| Explanatory Notes
1.1 The internal temperature of a capacitor increases in the following situations.
(1) Ambient temperature increase of the capacitor due to:
(a) High or increasing temperature outside of the device.
(b) High or increasing temperature within the device due to heat retention.
(c) Radiated heat from components such as power transistors, PTC thermistors
and cement resistors located near the capacitor.
(2) Excessive ripple current causing overheating due to the equivalent series
resistance (ESR).
(3) Excessive leakage current causing heat generated by the resistance of the
capacitor.
(4) Frequently repeated charge and discharge cycles.
(5) External heat transfer through the copper traces of the printed circuit board and
terminals into the capacitor.
(6) Excessive terminal heat generated by high contact resistance of poor terminal
connections.
(7) Applied voltages such as a reverse voltage, overvoltage and AC voltage exceed
the rated limits specified for the capacitor.
1.2 If the operating parameters of a non-solid capacitor exceed the maximum rated operating temperature and/or permissible ripple current specified in the product literature, an acceleration of the electrochemical reaction rate will produce gas generated by the electrolyte, raise the vapor pressure within the capacitor and, therefore, increase the internal pressure that will cause the can to bulge, vent or explode.
If an extremely high temperature or excessive current, for instance, are applied to a capacitor, there will be a rapid current increase or short circuit. The safety vent,
if any, of the capacitor will fail to open properly before the sharp increase of the inner pressure causes the capacitor to explode and expel the electrolyte and other internal materials.
2. For non-solid capacitors the lifetime (Lx) of a capacitor is estimated by the following equation.
Where: L : Assured lifetime (hours). This is specified in the product literature.
TO : Maximum rated operating temperature (&C) of the capacitor.
TX : Actual operating temperature (&C) of the capacitor.
A : Ripple current acceleration formula specified in the product literature.
(1) This equation applies only to the range of ambient temperature from ~40&C to
the maximum rated operating temperature of capacitors and means that every
10&C increase in ambient temperature reduces the lifetime of the capacitors by
one-half.
(2) Please note the following before using the ripple current multipliers specified in
the product literature for temperature and/or frequency.
(a) Even though capacitors will be used at a lower ambient temperature than the
maximum rated operating temperature, the temperature multipliers should
not be used if the permissible ripple current obtained is higher than the
rated maximum permissible ripple current specified in the product literature.
Applying a ripple current higher than the maximum rated limit to a capacitor
nullifies the extended lifetime which can be obtained by using the capacitors
at a lower ambient temperature. Also, the lifetime of the capacitors cannot
be estimated using the equation above when the ripple current exceeds the
rated maximum permissible ripple current.
(b) For frequency multipliers, follow the specifications in the product literature.
(3) In general, a maximum lifetime should be limited to approximately 15 years
due
to deterioration of the sealing materials of the capacitor.
|
Precaution