| Electronics > Beginners |
| 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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