypass Caps

• +V connects to NEG side of C1A.

• -V connects to POS side of C1B.


Capacitor Polarity Check

All silkscreen markings for polarized capacitors checked.

Each polarized capacitor was identified by reference designator, located using schematic diagram, parts list, layout and blank PWB boards, and its connectivity to ground verified with a DMM.

All capacitor markings (with exception noted for DAP – see PFR 040) were confirmed to be correct.

FGE board has a capacitor where the “+” symbol was slightly obscured by another part name. Flight board photo shows capacitor properly oriented.

Exhaustive list of all capacitors that were checked are provided in THM-SYS-063 THEMIS Capacitor Polarity Tech Memo.


Other Effects (Spin Bal; Mag)

• Spin Balance – NO EFFECT:

• No significant change in SPB or AXB mass due to rework and retesting.

• Magnetic Cleanliness – NO EFFECT:

• Measured moments at boom unit level were the same or lower than those measured during pre-integration magnetics checks.

• Lower moments probably due to explicit Probe-level Deperm operation during Magnetics testing at JPL.


Circuit Stress Analysis (1)

• Two boards of the THEMIS IDPU are directly connected to the power supply bypass caps on the EFI Preamp PWB:

• Boom Electronics Board (BEB) – Floating Ground Driver.

• Low-Voltage Power Supply (LVPS – +/- 10V Floating Supplies.



Circuit Analysis (Stu Harris – EFI Lead Electrical Engineer)• Unlikely that any other parts on the EFI BEB board have been stressed by reverse polarity

caps.

• The ultimate catastrophic failure mode of the caps is to short out, and potentially cause large current flow through the cap itself. None of flight devices in the flight units have experienced this yet, avoiding what would be the most potentially stressful situation. Although, even in that case, the current available from the Floating Power supplies is not enough to stress the preamp or supplies.

• Having avoided the catastrophic case, what has been seen is increased leakage current flow through the capacitors. This current flows from +V and -V, through their respective devices, and back to the Floating Ground. Estimate is that this current flow may be as high as 10 mA. The conduction path would not involve any other part.

• The capacitors are connected between the +V and -V terminals of the OP15 op-amp and Floating Ground. The op-amp could be stressed if over-voltaged, but there is no mechanism for that to occur in this case. The Floating supplies have also had to supply this additional current, but is also not enough to be a concern (see LVPS circuit stress analysis).

• The only unusual thing occurring is capacitor leakage currents flowing in the Floating Ground, which is carried in the Shield of the boom wire. Normally, no current flow would be expected in this conductor, but the magnitude of the current is not enough to stress [the floating ground driver] circuit.



LVPS Circuit Analysis (Peter Berg – LVPS Lead Electrical Engineer)• It is unlikely that any parts on the LVPS board have been stressed by reverse polarity caps.

• The Floating supplies on the LVPS had to supply any additional current seen. This additional current was very small on the flight boards (3-10mA). On the spare unit, external power supply current limited at 20mA [during] additional testing.

• 20 mA will result in 100 mW dissipation in the series transistor with 292 C/W theta j-a, equivalent to a 29 degree C rise in temperature. Adding this to the ambient temperature of the board, or at worst case the temperature during thermal vacuum (65C), gives 95C. This temperature is well below the 200 degree rating of the parts, and so no overstress is indicatted either on the Flight or Flight Spare boards.


Effects on Other EFI PFRs

Anomalous AXB Temp Sensor Reading (F4 AXB): Open found in connector of harness. Harness reworked to mechanically bias pins in sockets and

correct intermittent open circuit. THM-PFR-206 updated and re-closed.

Functional test Circuit Anomaly (F3 AXB): Fault traced to single pin connector in ACTEST line on AXB. Could not repair without full deploy/restow cycle, with

concomitant risk of damage to unit. THM-PFR-183 updated to include known cause of fault. Will be flown AS-IS.


End of Presentation

pre-ship iirt telecon efi 1 telecon, dec. 5, 2006 themis pre-ship iirt telecon efi preamp current...

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