FreeLists / si-list / [SI-LIST] Re: Model of Capacitors

["Bart Bouma" <bart.bouma@xxxxxxxxx>]

Hi Fred,
I already replied to Zhangkun (directly I believe) on his original post 
and explained the temp. and voltage dependency of high K-materials.
Modeling should be done at the applied voltage.

You're refering to X7R or worse class materials when saying +80/-20% 
tolerance. These type of capacitors are indeed used in decoupling apps etc 
and typically have large capacitance values (typ. 100nF and above), but 
are strongly dependent on voltage and temperature.
But they offer high capacitance-values, low inductance and (very-)low ESR, 
so quite suitable for decoupling/filtering. Change in capacitance is not 
critical here.
For resonant circuits however one should never use such X7R ceramic 
capacitors (also don't forget the piezo effect and the dielectric 
absorption/memory effect these exhibit).

Instead there are NP0/C0G type/class ceramic multilayer capacitors: low 
values (upto several nF) but very stable against temperature and voltage 
and showing very low dielectric absorption values.
These are used for microwave circuits, such as oscillators, resonance 
circuits, hi-speed S&H etc. and have tolerances down to 1%.

best regards, Bart






"Fred Townsend" <fred@xxxxxxxxxxxxx>

31-08-03 19:45
Sent by: si-list-bounce@xxxxxxxxxxxxx

Please respond to fred

 
        To:     <zhang_kun@xxxxxxxxxx>
        cc:     <si-list@xxxxxxxxxxxxx>
        Subject:        [SI-LIST] Re: Model of Capacitors
    Category: 



> Dear all:
> I have one question about decoupling effect of caps. When the voltage on
> the caps is changing, does the decouling effect of capacitor change?

Simple answer, Yes!  Lots!

This why anyone using a capacitor value as anything other than a first
order approximation is fooling themselves.  There are many effects based
upon temperature, voltage, history, age, diaelectric, etc.

This is also why a bypass type capacitor should never, ever, be used in a
critical value circuit (such as resonate circuits).

If you have any doubt about the classification of a capacitor take a look
at its tolerance.  Many bypass caps have a tolerance of +80/-20%!

Fred Townsend
>
> For example, there is one capacitor, 10uF/100V. I measure its model by
> means of VNA at low voltage. When the capacitor is used to decouple
> -48V, could the model of caps be used?
>
> Best Regards
>
> Zhangkun
> 2003.08.29
>
>
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