Dielectric strength at high voltages - Page 1
Forum section: electronics / projects

author: Barnee345
author_rank: Contributor
timestamp: March 14, 2023, 12:33:03 pm
content: I have two questions regarding the use of coupling capacitors in a classic circuit design. The main difference to the former design is the use of SMD technology. A coupling capacitor from WIMA of the type SMDTJ03100YA00MT00 is to be used, with the values 100 nF for the capacitance and 630 VDC for the voltage range. One side of the capacitor is connected to the anode voltage of a triode (ECC82) with 250 VDC to ground. The other side is connected to the grid of a pendode (EL34) via a series resistor (2k2), this capacitor side is also in contact with the negative grid bias of -70 VDC, so the voltage difference across the capacitor can be a maximum of 320 VDC. The dielectric strength of the capacitor should therefore be sufficient even with AC signals applied.Since an SMD capacitor is to be used, it is of course mounted on a circuit board using the appropriate technology. The printed circuit board is finished with a solder resist. The area on the board below the capacitor is isolated from its connection pads by corresponding free areas, but it carries ground potential.And now for the first question. The area has a size of 14 mm². This area could form another parasitic capacitor with the first plate of the capacitor. Is this likely and what could be the equivalent circuit of such an arrangement? I assume that this structure has an influence on the signal to be coupled via the capacitor.My last question is about the dielectric strength of the capacitor with respect to the ground plane of the PCB. I assume that the dielectric strength of the capacitor includes this complex.

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2023, 12:47:30 pm
content: use TH not,SMD components in valve / tube designs.derate all VDC of caps.Beware the voltage coefficient of ceramic MLC, check cap vs temp and cap.vs,voltage spec.We used only plastic capacitors axiaTH in our 1970s..1990s audience of valve designs.The ceramic MLC are,best for PS bypass, not to pass audio signalsJon

author: Barnee345
author_rank: Contributor
timestamp: March 14, 2023, 01:39:01 pm
content: Hello jonpaulnice to get an answer already. Regarding the capacitors, I have no problems, the WIMA capacitors I use for coupling audio signals are made of the best foil material. You should have a look what WIMA writes about it. Their SMD types are equivalent to the TH types. But that was not my question.My question is about the coupling of parasitic capacitances. Thanks for the answer anyway.

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2023, 02:04:24 pm
content: Barnee345,Yes, the WIMA, Siemens, Mial, EPCOS plastic caps are fine, choose the dielectric according to quality of audio and size / cost trade-off.the few pf of stray capacité SMD cap to PCB has little effect on overall frequency response.Stray cap is easy to estimate or measure, just 5......20 pf probably.Reduce that capacity by spacing the part off the PCB, and clear ground plane and tracés beneath or nearby.Anyway, you need some mm of PCB clearance for the voltages mentionedbon chancejon

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2023, 03:03:12 pm
content: To my knowledge, the best plastic-film capacitors (such as polypropylene) are not readily available in SMD packages, due to temperatures encountered during SMD soldering and assembly.The last time I looked into film SMD devices, the available higher-temperature dielectrics had considerably higher dielectric loss than polypropylene.

author: Barnee345
author_rank: Contributor
timestamp: March 14, 2023, 05:42:02 pm
content: I was particularly impressed by this statement because most film capacitors for the high-voltage range are manufactured by WIMA:QuoteThis is one of the reasons for me to believe that SMD capacitors are also suitable for the application.

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2023, 06:41:09 pm
content: Which dielectric films are available in those SMD capacitors?

author: Barnee345
author_rank: Contributor
timestamp: March 14, 2023, 08:34:49 pm
content: Polyethylene-terephthalate (PET)But you can also read the spec of the capacitor I mentioned in the first post.

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2023, 09:13:11 pm
content: PET, often called "Mylar" after the DuPont trademark, is a common plastic-film dielectric.It can be made with respectable voltage ratings.However, for critical applications, it is inferior to polypropylene in dielectric loss and dielectric absorption.Polystyrene (now virtually extinct) is also better than PET, but has even lower maximum temperature than polypropylene.For vacuum-tube circuits, the designer has a wider choice in TH capacitors than in SMT capacitors.

author: Barnee345
author_rank: Contributor
timestamp: March 15, 2023, 12:30:21 am
content: Hi TimThank you very much for the explanations. I will think hard about which choice to make. Ok, the dissipation factor is 4 times better (at 10 kHz) with polypropylene than with PET. If I choose a type with radial connections, then I also get rid of the problem of parasitic capacitances. I have no experience with vacuum tube circuits, but I do with "normal" electronics.

author: Someone
author_rank: Super Contributor
timestamp: March 15, 2023, 01:19:43 am
content: Quote from: Barnee345 on March 14, 2023, 12:33:03 pmNo.Quote from: Barnee345 on March 14, 2023, 12:33:03 pmThis may change depending on the orientation of the capacitor! Start with a worst case assuming the entire face of the capacitor is a plane connected to the least favourable terminal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2023, 03:05:00 am
content: A couple of points just to clear things up:- SMT film caps are expensive. I don't see any reasonable justification for this choice: the difference in assembly cost will be less than the part cost; size isn't an issue because the tubes are far bigger than the passives around them; and quality factor is irrelevant.PPS is a high-quality dielectric by the way, available in SMT chips, and is usable in place of PP more or less. This isn't an issue.The described application (coupling capacitor) is only sensitive to leakage current; the Q factor or even linearity of the part isn't important. (So, PET is fine.) Well, linearity a little, but only near the cutoff frequency -- which can be made low enough not to matter (say, a cutoff below 5Hz for an audio bandwidth of >20Hz). This might even suggest electrolytic or [type 2] ceramic, but these are probably excluded for leakage current or microphonic reasons. (C0G ceramic would be perfectly fine, but is also quite expensive in these large values. If grid resistors can be increased so that ~10nF is reasonable, it might be a strong contender.)- Uncoated stacked-film SMT chips are prone to leakage. Probably not enough to matter here, but it's worth mentioning. The problem is something like, inevitably the stack delaminates in places, letting in solder flux, which is impossible to wash out of the microscopic crevice. Or other environmental contamination is let in, in turn (ambient humidity, ions from ambient dust). PPS is otherwise an excellent material, and would be desirable for sample-and-hold applications for example, or extremely low frequency active filters (in compact applications where the SMT format is required), but this leakage frustrates such usage.- 400VDC or higher rating is acceptable, and 630V is adequate overhead. No surge rating is needed, and no film capacitors are prone to the voltage derating mechanisms of electrolytic, tantalum or [type 2] ceramic.- EMI type self-healing capacitors may actually be desirable toavoid, just because they tend to use such thin metallization that it oxidizes completely away in some decades of exposure to atmospheric moisture (i.e. moisture and oxygen absorb into the dielectric very slowly, over such time scales; the layer of aluminum is so thin it isn't self-protecting as with bulk metal, and eventually oxidizes through). These capacitors tend to have high ESR as well (again, since the metallization is so thin), not that that matters at all here.Plain bog standard PP or PET film caps, dipped, wrapped or boxed, will suffice. They may still be self-healing. They may still be X/Y rated (the above is not specific: there are plenty of low-ESR EMI caps as well). It really doesn't matter.To be clear, a tube circuit is more than robust enough to handle capacitor self-healing behavior, should it ever occur in the first place.Also for reference, such "high ESR" capacitors might have as much as tens of ohms. Compare to circuit impedances of 10s of kohm. ESR / Q factor just doesn't freaking matter. I don't know I can stress this enough: tube circuits areextremelynoncritical.- PCB materials have extremely high breakdown. The fiberglass material is treated as a "cemented joint" in most standards (e.g. IEC 60950-1; in this one, there are clauses specific to PCB material however), which can be used at rated electric strength with no creepage penalty through the material. Creepage is only an exposed (in-air) surface phenomenon.Typical 1.6mm 2-layer PCB stock has a breakdown voltage of... 10s of kV at least? You're much more likely to see sparks wrapping around edges/slots or along the surface, than through the stuff.Maintain adequate same-layer clearance, and avoid crossing traces under chip components. (Well, resistors are probably fine, just make sure they're not flip-chip types -- the bog standard ceramic chip resistor is top-metallized, bottom is pure alumina core: not an issue.)- Tube circuits are one case where I would recommend potentially removing ground pour entirely, or at least using much more sparingly. The low impedance (Zo ~ 150 ohms at most) and shielding value are not important at these impedances. The capacitance reduces bandwidth needlessly -- not that bandwidth is ever a concern for audio purposes, so treat this as a tweak, not an absolute. (Indeed,use only what bandwidth you need. Exclude out-of-band interference, particularly at outside connections (signal and power inputs/outputs), and you will also avoid potential rectification / mixing effects.)Which is not to say, be irresponsible with loop areas, proximity of signals, etc. By all means, continue to apply RF grounding / shielding / isolation principles to the system. Just allow more distance between signals and ground, and that means still more distance between different signal traces. System ground plane is still possible -- and desirable -- but it will be in the form of a metal chassis, a fixed distance away from the PCB. Keep separation distances relative to that, just as you would separate microstrip traces by the substrate height. When you need a "via" to ground-ground, simply place a plated screw hole there, and anchor with a standoff.Definitely do not get obsessive with star-grounding and make a convoluted mess. Typical attempts at this, that I have seen, result in far greater crosstalk at audio and high frequencies, compromising stability or making oscillators outright. Only the input ground reference is most critical, and that can be dealt with using a differential amplifier configuration instead.Indeed, coaxial inputs (e.g. RCA jack) can be treated differentially just as any other cable can be; you will still want a low RF impedance (to GND, i.e. many small bypass caps in parallel) to ensure good EMC, but at audio frequencies it can be modestly isolated from chassis ground (say with some "bleeder" resistance and a TVS) and a differential amplifier used to subtract out the common-mode voltage. Thiswholly eliminates ground loop induction at the input, within the compliance range of the amplifier (and TVS).It's only a little more work to do this with tubes (basically for-free with solid state, you're using an op-amp at the input regardless; tubes, you probably choose a LTP preamp), and delivers this actual bombshell of performance.Like... it's peculiar to me how little known this technique is in audio circles. Almost obsessively, they ground the input connector to chassis, get plagued by ground loops, go to extreme lengths to try and fix it, with only limited success (e.g. massive copper strap bonding equipment together, only to reduce it by some dB), and at great hazard (e.g. floating the chassis, using ungrounded power cables)! But, well, go figure.Or if this is something like a guitar amp, grounding may not be very important (the cable, instrument and player are probably isolated from ground themselves), but it certainly won't hurt anything to apply it in that case. And needless to say, it's cheaper than an audio isolation transformer -- but lacks the compliance range (isolation voltage), so one will still be required in case of cross-wired grounds for example (on the rare but severe occasion that a performer gets zapped from badly wired mics, lighting, instruments, etc.)....I suppose one could make a circuit to handle even that; tubes handle the voltage just fine, but you do need exceptional supply voltages (say +/-500V supplies for a generous margin on 240VAC common mode?), a step maybe not worth taking given the rarity of such conditions.Tim

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: March 15, 2023, 09:43:20 am
content: Hi Barnee345,can you post a picture of the layout/pcb? Just the area of interest.Remember to place the outside-foil/film of the capacitor (ring- or bar-marking) on the lower-impedance side.In your case this would be the ECC82-anode.Air is the best insulator (Allen Wright (RIP))Best regards and good luck

author: Barnee345
author_rank: Contributor
timestamp: March 15, 2023, 10:17:18 am
content: Hi Tim (T3sl4co1l)I am thoroughly impressed with your contribution. I immediately copied the post and saved it on my computer. It goes beyond my question in many ways - but probably my concerns are already so well known that you can answer the resulting questions in your sleep.Thank you very much for your answer.Hubert

author: jonpaul
author_rank: Super Contributor
timestamp: March 15, 2023, 10:59:23 am
content: Bravo to the OP to make a valve amp.suggest you avoid SMD and even a PCB for one off DIY.Valve amps are easily handwired with axial TH parts.SMD and PCB only complicate the projectIf OP can better set the objective can advise better the capsegHigh end audiophile amp?learn to work on valves amps?make products for a market?For 99% of non audiophile, almost any plastic cap is fine.The fine points of dielectric absorbtion, and distortion are really 2ond, third order effects hardly audibleJon

author: TimFox
author_rank: Super Contributor
timestamp: March 15, 2023, 01:55:55 pm
content: A few more details on film capacitors:PPS has a good temperature rating and its dielectric loss is between PET and PP.It's probably the best choice if a SMT film capacitor is really needed.In the modern legitimate vacuum-tube amplifier literature, it has been shown that capacitors can have measurable distortion, depending on both the dielectric and construction.Of course, the bad ceramics (Z5U, etc.) have bad values due to their strong voltage dependence.In general, PP seems to be the best practical choice when distortion is critical.PTFE is great, especially for dielectric absorption, but is expensive and rare.In my recent hobby work, rather than use PCBs, I do point-point wiring with PTFE stand-off insulators mounted on a single-sided 2 oz Cu on FR4 board at appropriate locations.

author: Barnee345
author_rank: Contributor
timestamp: March 15, 2023, 07:01:41 pm
content: Oh, that's a lot of information from a lot of friendly people.In regard to the interest shown byjonpaul, I would like to use a few clarifying words on my person. Actually, I would have to be younger to put such a project into action. I retired from professional electrical engineering about 12 years ago and, with a few minor exceptions, keep it the same in my private life. I have dedicated myself to working with wood for my retirement. A friend of mine, knowing this, approached me to help him build Klipsch corner speakers. He had a few plans, but they were rather something for the trash. I then did some research and, by a stroke of luck, had access to photocopies of drawings that Paul Wilbur Klipsch had made in 1945. It is important to know that Klipsch still sells these corner horns today. I studied these documents and was able to grasp the basic design as well as the construction principle of the exponential horn. Based on these documents and the knowledge gained from them, I made the drawing for the construction of the box. In the meantime, the first horn is finished and has already been heard in rehearsal. During this time the desire arose to build a tube amplifier, but not for use with these Klipsch horns.Over the last 12 years I have digitised pretty much everything I could get my hands on in the way of vinyl records. Much of it lies dormant on my hard drives. That's about to change. I want to stream the music from the hard drives and feed the amplifiers via LAN. To do this, I will use a network-compatible DAC that can also process 192/24 (kHz/bit) high-resolution data. This unit is already operational. Now I need 2 monoblocks + remote control. The monoblocks are thus the current topic.Since SMD technology is not so foreign to me - 6-pin DFN packages are not a horror for me - I don't think it is outlandish to realise the monoblocks with PCB and SMD. However, if a component is not available in SMD with the required parameters, then a conventional component is not considered a problem either. I work with DipTrace and have done some excellent things. The division into 2 monoblocks is decided, I just ordered the toroidal transformers with shield winding.And now toRoehrenonkel. Should you have the opportunity to read in the German HeiFei forum? There the topic is being trampled on to the point of ridicule. What a pity. That makes me sad.But thank you for the hint to install the capacitors in this way.Now I have already answered some of what is in jonpaul's interest. I am good with SMD and PCB. I think I've already guessed correctly about having to be extra careful when choosing the coupling capacitors.I also thankTimFox, of course. In the meantime I have understood which foils I should use for capacitors.T3sl4co1lalso played a significant role in helping me to make the right decisions. In any case, I have considered not using SMD types for the coupling capacitors, but normal print types. Such a decision can solve two problems at the same time: no limited choice and no parasitic capacitors on the PCB.