Page 30 - PEN Ebook May 2021
P. 30
Power Supply Power Supply
illustrates at a high level some of the initial con- ▶ 85°C degrees ambient temperature + option-
siderations to be made. For instance, if the final al cooling system (-15°C)
application dictates a need for a higher back-
up time, then a high impedance solution from The equation for the requested capacitance is
KEMET’s FG, FY, FC, FM, and FR series would given by:
represent the best place to start.
Shorter backup time requires low impedance
and that would mean a different set of solutions, From this rough calculation, you would need a
primarily from the FA, FE, FS, FT, and FM family supercapacitor with a capacitance of around 0.1F.
of products. Alternatively, if the key application However, there are additional parameters that
requirement was high power, then a very spe- would need to be considered. These include a
cific range such as HV Series would provide the full range of parasitic effects such as R series -
answer. These are important considerations from DC resistance and ESR; self-discharge character-
the outset, though, before additional thought is istics; leakage current and ambient temperature.
applied. In each case, KEMET can help design engineers
calculate the impact of these parasitic effects to
In addition to backup time, a more comprehen- establish more accurate product selection.
sive list of parameters that need to be defined
before choosing a supercapacitor could also in- Furthermore, it is important to remember the
clude required minimum and maximum operating ‘life estimation’ of the supercapacitor (Figure
voltage; operating temperature; required dimen- 4). End of life is defined as the point at which
sions; and mounting type (surface or through- capacitance is reduced to 70% of the initial value.
hole). Then, with these details to hand, a rela- Typically, the lifetime of supercapacitors at an
tively simple formula can be applied to enable ambient temperature of 25-degree C would be
the rough calculation of the capacitance needed ten years. However, high-temperature load life
for the task at hand. tests show that this is reduced by half with an
increase of 10-degree C.
As an example of the calculation process, in the
project example below the customer needs a So, in many cases, if the supercapacitor is need-
supercapacitor that will be able to withstand 150 ed for a high-temperature application, it needs to
hours of backup time under the following condi- be cooled to reduce the ambient temperature to
tions: increase the life expectancy. An informed judg-
ment call needs to be made here, though: for
▶ V min (minimum operating voltage) =2.5 V very high-temperature use-cases, other technol-
ogies such as lithium battery cells might provide
▶ V max (maximum operating voltage) = 5.5 V a better answer.
▶ I backup (required back up current) = 540nA
▶ requested back up time T > 150 hours,
Figure 4: Lifetime of supercapacitors.
30 MAY 2021 | www.powerelectronicsnews.com MAY 2021 | www.powerelectronicsnews.com 31
