REG NASA-LLIS-0417--1996 Lessons Learned - High-voltage Capacitor Used in Voltage Doubler Circuits for Space Applications (~1978).pdf

1、Lessons Learned Entry: 0417Lesson Info:a71 Lesson Number: 0417a71 Lesson Date: 1996-07-01a71 Submitting Organization: JPLa71 Submitted by: J.A. RobertsSubject: High-voltage Capacitor Used in Voltage Doubler Circuits for Space Applications (1978) Abstract: Voyager circuit design verification showed t

2、hat capacitors that were appropriate for use at atmospheric pressures, contained voids that could lead to corona and possible failure in space. This deficiency might not have shown up during testing. The possibility of critical pressure must be eliminated from all circuit parts capable of causing co

3、rona or arcing. Testing should not be relied on to catch all design weaknesses.Description of Driving Event: (Relevant Historical Lesson(s) Learned)A deficiency was discovered during circuit design verification which might not have shown up during testing. The condition involved capacitors having a

4、large AC voltage between the capacitor body and its surroundings, and the capacitors having air pockets or voids between the body and the outside jacket. The finding is significant because this condition can remain harmless at atmospheric pressure, yet lead to corona and possible failure after month

5、s or years in the vacuum of space. To solve an interaction problem between the Voyager Traveling Wave Tube (TWT) and the power supply, the designer added a voltage doubler circuit having capacitors as integral parts of the circuit. The standard high-voltage capacitors initially used have the outside

6、 of the body taped to provide a moisture barrier. The doubler operates at 7.2 kHz with an input higher than 300 volts AC. The action of a voltage doubler is such that the two plates of the capacitors used change potential each time the inverter switches. The transformer winding that drives the doubl

7、er capacitors is referenced to the TWT collector through a diode bridge. Each winding output is, therefore, near collector voltage on one half cycle and changes by the transformer windings output voltage (about 700 V) on the next half cycle. When the combined problem of the high AC voltage and the t

8、rapped air pockets was realized, the designers specified that modified capacitors, not having the tape wrapping, be procured. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-These were then specially prepared by vacuum coating with Scotchcast 281.Les

9、son(s) Learned: 1. Standard circuit designs or parts which operate well at atmospheric pressure need to be verified for vacuum.2. Circuit changes to solve other problems may induce their own problems. The original problem was an instability in the TWTA power supply caused because the TWT reacted as

10、a negative impedance load, and the power supply helix output voltage was fed back to control the voltage regulator. This problem was corrected by special focusing of the TWTs to minimize their negative load impedance characteristics and by adding a voltage doubler circuit to reduce the power supplys

11、 helix voltage output impedance. The changes that corrected the instability problem produced the large AC component on the high-voltage capacitors.Recommendation(s): The space environment is a vital consideration in circuit design. Voids will slowly bleed down to critical pressure, thus allowing cor

12、ona inception, which occurs at lower AC voltages than DC voltages. The possibility of critical pressure must be eliminated from all circuit parts having DC or AC potentials capable of causing corona or arcing. Many design weaknesses can be discovered through testing; however, testing should not be r

13、elied on to catch all. Independent analytical review of the circuit design and parts application benefitted the Voyager TWTA voltage doubler design.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Environmenta71

14、 Hardwarea71 Test & VerificationProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional Info: Approval Info: a71 Approval Date: 1995-10-26a71 Approval Name: Carol Dumaina71 Approval Organization: JPLa71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-