Another Ceramic Cap Anecdote

Aug 07, 2019 2 Replies
T

Anecdote, since the plural of "anecdote" is not "data". It is suggestive though...


0.47uF 16V X7R 0603, Samsung CL10B474KO8NNNC. Withstands over 120V without breakdown.

Tested in a LLC resonant converter. Variable range quite large, and variability doesn't go away when hot. By "hot", we're talking on the order of 20 VAR.



Startup of the converter is interesting. It drifts severely at first, as the capacitors heat up. It reaches equilibrium, seemingly somewhere below Curie temperature. Possibly the core temperature is over, and varying amounts of chip are depolarizing?



Even when hot, the adjustable range is quite good. Hm, didn't seem like there was a big tail after adjustment, but there should because of the change in reactive power? May've been masked by the large time constant I used for biasing.



Tim


Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
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G

What's VAR?

Curie temperature makes me think of ferro-magnetics.. but I assume you mean the curie temp of the ceramic. (And what is a ceramic Curie temperature?)

George H.

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T

Volt-amps reactive. It's power, but sideways (i.e., imaginary). It has units of power but it's actually energy -- if you know the frequency it's being "sloshed" at (give or take some pi, E = VAR / (2*pi*F)).

Analogous to torque having the units of work (i.e., N.m), but being "sideways", so it's not real work, it's imaginary work. In other words, rotating force.

Yes, exactly: ferroelectricity is so named because the physics is almost identical to ferromagnetism. It's just with electric charges rather than magnetic spins.

So, that's the temperature where the charges are unable to maintain spontaneous, collective polarization. Also relevant: leading up to that temperature, the field strength that causes saturation drops exponentially (hrm, actually, hyperbolically shouldn't it?). Meaning that the C(V) curve should be shifted left as T --> Tc.

I guess X7Rs being good up to 125C means they're Tc >= 150C or something like that. Probably about as high as BaTiO3 and relatives go, from what I understand?

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
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