Input rectifier cap [Design]

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From: Hammy on 9 Jun 2010 09:16

If I calculated a 120Hz, 1.3Arms ripple current for an input rectifier
cap for a flyback, how could I calculate the core temp based on a
given maximum ambient temperature (50C)?

I'm interested in using one Panasonic 270uf 105C cap rated at 1.42
Arms @120Hz.

http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=EETED2E271BA

There's no way that the commercial SMPS's I've looked at are using the
105C rms rateing to select the input cap. Most I've taken apart have
either one cap or two in series for a switch doubler including the
350W SMPS powering my computer. They must be increasing the rms
handling capability of a cap based on a reduced operating temperature
then the 105C specified in the datasheet.

Basso's book mentions a multiplier for determining max rms current for
reduced operating temperature. I've looked at several datasheets for
caps and haven't seen one. Is there a rule of thumb? Also is there a
reasonable estimate for including the converters high frequency rms
contribution to the capacitor?

From: MooseFET on 9 Jun 2010 10:41

On Jun 9, 9:16 pm, Hammy <s...(a)spam.com> wrote:
> If I calculated a 120Hz, 1.3Arms ripple current for an input rectifier
> cap for a flyback, how could I calculate the core temp based on a
> given maximum ambient temperature (50C)?
>
> I'm interested in using one Panasonic 270uf 105C cap rated at 1.42
> Arms @120Hz.
>
> http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keyword...
>
> There's no way that the commercial SMPS's I've looked at are using the
> 105C rms rateing to select the input cap. Most I've taken apart have
> either one cap or two in series for a switch doubler including the
> 350W SMPS powering my computer. They must be increasing the rms
> handling capability of a cap based on a reduced operating temperature
> then the 105C specified in the datasheet.
>
> Basso's book mentions a multiplier for determining max rms current for
> reduced operating temperature. I've looked at several datasheets for
> caps and haven't seen one. Is there a rule of thumb? Also is there a
> reasonable estimate for including the converters high frequency rms
> contribution to the capacitor?

Things can be extra ugly when you have a mixture of high frequency
and low frequency ripple. Many capacitor makers specify the ripple
current at each. What I usually do is make sure never to exceed the
100% mark when I add the percentages of each that I have.

If you add a few ceramics to the design, you can have the ESR and ESL
keep the high frequency stuff mostly out of the big guys.

Remember that the heat flows are nearly linear in the case of a
capacitor
on a PCB. The RMS current and the ESR mostly make the heat.

From: Grant on 9 Jun 2010 11:05

On Wed, 09 Jun 2010 09:16:20 -0400, Hammy <spam(a)spam.com> wrote:

>If I calculated a 120Hz, 1.3Arms ripple current for an input rectifier
>cap for a flyback, how could I calculate the core temp based on a
>given maximum ambient temperature (50C)?
>
>I'm interested in using one Panasonic 270uf 105C cap rated at 1.42
>Arms @120Hz.
>
>http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=EETED2E271BA
>
>There's no way that the commercial SMPS's I've looked at are using the
>105C rms rateing to select the input cap. Most I've taken apart have
>either one cap or two in series for a switch doubler including the
>350W SMPS powering my computer. They must be increasing the rms
>handling capability of a cap based on a reduced operating temperature
>then the 105C specified in the datasheet.

A lot of them only want 12 months life -- don't care what happens after
warranty expires :(
>
>Basso's book mentions a multiplier for determining max rms current for
>reduced operating temperature. I've looked at several datasheets for
>caps and haven't seen one. Is there a rule of thumb? Also is there a
>reasonable estimate for including the converters high frequency rms
>contribution to the capacitor?

Odd you don't have datasheet numbers for this, it was on datasheets
I looked at a few weeks ago. Look for a table of multipliers buried
in the details, the tables are small, perhaps easy to miss?

An example. Recently I worked through ripple current adjusted for
both temp. and frequency for two different brands of caps and the
multipliers are a little different. These are 85'C caps.

I built a bank of caps for about 600A ripple current for <45'C, but
it's down to about 350A at 85'C (~500A at 60), bank is 20 x 10mF/50
plus 60 x 4700/50.

First brand is 1.73x < 45'C and 1.5x at 60'C, second is 1.55x <45'C
and 1.30x at 60'c (both 1.0x at 85'C).

Multipliers for frequency varied more, first is 1.08x for 10kHz,
second is 1.20x for 10kHz (both 1.0x at 120Hz).

Grant.
--
http://bugs.id.au/

From: Hammy on 9 Jun 2010 12:27

On Thu, 10 Jun 2010 01:05:03 +1000, Grant <omg(a)grrr.id.au> wrote:

>>There's no way that the commercial SMPS's I've looked at are using the
>>105C rms rateing to select the input cap. Most I've taken apart have
>>either one cap or two in series for a switch doubler including the
>>350W SMPS powering my computer. They must be increasing the rms
>>handling capability of a cap based on a reduced operating temperature
>>then the 105C specified in the datasheet.
>
>A lot of them only want 12 months life -- don't care what happens after
>warranty expires :(

Mine has been going for 8yrs now. I doubt it could deliver 350W
continuous power.

I also noticed with mine when cleaning it , it has no Common mode
choke!! This explained why a TV operating off the same line would get
snowy when the PC was on simultaneously. Oddly enough it has all these
regulatory approval stickers and it's a Antec supposedly.

>>Basso's book mentions a multiplier for determining max rms current for
>>reduced operating temperature. I've looked at several datasheets for
>>caps and haven't seen one. Is there a rule of thumb? Also is there a
>>reasonable estimate for including the converters high frequency rms
>>contribution to the capacitor?
>
>Odd you don't have datasheet numbers for this, it was on datasheets
>I looked at a few weeks ago. Look for a table of multipliers buried
>in the details, the tables are small, perhaps easy to miss?

Your right it is there. I can double the rms current for 70C ambient.
I always thought the 105C specification was for core temperature not
ambient. I have been grossly derateing my caps! Oh well live and
learn. :-)

>An example. Recently I worked through ripple current adjusted for
>both temp. and frequency for two different brands of caps and the
>multipliers are a little different. These are 85'C caps.
>
>I built a bank of caps for about 600A ripple current for <45'C, but
>it's down to about 350A at 85'C (~500A at 60), bank is 20 x 10mF/50
>plus 60 x 4700/50.

Christ what's it for an industrial egg friar. ;-)

>First brand is 1.73x < 45'C and 1.5x at 60'C, second is 1.55x <45'C
>and 1.30x at 60'c (both 1.0x at 85'C).
>
>Multipliers for frequency varied more, first is 1.08x for 10kHz,
>second is 1.20x for 10kHz (both 1.0x at 120Hz).
>
>Grant.

From: Jim Thompson on 9 Jun 2010 12:32

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