Electronics > Beginners
High frequency capacitor selection
ZeroResistance:
I need some capacitors for high frequency resonant oscillators, these capacitors also need to handle high frequency resonant currents and should be of low loss. The frequency range I am looking at is between 1 Mhz to 15 MHz.
I was looking into polypropylene capacitors for these and while looking into data sheets I found something strange.
The AC Voltage rating of the capacitor drops with frequency. So If I select a 250VAC / 1000VDC, 1nF capacitor. At a frequency of 1Mhz the AC voltage rating drops to 90VAC rms.
I want a capacitor at least for 13.56 MHz frequency with AC voltage of 250V rms. What would be a suitable capacitor for this frequency?
There are a few graphs in the datasheet that I have attached.
Also a equation that doesn't make sense to me "AC voltage: f <= 1000 Hz; 1.4 x Urms + UDC <= Ur"
sourcecharge:
V-caps are teflon and copper.
They are expensive though, and the lowest cap is 10nF.
Never tried them but I'm guessing they would do the trick.
https://www.v-cap.com/cutf-capacitors.php
ZeroResistance:
--- Quote from: sourcecharge on August 14, 2018, 05:39:21 am ---V-caps are teflon and copper.
They are expensive though, and the lowest cap is 10nF.
Never tried them but I'm guessing they would do the trick.
https://www.v-cap.com/cutf-capacitors.php
--- End quote ---
That's interesting never heard of these before, and I thought that polypropylene capacitors had the lowest loss particularly for high frequency applications. Aren't there any polyproylene capacitors that work at this frequency range with a suitable volatge range of 250Vrms. Or is connecting capacitors in series to get the desired voltage rating the only way out?
Twoflower:
I think the cap selection of choice for high frequency requirements are Silicon Caps: https://www.murata.com/products/capacitor/siliconcapacitors
helius:
--- Quote from: ZeroResistance on August 14, 2018, 05:30:23 am ---Also a equation that doesn't make sense to me "AC voltage: f <= 1000 Hz; 1.4 x Urms + UDC <= Ur"
--- End quote ---
It is talking about peak instantaneous voltage. 1.4 x Vrms + Vdc is the (positive) peak of a sinusoid.
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