High frequency capacitor selection - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 05:30:23 am
content: 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"

author: sourcecharge
author_rank: Regular Contributor
timestamp: August 14, 2018, 05:39:21 am
content: 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

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 05:52:34 am
content: Quote from: sourcecharge on August 14, 2018, 05:39:21 amThat'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?

author: Twoflower
author_rank: Frequent Contributor
timestamp: August 14, 2018, 06:29:24 am
content: I think the cap selection of choice for high frequency requirements are Silicon Caps:https://www.murata.com/products/capacitor/siliconcapacitors

author: helius
author_rank: Super Contributor
timestamp: August 14, 2018, 06:39:00 am
content: Quote from: ZeroResistance on August 14, 2018, 05:30:23 amIt is talking about peak instantaneous voltage. 1.4 x Vrms + Vdc is the (positive) peak of a sinusoid.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 07:06:11 am
content: Quote from: helius on August 14, 2018, 06:39:00 amOK, so if Urms = 250VAC and if I'm correct UDC is the DC level on which this Sine wave is superimposed on? correct?Then that equation is 1.4 * 250 + UDC (lets say UDC is 0) so that comes to 350V.So is that equation trying to say that 350 should be <= Ur for a frequency of less that 1000Hz? What about frequencies above 1000Hz?And what is Ur? is it the max rated voltage of the capacitor?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 07:09:54 am
content: Quote from: Twoflower on August 14, 2018, 06:29:24 amYes they seem to be for high frequencies in the GHz range.. but can they handle high sinusoidal currents running thru them? Something in the order of 10 to 20Amps peak.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 14, 2018, 08:06:35 am
content: http://www.atceramics.com/CheersAlex

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 09:58:33 am
content: OK, so good alternatives available from Silicon, Teflon and ceramic capacitiors. However I would like to know why does the capacitor rated AC volatge decrease as freqency increases? Anyone has any info regarding this?

author: helius
author_rank: Super Contributor
timestamp: August 14, 2018, 10:02:12 am
content: Quote from: ZeroResistance on August 14, 2018, 07:06:11 amAffirmative.QuoteThe permissible peak voltage typically decreases with higher frequencies, as the capacitor's impedance decreases until the resonant frequency is reached. The specific phenomenon that weakens the voltage withstood is called "dielectric loss".Quote from: From Kemet's white paper What Is A Capacitor?QuoteYes. It will be spelled out in the datasheet: capacitors can have a Surge Rating higher than the Working Voltage rating (only the latter is usually printed on it), which would be the limit in this case.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 10:21:00 am
content: Quote from: helius on August 14, 2018, 10:02:12 amOk, so all capacitors will face this roadblock right? At higher frequencies eventually their Xc will be a limiting factor right? For eg. for a 1nF capacitor the Xc comes to 160 ohm for a frequency of 1Mhz. So how come the ceramic capacitors are used for RF power at frequencies in the 10's of Mhz to 10's of Ghz?

author: iMo
author_rank: Super Contributor
timestamp: August 14, 2018, 10:40:23 am
content: QuoteAll types of capacitors have a quality Q. The quality depends on "parasitic" serial/parallel R, L, and frequency. The same with ceramics for 0.01-100GHz.The 1nF polypropylen bulk Wimo could have 500nH L, 2000ohm R (because of its mechanical construction).The ceramics for xxxxMHz are of different construction - those are leadless pieces of ceramics of special shapes. The 1nF may have 0.xxnH L and 0.xxohm R for example.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: August 14, 2018, 10:44:09 am
content: Quote from: ZeroResistance on August 14, 2018, 10:21:00 amhttp://www.atceramics.com/technical-notes.aspxCheersAlex

author: iMo
author_rank: Super Contributor
timestamp: August 14, 2018, 11:03:21 am
content: @ZeroResistance: better you define the required capacity (in nF/uF) at specific frequencies, max currents (Ip-p), voltages(Vp-p), and other params you are aware of, and the experts here may identify the proper caps for you easier.

author: dmills
author_rank: Super Contributor
timestamp: August 14, 2018, 11:43:40 am
content: The reason the applied voltage limit drops with frequency, is that you are really hitting a current limit!Xc drops with increasing frequency, so the voltage you can apply without exceeding the maximum rated current (Yes, there always is one) drops with increasing frequency.What you probably want are 'doorknob' capacitors used in high power transmitters, these are specified for RF current handling and voltage.ATC make excellent parts but mainly for higher then the wiggly DC that is 13.56MHz.Regards, Dan.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 14, 2018, 12:55:49 pm
content: Quote from: imo on August 14, 2018, 11:03:21 amProblem being I looking for various values from 1000pf to 100pf, i might connect these in series or parallel to get the desired tuning.

author: Twoflower
author_rank: Frequent Contributor
timestamp: August 14, 2018, 07:38:36 pm
content: I was suggesting that you should get in contact with Murata to get a complete data sheet. Unfortunately I don't have the max peak current or other values than shown on the web. They seem to be very stable over a wide range of values. But your capacitance ranges might strike them from the list.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2018, 12:44:56 am
content: C0G ceramic or silvered mica.If you're doing that much voltage into any significant amount of capacitance (let alone 1nF), you're probably looking at doorknob or pot type capacitors, possibly with water cooling.Maybe just salvage a diathermy machine (that sounds like what you're doing, anyway!).Tim

author: bson
author_rank: Unknown
timestamp: August 15, 2018, 03:49:42 am
content: A 90V 15MHz resonator? That sounds like volcano-dwelling mad scientist stuff to me...

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 15, 2018, 06:21:15 am
content: Quote from: dmills on August 14, 2018, 11:43:40 amAt 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?

author: JS
author_rank: Frequent Contributor
timestamp: August 15, 2018, 06:42:32 am
content: Quote from: ZeroResistance on August 15, 2018, 06:21:15 amPulses 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

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2018, 07:19:56 am
content: 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.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 15, 2018, 07:37:46 am
content: Quote from: imo on August 15, 2018, 07:19:56 amOk, 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.

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2018, 07:42:11 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2018, 07:42:24 am
content: 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

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: August 15, 2018, 01:19:20 pm
content: Quote from: imo on August 15, 2018, 07:42:11 amThat's an awesome plot "imo", If the cap is operating in resonant mode can we expect the same levels of current, or would they be lower?Quote from: T3sl4co1l on August 15, 2018, 07:42:24 amAgreed, at 1Mhz skin depth of copper is around 65microns and Aluminium 82 microns. I was looking at film-foil capacitors and not the metalilized types? So if we want to get a 1nF cap running at 1Mhz and higher. Considering the Xc of 160 ohm, and keeping voltage as 250VAC rms would it mean thatWider foil with around the 80 micron thichness to give a cross sectional area for passing the desired continuous current should do the job? A Wider foil might obviously affect other parameters, or would it not??I understand that film-foil capacitors construction is such that it can handle large currents, how do these COG type ceramics handle the same levels of current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 15, 2018, 04:02:15 pm
content: Quote from: ZeroResistance on August 15, 2018, 01:19:20 pmGood question. How could we predict such things?QuoteTLDR: find ESR (or loss tangent, which is ~= 1/Q) and solve for power dissipation.Tim