Capacitively isolated power supply - Page 1
Forum section: electronics / beginners

author: epaelectronic
author_rank: Contributor
timestamp: February 03, 2021, 08:22:44 pm
content: I want to design a capacitively isolated power supply. I am looking for functional isolation.I made up a simple circuit that consists of a half bridge driver, isolation capacitors and rectifier diodes (see attachment). I have no idea if the circuit would even work the way it is currently.Can it even be done with a half bridge? or should a H bridge be used?The power supply is meant for a low voltage application 3.3-5v.Could someone push me on the right track?Any help would be appreciated.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 04, 2021, 04:59:11 pm
content: Do you have to comply with any electrical or medical device directives?It is generally not a good idea because the physical width of the isolation barrier is set by the thickness of the capacitor's dielectric.Even if you have 100Kv caps you can't comply with creapage and clearance requirements.For calculations for your proposed scheme seehttps://www.eetimes.com/build-isolated-power-supply-using-capacitors/

author: Benta
author_rank: Super Contributor
timestamp: February 04, 2021, 05:04:15 pm
content: OP is talking about functional isolation, not safety.Take a look at the NIS6201 from ON Semi. It's obsolete, but the datasheet might give you some ideas.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 07, 2021, 05:39:29 pm
content: It good that you speak for OP, are you his keeper?If you'd bothered to read my post you'd have see I offered a solution using a MAX256 which is not obsolete.Its not cheap but the note describes precisely how to use a this or another H bridge driver in this application.

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2021, 07:21:50 pm
content: Quote from: Terry Bites on February 07, 2021, 05:39:29 pmPipe down, relax and get a feel for this forum first before going in with blazing cannons.Cheers.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 07, 2021, 08:42:08 pm
content: So you still didn't read but you're happy to make presumptions about where I'm coming from as well?EOT

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 07, 2021, 09:01:21 pm
content: Quote from: Terry Bites on February 07, 2021, 05:39:29 pmNo, he just read the OP's requirements?

author: pqass
author_rank: Super Contributor
timestamp: February 07, 2021, 09:34:21 pm
content: See "Floating Supply for LCD Panel Meter" at bottom of the page here:http://techlib.com/electronics/regulators.htmlIt works. I built it for the purpose of floating a cheap panel meter but the meter never read the correct value and I didn't investigate further.Maybe you can adapt it to your output voltage and power needs.eg. larger isolation caps, dual H-bridge, different output regulator (vs zener), etc.

author: pqass
author_rank: Super Contributor
timestamp: February 07, 2021, 11:51:17 pm
content: Quote from: pqass on February 07, 2021, 09:34:21 pmSo I dug-out the circuit and looked at the output with my scope. I found that at Vcc=12V, it produces 9.7V output but with huge 2Vp-p bursts @27Mhz (~10us wide) every 200us (5KHz; the oscillator frequency) whether loaded down with a 6K8 resistor or not. Maybe that's why the meter never worked right with it. FYI: I used a TC4011BP CMOS NAND with 10K and 10nF timing passives.Instead, maybe you should adapt a circuit that doesn't rely on switching two outputs simultaneously without some deadband time.See "Floating Power Supply" half way down this link:http://www.repeater-builder.com/astron/digital-metering/digital-metering.htmlIt just uses a single 555 output into the isolation cap. I'm sure you can substitute any squarewave oscillator that can both source and sink current.

author: JoeyG
author_rank: Regular Contributor
timestamp: February 08, 2021, 03:11:20 am
content: If it's just a communication over a barrierTI and Silicon Labs have capacitive coupled IC's.Analog Devices has transformer coupled ICsThen there are the classic optocouplersIt's then a question, do you design, build it OR just buy it (buy the IC that is)

author: pqass
author_rank: Super Contributor
timestamp: February 08, 2021, 05:40:47 am
content: Quote from: pqass on February 07, 2021, 11:51:17 pmOkay, I just couldn't leave the odd noise burst problem alone so down the rabbit hole I went...I changed my NAND oscillator to just use one output into the isolation cap and other cap grounded. RESULT: NO MORE NOISE!I then used it as a power source for my LCD panel meter in my analog current load and it now reads correctly!I don't know your power requirments; mine were only <2mA. However, see below for my attempt at a 5W (700mA@7V) source using the same design. Note: I've increased the frequency to 10KHz, increased the isolation caps from my 100nF to 220uF, bulk cap from my 3.3uF to 1000uF, and added a half H-bridge (drain resistors get hot at 3W). The zener is a stand-in for a down-stream regulator.Power supplies based on switched capacitor/charge pumps will never keep-up with inductor-based designs.https://electronics.stackexchange.com/questions/216094/why-are-charge-pumps-only-used-for-low-current-applicationsYou can play around with the falstad.com model here:http://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3EYFYBMBOALBtcmQwOxwDMIeZZkAUGEhALQAck4BAbCMSqxy92PxRUA7iCYswabuPBsW1UTLByxzEGl5UATqpYbO3fRPjVaEDIx6yJ7KCG4ShorlYvWoIg68uSHnl-ouYLbUOgEcynq84CaekeoRKsGaZlbJ9ow+IfbggnFJtunxCu5FhVIeimr6Mm7UAMa6IG7xdc3QRCZd8GAApvRoYhjQaBpoBERsKCiMcoMwCPksLSpG-sQRthjKVg3NO+kWNtELcNPdF5CkYNDEqAQobMQPGGxTjIiwkGBUACbNanSSAKHG4v16ADMAIYAVwANgAXP4AvgYZaA2xgyGwxGeI72NFkFQoQl7YHHImo+RQL4QAjQWhvYHEWjbD6MAhIGmLRrbUGE-Ek6mnSDnS5dUj0G7ERiPMCzSQbAjyyCWU4-ADm+35LBlGAJws8U31QpARBNpM85qsnP4ISt+ANZtQTuoAAdnXaOLanYgPM8WMabY6inZQmaQ7ZrabjFRiPqAF69AB2vS0AwZ4HAHTAGDuM18H3gKH6BDsKC+4vgjH1aEzPwTICDpqD6W4SdT6brP1E5KdzctvZBTZJuyoACVOBsAVPEmHmo558gqFriNPiI61xwuIadDMfAIMpY2mBYnvMk795xHcZ4NpZ7lLFvduB78-0s-pvqJJ4r25n3U6wcP+05rOeWSgheYCHre95Xm2F4xjEJRXhu+rPmhlQPphn7Ah4EJHk6n6EkE0BcjAUhUAR8GHu+tHXGRNKUQA9rIWbLBs3AoBI0DRPw24gAAsgA8gAygAYgAogAKgAOgAzsJACq0kAAoqVQrEoGWfp5kgXKnjSfoQMpakqQp0nCQpAAywkAIIACKac0ERms0dxctxTbQPqEAQBWXJKapqmSeOCkiRJMkKapwkAOqhc5GAGW5emebq0D8FmAUgLZ8VhfJEVSXJ8kxXlVBAA

author: JackJones
author_rank: Regular Contributor
timestamp: February 08, 2021, 05:57:37 am
content: Something similar has been made for VFD filaments with LM4871, or H-bridge.https://www.eevblog.com/forum/projects/vfd-filament-power-using-lm4871-replacement-for-obsolete-lm9022/

author: pqass
author_rank: Super Contributor
timestamp: February 08, 2021, 06:16:47 am
content: CA$129.67 just to avoid an inductor. Wow!https://www.digikey.ca/en/products/detail/helix-semiconductors/MXC-274C1-EB-10/9998813The other eval boards are a more modest CA$86 but the chips themselves are at least realistic at ~CA$10.https://www.digikey.ca/en/product-highlight/h/helix-semiconductors/efficient-ac-dc-and-dc-dc-power-solutions

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 10, 2021, 01:11:47 pm
content: I'm sorry, perhaps I did not understand the essence of the question, but I will offer the option of using a flying capacitor to isolate the power supply using an analog switch for 2 groups of contacts.

author: perieanuo
author_rank: Frequent Contributor
timestamp: February 11, 2021, 11:46:13 am
content: Quote from: Ice-Tea on February 07, 2021, 09:01:21 pmyep, the op's requirements are just incomplete.when you design a ps, you speak current, voltage, how much isolation are we talking aboute etc etcI see only voltageare we talking about 10amps or 10 Ma?well, the solution proposed depends on this specs, which are for the moment almost ZEROso, first we have to orient the direction, then propose