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Re: [TCML] Voltage - Gap



In addition to Jim's site, CapSite2009 provides some additional formulas to estimate capacitor life as a function of combined voltage and temperature for a variety of capacitor technologies.
http://my.execpc.com/~endlr/reliability.html

1. For film caps:

L=LR*((ER/Eo)^7)*2^(deltaT/10)

Where:
L = operating life under stated temperature and voltage
LR = life at rated temperature and voltage
ER = rated voltage limit
Eo = operating voltage
deltaT = difference between rated operating temperature and capacitor core temperature in C.

Note the use of a significantly lower exponent (7) for film caps than MacDougall showed (15) in the previously referenced paper. In any event, keeping voltages and currents within spec are the trick to long cap life!

2. For Ceramic caps:

L=LR*((ER/EO)^3)*(TR/TO)^8

Where:
L = operating life under stated temperature and voltage
LR = life at rated temperature and voltage limits
ER = rated voltage limit
EO = operating voltage
TR = rated temperature limit
TO = operating temperature in C (usually assumed to be same as ambient)

For Aluminum Electrolytic caps:

LT=LR*(ER/EO)*2^(deltaT/10)

Where:
LT = operating life under stated temperature and voltage (capacitor core temperature, not just ambient)
LR = the life at rated limits
ER = rated voltage limit
EO = operating voltage
deltaT = difference between rated operating temperature and capacitor core temperature in C

Bert

Jim Lux wrote:
On 2/3/12 9:27 PM, Carl Noggle wrote:
True. Such things often have interesting power dependencies. The life if
an incandescent light bulb is inversely proportional to the thirteenth
power of the applied voltage. Bandsaw blade wear, the seventh power of
the excess cutting speed. Wonder what the dependency is for cap voltage,
rep rate, etc.?



life is in cycles, for the most part, because the "wear out mechanism"
is the stress on the dielectric/plate system. There are also some
factors that are temperature related (whether external environment or
due to internal dissipation)

Most all is revealed at
http://home.earthlink.net/~jimlux/hv/caplife.htm
with some examples...

Lx = Lref * (Qref/Qx)^1.6 * (Vref/Vx)^7.5

Q = sqrt( 1 + 1 /( 2 /pi*ln(VR))^2)

There are some differences in the life prediction equations depending on
the actual dielectric being used and the construction. So what works for
a foil/poly as used in the Maxwell plastic cased (white brick)
capacitors won't necessarily be valid for paper/castor oil (big metal
can pulse caps) or the PP film caps used in a MMC.

Somewhere back in the archives (really) there was a list of the "age
derating" exponents for some of the other dielectrics.
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