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'[PIC] Capacitor derating'
2006\10\03@060616 by Mohit M. (Lists)

picon face
Hi,

Could somebody tell me if I need to derate multi-layer ceramic
capacitors? Specifically, can 10uF/16V MLCC be used in filtering the
ouput of a 12V PWM output from an H-bridge?

Thanks,
Mohit.

2006\10\03@144642 by Richard Prosser

picon face
Mohit,
You need to supply more information. Along the lines of ripple
current, load type, other filter components, the capacitor types you
are considering, working temperature, consequences of failure etc.
etc. And a hint of the actual application may help also iin case there
are special requirements you don't know about yet.

RP

On 04/10/06, Mohit M. (Lists) <spam_OUTmohit.listsTakeThisOuTspamgmail.com> wrote:
> Hi,
>
> Could somebody tell me if I need to derate multi-layer ceramic
> capacitors? Specifically, can 10uF/16V MLCC be used in filtering the
> ouput of a 12V PWM output from an H-bridge?
>
> Thanks,
> Mohit.
>
> -

2006\10\03@155652 by Mohit M. (Lists)

picon face
Richard,

Thanks for your reply. The circuit looks something like this:

                       ____ Peltier (1.7ohm)
                  ----|____|----
                  |            |
                  |            |
        L (100uH) |       C1   |   L (100uH)
             ___  |     ||     |  ___
      V1 o---UUU--o-----||-----o--UUU---o V2
                  |     ||     |
                  |            |
                  |            |
              C2 ---          --- C2
                 ---          ---
                  |            |
                 ^^^          ^^^
                 GND          GND


C1,C2 are 10uF. V1 and V2 are outputs from an H-bridge running at
10kHz (12Vp-p, 10A). Direction of current into peltier can be changed
(V1-V2 and V2-V1) to either heat or cool. Aim of this LC filter is to
filter output of H-Bridge to less than 10% voltage ripple.

> ripple current
Calculated to about 150mA AC. The datasheet from manufacturer suggests
up to 200mA is okay.

>load type
Basically resistive. Peltier module. Manufacturer's specs says
1.70ohm.

> other filter components,
Two Inductors. Differential LC filter.

> capacitor types you are considering,
Murata: 16V,10uF in SMD1210. Dielectric X7R. Tolerance +/-10% and
change across -55C to 125C +/-15%.

> Working temperature
Enormous amounts of forced-air cooling. So working temperature should
be <<50C.

> consequences of failure
Ripples in the voltage across peltier, leading to quicker failure of
module.

> actual application
Peltier heater/cooler.

Looking forward to your help,
Mohit.

{Original Message removed}

2006\10\03@174101 by Richard Prosser

picon face
Mohit,
I'm not sure I can add useful information but I felt that those who
would be able to assist would need a bit more info to get interested.
However - I have set up a simplistic simulation in PSpice.
With the values shown and 100mOhm inductor resistance I get about
480mA ripple at 5A of current (10% duty cycle).. This is not the worst
case so you may want to increase the inductance and / or capacitance
somewhat.

The current ripple through C1 is greater than C2 and is about 670mA
pk-pk (350mA for C2) so you may need to check this against the spec.

With respect to your origonal question  - I don't know  - but
paralleling capacitors will increase the total capacitance and
decrease the ripple current and resistive loss,  through each so can
only be a good thing.
I'm not aware of any reccomendations wrt derating MLC caps when
paralleling. In this case I'd make it up and monitor the capacitor
(and inductor) temperatures under various running conditions.

Richard P

On 04/10/06, Mohit M. (Lists) <.....mohit.listsKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

2006\10\03@182122 by Bob Blick

face picon face

> C1,C2 are 10uF. V1 and V2 are outputs from an H-bridge running at
> 10kHz (12Vp-p, 10A). Direction of current into peltier can be changed
> (V1-V2 and V2-V1) to either heat or cool. Aim of this LC filter is to
> filter output of H-Bridge to less than 10% voltage ripple.

> Murata: 16V,10uF in SMD1210. Dielectric X7R. Tolerance +/-10% and
> change across -55C to 125C +/-15%.

If by 12V p-p you mean the h-bridge can supply +6 or -6 volts across the
peltier, then I'd say you are marginally OK. But if you can get + or -12
volts to the peltier then you should probably have higher voltage caps.

Capacitor data sheets don't represent the type of stress you will put
these to... Ceramic caps are only one step up from electrolytics in terms
of perfection. They really like to have the same polarity all the time, or
else low ripple current. But you are reversing polarity and having high
ripple current.

Cheerful regards,

Bob


2006\10\03@182312 by Bob Blick

face picon face
>
>                         ____ Peltier (1.7ohm)
>                    ----|____|----
>                    |            |
>                    |            |
>          L (100uH) |       C1   |   L (100uH)
>               ___  |     ||     |  ___
>        V1 o---UUU--o-----||-----o--UUU---o V2
>                    |     ||     |
>                    |            |
>                    |            |
>                C2 ---          --- C2
>                   ---          ---
>                    |            |
>                   ^^^          ^^^
>                   GND          GND

Another thing to look at is you inductors - what is the inductance right
below maximum current? What will this do to your ripple?

Cheers,

Bob


2006\10\04@003409 by David VanHorn

picon face
I don't know if I can quickly put my finger on it, but Jim Williams of
Linear had published a rather good formula to predict capacitor lifetime
based on temperature, ripple current, applied voltage, etc..  I used it to
select output caps for a very unconventional printer power supply that had
to produce 19A pulses at 32V, and surprisingly, it predicted that the
expensive low ESR caps would fail early, but a triad of cheap caps would
work fine.  That's been borne out now in over a decade of real-world use.

--
Feel the power of the dark side!  Atmel AVR

2006\10\04@085406 by Gerhard Fiedler

picon face
David VanHorn wrote:

> I don't know if I can quickly put my finger on it, but Jim Williams of
> Linear had published a rather good formula to predict capacitor lifetime
> based on temperature, ripple current, applied voltage, etc..  

That would be something really useful. I tried to find it through Google,
but no luck. Can you add any information that might help in finding?

Gerhard

2006\10\04@091236 by Daniel Dourneau

flavicon
face
MIL HDBK 217E

Quoting Gerhard Fiedler <listsspamKILLspamconnectionbrazil.com>:

{Quote hidden}

> -

2006\10\04@104316 by Marcel Duchamp

picon face
David VanHorn wrote:
> I don't know if I can quickly put my finger on it, but Jim Williams of
> Linear had published a rather good formula to predict capacitor lifetime
> based on temperature, ripple current, applied voltage, etc..  I used it to
> select output caps for a very unconventional printer power supply that had
> to produce 19A pulses at 32V, and surprisingly, it predicted that the
> expensive low ESR caps would fail early, but a triad of cheap caps would
> work fine.  That's been borne out now in over a decade of real-world use.
>

If you can find it, I would love to see a copy.  I have had a few
capacitor failures and would appreciate some insight on the topic.
Thanks.

2006\10\04@124149 by Russell McMahon

face
flavicon
face
>> I don't know if I can quickly put my finger on it, but Jim Williams
>> of
>> Linear had published a rather good formula to predict capacitor
>> lifetime
>> based on temperature, ripple current, applied voltage, etc..

> That would be something really useful. I tried to find it through
> Google,
> but no luck. Can you add any information that might help in finding?

It's probably only for "wet" aluminum electrolytics.

Some rules of thumb:

>From design life and temperature double life for every 10 degrees C
reduction in actual operating temperature.

Operating unpowered at temperature uses up life at 2+ times the rate
when powered.

People publish ripple current lifetime derating figures.
Increase ripple current by 2 to 3 times over spec and your lifetime
comes own to rated lifetime at max temperature - which is short!.

Ripple current spec is mainly a heating one and if you heatsink your
capacitors (!!!) you can get extra ripple current capacity or life.

Voltages much below nominal REDUCE lifetime.



       Russell





2006\10\04@151126 by Gerhard Fiedler

picon face
Daniel Dourneau wrote:

>> David VanHorn wrote:
>>> I don't know if I can quickly put my finger on it, but Jim Williams of
>>> Linear had published a rather good formula to predict capacitor lifetime
>>> based on temperature, ripple current, applied voltage, etc..
>>
>> That would be something really useful. I tried to find it through Google,
>> but no luck. Can you add any information that might help in finding?
>>
> MIL HDBK 217E

Thanks; good hint -- lots of results with Google :)

See http://www.sqconline.com/reliability/capacitor1.html for an online
version.


For Russell: the capacitor types covered are (the full document probably
contains the full description of the abbreviations):

CP: Capacitor, Paper Dielectric, DC (Hermetica...
CA: Capacitor, By-Pass, Radio-Interference Red...
CZ, CZR: Capacitor, Feed through, Radio-Interf...
CQ, CQR: Capacitor, Fixed Plastic (or Paper-Pl...
CH: Capacitor, Fixed, Metaliized (Paper, Paper...
CHR: Capacitor, Fixed, Metallized Paper, Paper...
CFR: Capacitor, Fixed, Plastic (or Metalized P...
CRH: Capacitor, Fixed Supermetallized Plastic ...
CM: Capacitors, Fixed, Mica Dielectric...
CMR: Capacitor, Fixed, Mica Dielectric, Establ...
CB: Capacitor, Fixed, Mica Dielectric, Button ...
CY: Capacitor, Fixed, Glass Dielectric...
CYR: Capacitor, Fixed, Glass Dielectric, Estab...
CK: Capacitor, Fixed, Ceramic Dielectric (Gene...
CKR: Capacitor, Fixed, Ceramic Dielectric (Gen...
CC, CCR: Capacitor, Fixed, Ceramic Dielectric ...
CDR: Capacitor, Chip, Multiple Layer, Fixed, C...
CSR: Capacitor, Fixed, Electrolytic (Solid Ele...
CWR: Capacitor, Fixed, Electrolytic (Tantalum)...
CL: Capacitor, Fixed, Electrolytic (Nonsolid E...
CLR: Capacitor, Fixed, Electrolytic (Nonsolid ...
CRL: Capacitor, Fixed, Electrolytic (Nonsolid ...
CU, CUR: Capacitor, Fixed, Electrolytic (Alumu...
CE: Capacitor, Fixed, Electrolytic (DC, Alumin...
CV: Capacitor, Variable, Ceramic Dielectric (T...
PC: Capacitor, Variable (Piston Type, Tubular ...
CT: Capacitor, Variable, Air Dielectric (Trimm...
CG: Capacitor, Fixed or Variable, Vacuum Diele...

Gerhard

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