Isolated power supplies - Page 1
Forum section: electronics / beginners

author: epaelectronic
author_rank: Contributor
timestamp: January 15, 2021, 11:05:52 am
content: Hello.I am experimenting with isolated power supplies for some ic's.Recently I remembered about wireless charging systems.Would two coils with inductive power transferring provide the same kind of galvanic isolation as a transformer?So I'm thinking would it be possible to embed the coils directly insidea multi layer PCB if board space wasn't a constraint and the efficiency didn't matter too much?So basically having two coils on top of each other inside a PCB.Let's say the ic draws around 600mA max.Are there any sort of calculations/calculators for coils directly on a PCB orwill I have to experiment and measure to find out the coil's resonant frequencies etc?How about optimizing the coils when designed directly into a PCB?Could the transmitting coil be driven by a flyback converter for example? If not, what would be ideal for a power supply?

author: ledtester
author_rank: Super Contributor
timestamp: January 15, 2021, 11:32:07 am
content: No experience with it, but this program says it can design flat pcb coils:https://coil32.net/However, in this video the guy says he just measured the inductance of the coil after it came back from the fab house:https://youtu.be/3QEGdm6vfUwBtw, this guy's been doing some cool stuff with PCB coils:https://www.youtube.com/channel/UCdxTCCRnQgfi2vr2fZupYIQ

author: Michaelaudio
author_rank: Regular Contributor
timestamp: January 15, 2021, 12:24:17 pm
content: Two coils with inductive Power transfering basicslly IS a transformer and will have the same galvanic isolation. (As Long as u have an air gap between or another Isolator)I have a PSU where the coils of transformer are inside a small PCB so its possible but i dont know any simulation programm.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 15, 2021, 05:48:55 pm
content: An airgap wont necessarily help. You need a properly rated transformer with the right isolation voltage. You could use flat coils on either side of a PCB but its a waste of space and an EMI headache you don't need.https://resources.altium.com/p/how-create-planar-transformer-pcb-layout.Small (even kV) isolated DC_DC power supplies are cheap as chips.First work many volts isolation barrier needs to cope with.Design your magnetic circuit.Choose a dielectric material of the right dimensions.Factor in creepage and clearance.Then order one off the shelf.

author: Benta
author_rank: Super Contributor
timestamp: January 15, 2021, 06:06:20 pm
content: Search for "planar transformer".

author: epaelectronic
author_rank: Contributor
timestamp: January 15, 2021, 06:23:24 pm
content: Could you please explain what causes a planar transformer to be more an EMI headache than a regular discrete transformer?

author: TimFox
author_rank: Super Contributor
timestamp: January 15, 2021, 06:26:45 pm
content: Leakage inductance determines the fraction of the input flux in the primary that leaks out into the environment instead of being coupled to the secondary circuit. The lowest leakage inductance is in a good toroidal transformer, but two coils separated from each other without an iron core will have substantial leakage inductance compared with the mutual inductance between the coils.

author: epaelectronic
author_rank: Contributor
timestamp: January 15, 2021, 06:33:16 pm
content: Does the leakage inductance make a iron-core-less planar transformer unusable in the terms of EMI? or could it still work well with careful design somehow?

author: TimFox
author_rank: Super Contributor
timestamp: January 15, 2021, 08:48:31 pm
content: Careful overall magnetic shielding can work: at high frequencies, continuous conductive shields are the usual approach. The devil is in the details, and the problem is quantitative.

author: epaelectronic
author_rank: Contributor
timestamp: January 15, 2021, 09:27:40 pm
content: I've gotten the impression that shielding for magnetics is pretty hard. Is around 200-300Khz considered high frequency when it comes to planar transformers? Would continuous conductive shields even function at these frequencies?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 17, 2021, 06:28:01 pm
content: It depends on your "conductive material". There are ferromagnetic metal alloys made for the job Look up "mumetal".A simple conductor will only shield you from electric fields- the Faraday Cage.

author: Dennis Frie
author_rank: Contributor
timestamp: January 17, 2021, 07:57:36 pm
content: An inductor without a ferromagnetic material (typically air core inductor) gives very little inductance and is really not usable for anything but very high frequencies.As you need the core anyway, it's a lot easier to either used an off-the-shelf coupled inductor or design a transformer.While there's some interesting applications for planar transformers, on a hobby project, I would only go that way, if you do it out of interest.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 08:00:17 pm
content: Quote from: epaelectronic on January 15, 2021, 06:23:24 pmIt does not have a magnetic core that concentrates the magnetic field.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 08:24:03 pm
content: Is it necessary to use a transformer for power transmission? The capacitor can also act as an insulator. It can be a flying capacitor or it can be an insulating capacitor on the board.

author: Benta
author_rank: Super Contributor
timestamp: January 17, 2021, 08:58:20 pm
content: Quote from: S. Petrukhin on January 17, 2021, 08:24:03 pmHere's an experiment for you: take a standard 230 V plug and wire one end of a 100 nF capacitor to the live prong.Insert it into a live outlet.Grab hold of something that's earthed with one hand (heating, water tap...).With the index finger of your other hand, touch the other end of the capacitor.Unpleasant, no?And not really viable for a galvanically isolated power supply.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 09:25:34 pm
content: Quote from: Benta on January 17, 2021, 08:58:20 pmFirst, you don't understand or don't know what a flying capacitor is.I assure - you can take it as much as you want with one hand and the other for PE.Secondly, it is not necessary to be the arm of the capacitor divider and take over the entire charge current - this arm can be the second capacitor.Third, I was talking about a capacitor made of layers of copper on a board, such a capacitor has a very small capacity and you are not in danger of getting a charge at a low frequency of the supply network.

author: Benta
author_rank: Super Contributor
timestamp: January 17, 2021, 10:48:19 pm
content: I just know what a flying saucer is. You seem to make some very direct translations to English from your native language.And capacitive coupling from one layer of a PCB to another will not transfer very much power.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 17, 2021, 11:37:21 pm
content: Quote from: Benta on January 17, 2021, 10:48:19 pmI thought the term flying capacitor is quite common and well-known in the English language. However, in the language of schemas, I'm sure you will be completely clear. See attachment.Of course, you will not be able to power the welding machine through the capasitive between the layers, but for some isolated devices, for example, the power of an isolated probe, it is quite likely.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 17, 2021, 11:40:10 pm
content: Quote from: S. Petrukhin on January 17, 2021, 11:37:21 pmDo you mean Fly Back? As in a capacitor within a flyback convertor circuit.edit: changed wording.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 17, 2021, 11:57:24 pm
content: https://www.analog.com/media/en/training-seminars/tutorials/MT-071.pdf

author: TimFox
author_rank: Super Contributor
timestamp: January 18, 2021, 12:10:18 am
content: Quote from: S. Petrukhin on January 17, 2021, 11:37:21 pmThis circuit is not common, but is usually called a “flying capacitor”, possibly analogous to a flying trapeze at the circus. For galvanic isolation, the insulation and break-before-make of the switch are critical.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 18, 2021, 12:11:06 am
content: Quote from: Ed.Kloonk on January 17, 2021, 11:40:10 pmI may be wrong about the English name. Look at the diagram.