[time-nuts] Low temperature coefficient capacitors for DMTD

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Tue Jan 26 00:54:44 UTC 2010


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Poul-Henning Kamp
> Sent: Monday, January 25, 2010 2:48 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Low temperature coefficient capacitors for DMTD
> 
> In message <ECE7A93BD093E1439C20020FBE87C47FED2B80A552 at ALTPHYEMBEVSP20.RES.AD.J
> PL>, "Lux, Jim (337C)" writes:
> 
> > 
> 	In 1990, the conclusion of a polycarbonate film capacitor
> 	paper[1] stated, "both the orientation and crystal structure
> 	of PC (polycarbonate) film affects its mechanical properties
> 	and electrical dissipation factor". The paper was a cooperative
> 	investigation by the Jet Propulsion Laboratory and Electronic
> 	Concepts' film manufacturing division,
> 
> Why would JPL study obscure production details of polycarbonate
> film capacitors in 1990, if they fell of the market six years earlier ?

Oh my.. a bit of research in Xplore finds that all the gory details were in the very section I work in at JPL (we build radios for deep space missions)

Well, the original incident triggering the study was in 1978 on Voyager 2, when they weren't able to get an uplink lock to the spacecraft.  The telemetry from the spacecraft indicated that the loop filter in the carrier tracking loop was having problems, most likely from leakage across the 75 uF capacitor. They figured out a workaround to estimate the best lock frequency using temperature and Doppler estimates, and Voyager continues on its happy way out of the solar system.  (FWIW, the carrier tracking loop has a bandwidth of a few Hz, I think.)

In fact, my office mate says that the former occupant of the chair I am sitting in as I type this spent months checking the best lock frequency of Voyager to develop that estimation approach.


Frank Ott (a coworker who retired a couple years ago) did the failure analysis and figured out a way to test the capacitors and published a paper in 1985 describing it, and that paper was the basis for the 1990 Yen and Lewis paper cited in the white paper.

So, JPL actually did the studying in the 80s, right around when the market for polycarbonate caps was going away.

Interesting conclusions in Ott's paper: "Conditions leading to a capacitor failure can occur without voltage being applied.  JPL believes the Voyager in-flight failure occurred during six months of non-operation."



The actual reference (from IEEE Xplore) is:
"Effect of structure and morphology on thermal and electrical properties of polycarbonate film capacitors"
Yen, S.P.S.    Lewis, C.R.  
Jet Propulsion Lab., California Inst. of Technol., Pasadena, CA, USA;
This paper appears in: Power Sources Symposium, 1990., Proceedings of the 34th International
Publication Date: 25-28 June 1990
On page(s): 387 - 391
Meeting Date: 06/25/1990 - 06/28/1990
Location: Cherry Hill, NJ
ISBN: 0-87942-604-7
INSPEC Accession Number:4111725
Digital Object Identifier: 10.1109/IPSS.1990.145871
Current Version Published: 2002-08-06


Abstract
Research is reported to identify polycarbonate (PC) film characteristics and fabrication procedures which extend the reliable performance range of PC capacitors to 125°C without derating, and establish quality control techniques and transfer technology to US PC film manufacturers. The approach chosen to solve these problems was to develop techniques for fabricating biaxially oriented (BX) 2 μm or thinner PC film with a low dissipation factor up to 140°C; isotropic dimensional stability; high crystallinity; and high voltage breakdown strength. The PC film structure and morphology was then correlated to thermal and electrical capacitor behavior. Analytical techniques were developed to monitor film quality during capacitor fabrication, and as a result, excellent performance was demonstrated during initial capacitor testing

---
And the paper's first paragraph says why JPL would do this:

In March 1978, a 78 uF PC capacitor failed in a receiver tracking loop filter aboard Voyager I. This failure led to a series of investigations to duplicate the failure mode, determine the failure mechanism and establish a viable screening technique. The research was completed in 1982 [1-3]. Sporadic failures of metalized 2 micron polycarbonate (PC) film capacitors in low voltage high impedance circuits indicated lack of reliability above 100°C. Although the failure mechanism was not identified, a ramp test was implemented as a standard screening test[3], and 100°C was set as the upper temperature limit for full rated voltage use. For 125'C applications, a 50% voltage derating was recommended.


You can find the paper in NASA Technical Reports Server (NTRS) at ntrs.nasa.gov... The record there says it's not copyrighted, but the electronic copy I have has the IEEE copyright notice on it.

NTRS has another paper listed "Review of the NASA Voyager spacecraft polycarbonate capacitor failure incident"
Summary: 	The premission failure of a Voyager spacecraft capacitor has prompted an investigation into the use of polycarbonate capacitors in high impedance circuits, during which capacitor failures were induced by thermal cycling together with extended periods at high temperature. Measurement of leakage path temperature coefficients indicates that there are two distinct leakage types whose mechanisms are complicated by movement within the capacitor during temperature changes. A novel system for pulse detection during capacitor burn-in and ramp testing has proven to be beneficial.
Feb 1, 1985
	IEEE Transactions on Electrical Insulation (ISSN 0018-9367), vol. EI-20, Feb. 1985, p. 47-54.

Frank Ott, the lead author on the latter, retired about 2 years ago, and coauthored a NASA Tech Brief on a technique for "determining internal connections in capacitors" in 1986 (presumably from the same incident).  NPO-16499 and a "Capacitor Test System" (NPO-16485) which was a microprocessor controlled system simultaneously monitoring 80 capacitors (which is how they got those millions of hours). 


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