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High frequency capacitor selection

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JS:

--- Quote from: ZeroResistance on August 15, 2018, 06:21:15 am ---At 1Mhz the Xc of a 1nF capacitor is 160 ohms.
So If I take the actual rated voltage of 250V rms the current is 250/160 = 1.5A rms.
and If I take the max allowed volatge of 90V rms @ 1Mhz the current is 90/160 = 0.56A rms.

I still don't see why 1.5A * 1.414 = 2A peak should cause damage to a capacitor, especialy a pulse rated capacitor. What am I missing here?

--- End quote ---

Pulses are short duration and every once in a while? As opposed to constant RMS rating... Most devices have a very different spec for pulse than for nominal value, you will find, for some semiconductors, the SOA in the datasheet, which shows the different ratings at different exposure times.

JS

iMo:
The pulse current through a capacitor is mainly given by the parasitic resistance/inductance of the capacitor, impedance of the pulse source, rising/falling times of the pulse's edges and the amplitude of the pulse. With a "perfect capacitor" and a "perfect pulse source" the current is unlimited high.

ZeroResistance:

--- Quote from: imo on August 15, 2018, 07:19:56 am ---The pulse current through a capacitor is mainly given by the parasitic resistance/inductance of the capacitor, impedance of the pulse source, rising/falling times of the pulse's edges and the amplitude of the pulse. With a "perfect capacitor" and a "perfect pulse source" the current is unlimited high.

--- End quote ---

Ok, So if I connect the said 1nF capacitor to a 1Mhz AC signal source at supply 230Vac rms wavefrom to it what kind of current waveforms should I expect? would'nt it be just a plain old sinusoidal current waveform.

iMo:
This is a 1nF capacitor with R=10mOhm, L=10nH, source is 0/100V square wave (100Vp-p) of 10MHz with 1ns r/f edges and internal resistance of 10mOhm.
The current is ~1200Ap-p.

T3sl4co1l:
Ballpark speaking, the RMS current rating is usually 1/5 to 1/100 the pulse rating.  If a pulse or dV/dt rating is provided at all.

RMS current limit is thermal, obviously.  It can be worse at higher frequencies, due to skin effect and such; CDE 940C caps for example are quite prominent in this way (rated for lots of peak amps, but not very much RMS at 100kHz, and even less at 400kHz -- I know, I've melted them before).

Mind that ceramic chip caps aren't very conductive (they aren't made of Al2O3, or much metal!), so they can't handle the same power level that a chip resistor (typically Al2O3 or metal body) of the same size would.

I have plans to build an induction heater with C0Gs; it'll take about qty 300, wired in parallel.  It's a pretty good bit of reactive power handling, though: 100 VA each (30kVA total) isn't bad at all for parts so small!

Tim

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