[AMPS] Ripple current ratings of high voltage caps.

[John Fielding]

A thread which has been going on for a few days now deserves some added points.

The original question was whether some caps advertised for sale were suitable for high
voltage power supply duty in amplifiers.  Whilst the topic may be "old hat" to most I
feel it necessary to make a few comments in case you are not aware of some of the quirks
of high voltage electrolytics, and especially for the benefit of some less experienced
hams.

The Nichicon LLK2W271MHBS and LGQ2W471MHSC types were among the capacitors discussed. 
Fortunately I have  old Nichicon catalogs so I could look up the parameters.  By old I
mean going back to 1996. 

The LLK series is the LK type in the catalog, (discard the first L to arrive
at the part type in the catalog) and is not listed in the 1998 or 1999 catalog and, after
checking with the local agent, I can tell you it is now obsolete.  The (L)GQ series is
still current and probably will remain so for some time.

Merit W6NQ in a post erroneously informed us that the (L)LK series is rated at 105C, in
fact it is a 85C rated part.  The (L)LQ series is also only rated at 85C, however the
(L)GQ
series is a 105C part and here is where it gets interesting!!

If you examine the ripple current rating of these 3 types you will notice that the 105C
part has a substantially LOWER rating than the 85C parts!!

Why is this?  Well it is fairly simple to explain.  All these caps for the 450V rating
have a case diametre of 35mm and vary from 40 to 50mm in height.  They are all pc mount
with snap-in lugs.

Ripple current rating is a function of the esr of the capacitor and esr increases
(generally in a linear fashion) with increasing temperature, like a resistor with a
positive temperature coefficient. (At very low temperatures - below -20C the esr also
 increases, but this is due to a different mechanism, which is the electrolyte freezing).

If you take the 3 capacitor types and examine the ripple current rating you will see that

for 120 Hz ripple:

LQ series	85C	270uF @ 450V  Ir = 1,80A  (maximum value is 470uF @ 450V)
LK series	85C	270uF @ 450V  Ir = 1,61A  (maximum value available is/was 390uF)
GQ series	105C	270uF @ 450V  Ir = 1,19A  (maximum value is 470uF @ 450V)

Taking just the LQ and the GQ series for  470uF @ 450V the data is:

LQ	Ir = 2,47A @ 85C
GQ	Ir = 1,74A @ 105C

Which is a difference of about 30%.

The ability of an electrolytic to sustain ripple current is primarily the external
surface area available to dissipate the heat.  Both these caps are identical size (35mm x
50mm) so have exactly the same surface area.  But the GQ is rated at a case temperature
of 105C. 

I think that the trade off is in the expected temperature the case will achieve
in a practical design.  The bottom line is this - if you require high ripple
current rating and high temperature operation then you need to increase the capacitor
value by some 30 to 50% to arrive at the same ripple current capability of a 85C rated
part.

Hope this of some interest as I feel that spending $$ un-necessarily on 105C rated
capacitors to end up with less ripple current capabilty is a pointless fact.

John	ZS5JF	

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