[SI-LIST] Re: Model of Capacitors
- From: <fred@xxxxxxxxxxxxx>
- To: <bart.bouma@xxxxxxxxx>
- Date: Tue, 2 Sep 2003 09:54:20 -0700 (PDT)
> 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%.
Bart I totally agree and I am aware of all these effects and even more
like the effects on teflon, mica, and glass dielectrics. The point I was
trying to make was the original question is not or should not be relevant
in modeling because the gross variables mean bypass caps can only be
treated as minimum values, approximations at best. Kind'a reminds me of my
students that accquire data with a scope and turn in data with 6 places to
the right of the decimal point because their calculators have 6 places. A
waste of good pencil lead.
Fred
>
> 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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