DC-10MHz isolation design - Page 1
Forum section: electronics / projects

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 21, 2011, 03:21:00 pm
content: Has anyone designed an isolation circuit for measuring switching power supplies using an oscilloscope? The frequency range is DC-10MHz or so and the isolation needs to be at least 400-600V CAT II.I have thought about amplitude modulating a 100MHz or so (or optical) carrier, sending it through a transformer, and then synchronous rectifying that to get back the original signal. But that would require careful RF design to work properly.Another idea is to use a transformer to couple the high frequencies and a some other isolation method to couple the DC and low frequencies, but would that require careful adjustments to ensure a flat frequency response? (Curves are for models, not linear isolation circuits!)

author: scrat
author_rank: Frequent Contributor
timestamp: January 21, 2011, 04:26:01 pm
content: The other big problem I see is measuring such a high voltage signal (= using high impedance) up to that frequency, since, for example, at 10MHz cutting frequency, 100kohm input resistance means 0.16pF in parallel.However, maybe I'm saying a stupid thing, but... what about acquiring data on the measure side and transferring it digitally to the other?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 21, 2011, 04:46:20 pm
content: Switching power supplies are not going to be affected by a few pF of loading.Digitizing the data would basically mean building an oscilloscope from scratch, not exactly practical. That would indeed make the isolation part easy.

author: jahonen
author_rank: Super Contributor
timestamp: January 21, 2011, 04:51:49 pm
content: How about modifying a 2.4 GHz video link to accept your desired signal voltage range? Well, maybe not up to 10 MHz, but might go near.Regards,Janne

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 21, 2011, 05:18:47 pm
content: I'm pretty sure video is AC coupled with automatic zeroing so that wouldn't work.

author: scrat
author_rank: Frequent Contributor
timestamp: January 21, 2011, 05:51:16 pm
content: Quote from: NiHaoMike on January 21, 2011, 04:46:20 pmI was worried about achieving such a bandwidth at high impedance (which should be needed for high voltage measuring).It seems that the solution exists, since there are voltage probes up to the 100MHz range...

author: Unknown
author_rank: Unknown
timestamp: January 21, 2011, 06:13:31 pm
content: Quote from: scrat on January 21, 2011, 05:51:16 pmThe voltage is usually derated with frequency, though, they won't survive their max voltage at 100MHz. Think about the amount of power dissipated by high-voltage, high-frequency in that few pF.You don't need galvanic isolation for this I think, just a high-voltage differential probe that can survive the required voltages is enough. I'd probably buy one, designing one is not trivial.Tektronix had an isolation amplifier in the past (A690x?), there's a service manual with schematics available somewhere, but it's not easy to clone. I think it used different methods for low and high frequency, one of them was modulating the signal as RF and a transformer. I believe the Tek TPS2xxx series uses optical for the low frequencies, and transformers for high frequencies.

author: tyblu
author_rank: Frequent Contributor
timestamp: January 21, 2011, 06:41:56 pm
content: Run the 'scope on batteries. Don't touch it while taking measurements.

author: scrat
author_rank: Frequent Contributor
timestamp: January 21, 2011, 06:51:38 pm
content: Quote from: alm on January 21, 2011, 06:13:31 pmThanks, I should have imagined this. Better, I remember I looked at those incredible specs once more some time ago, and found that there was an obvious derating. Physics is the same, also for Tek or other specialists...

author: Unknown
author_rank: Unknown
timestamp: January 21, 2011, 07:01:20 pm
content: Quote from: tyblu on January 21, 2011, 06:41:56 pmSay goodbye to loved ones. Take out life insurance policy. People have been killed this way, and all reputable scope vendors strongly recommend against it (excludes isolated scopes like Fluke Scopemeters or Tek TPS2000 series). And that's not just so they sell more $$$ differential probes that you need to do it properly (marketing research has shown that alive customers buy more than dead ones).

author: scrat
author_rank: Frequent Contributor
timestamp: January 21, 2011, 08:12:37 pm
content: Quote from: alm on January 21, 2011, 07:01:20 pm

author: Unknown
author_rank: Unknown
timestamp: January 21, 2011, 08:13:38 pm
content: Quote from: NiHaoMike on January 21, 2011, 04:46:20 pmSo buy a USB scope and an isolated USB hub. Not sure what they do with power supplies on isolated hubs but DC-DC converters with good isolation are easy enough to find.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 21, 2011, 08:31:22 pm
content: Quote from: Fritz on January 21, 2011, 08:13:38 pmYeah, buy some USB junk and gamble that a kludge made from a funny USB scope, connected via a funny USB cable to a funny isolated hub, won't let the juice out somewhere. E.g. if you accidentally touch the metal on the USB cable at the hub's end. Or the non-isolated probe BNC connection.These properly CAT ratedisolateddifferential probes are just for wimps. Real man love the smell of electrocuted flesh in the morning.

author: tyblu
author_rank: Frequent Contributor
timestamp: January 21, 2011, 08:37:16 pm
content: Relevant question:http://electronics.stackexchange.com/q/8534/2118

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2011, 08:47:28 pm
content: An opto-coupler and high speed op-amp?The HCNR201 has a -3dB bandwidth of 9MHz, with a fast enough op-amp you might be able to push it to 10MHz with negative feedback compensating for both the non-linearity and the loss.http://www.secomtel.com/UpFilesPDF/PDF/Agilent/PDF_DOCS/ISONCONT/01_OPTO/1_418_33.pdf

author: RayJones
author_rank: Frequent Contributor
timestamp: January 21, 2011, 09:02:30 pm
content: Isn't the traditional method for diagnosing "live chassis" equipment to run the unit via an ISOLATION TRANSFORMER?This isolates the equipment from ground, and thus allows you to attach your earthed CRO to any point in the unit without introducing a short.I recently repaired a plasma TV's supply and was getting nowhere fast until I borrowed an isolation transformer.However I admit the full circumstances have not been fully described - can the power supply be isolated in this manner?If it is part of fixed equipment, yes you will need a differential probe.

author: Unknown
author_rank: Unknown
timestamp: January 21, 2011, 09:13:55 pm
content: Quote from: BoredAtWork on January 21, 2011, 08:31:22 pmThe point of an isolation barrier is that one side is safe and the other isn't. If you are too stupid to understand that you are too stupid to be working with live equipment.Being USB controlled there isn't anything you need to touch on the scope so you can package the whole lot in an insulated (or earthed) enclosure. The isolated hubs are not funny or junk, a common application is for connection of patient facing medical equipment to otherwise unsafe computers.You can get a a 50Ms/s USB scope and isolated USB hub for less than the price of the cheapest Tek differential probe and it gives you two channels.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2011, 10:21:47 pm
content: Lots of modern 'scopes are double insulated, which means they don't have an earth connection so there won't be a short and are safe to use, as long as you don't touch any metal parts whilst the equipment is on.You can also use the differential setting on either earthed or non earth 'scopes with live chassis equipment and no isolation transformer.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 21, 2011, 10:24:22 pm
content: Quote from: Fritz on January 21, 2011, 09:13:55 pmI officially declare you a clueless idiot. Everything that can carry live voltage and can easily come apart is unsafe - like using flimsy USB connectors on a "hot" USB scope. So with all due respect, stick your stupid USB "solution" up your arse. That's anyhow the place from where you pulled the suggestion.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 21, 2011, 10:32:01 pm
content: Quote from: Hero999 on January 21, 2011, 10:21:47 pmBut the problem is you still need to reference your signals against some sort of reliable "earth" which is what starts the problem on live chassis equipment. Looking at a signal without a proper reference produces crap.The difference of two craps will still be crap.To do a proper differential probe, you are essentially looking at the instrumentation amplifier topology, which looks for the true difference between the signals, not the difference of each signal relative some unknown, possibly not related, earth.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2011, 12:20:01 am
content: Quote from: BoredAtWork on January 21, 2011, 10:24:22 pmWhat kind of idiot is someone who declares all mains leads unsafe?Quote from: BoredAtWorkToo stupid to understand isolation barriers. The safety afforded by an isolation barrier does not depend on anything being connected, it depends on no connection breeching the barrier.

author: Unknown
author_rank: Unknown
timestamp: January 22, 2011, 07:44:04 am
content: Quote from: Hero999 on January 21, 2011, 10:21:47 pmLots? I'm aware of portable scopes like the Fluke Scopemeters (I think Tek and Agilent may also have made competitors at some point), some cheap alternatives by companies like Velleman and Uni-T that I wouldn't trust anywhere near CAT II 400-600V circuits, and the Tek TPS2000 scope with isolated channels. Could you point at those lots of alternatives that are safe for high(ish) energy circuits?Quote from: RayJones on January 21, 2011, 10:32:01 pmThat's how every differential probe works, you don't usually connect the common lead either. It's just that the CMRR and common-mode range of a scope front-end sucks compared to a real differential probe (usually something like 20:1-50:1, as opposed to something like 10000:1 or 1000:1 with real differential probes), because the front-end of (most) scopes is not a proper differential amplifier. Yes, the ground may not be stable relative to the DUT, and the limited CMRR and slew rate will be unable to compensate for this. Commercial designs have the same issues, BTW, I've also heard complaints about using the Tek TPS2000 for high-side FET measurements in switchers.Quote from: Fritz on January 22, 2011, 12:20:01 amEver compared mains connectors with USB connectors? Looked at the voltage rating for USB cables compared to mains?Quote from: Fritz on January 22, 2011, 12:20:01 amSafety also depends on keeping the area of exposed dangerous conductors as small as possible. Low voltage cables like USB are especially dangerous, because they may seem safe, but are not at CAT II 400-600V. This has been tried, people have put scopes in plexiglass cages, still dangerous.It's also false economy: a cheap high voltage differential probe (eg. Instek) costs something like $300. USB scope plus USB isolator rated for CAT II 600V is likely to be more expensive, especially since most USB scopes have a fairly limited voltage range, so you may need 100x probes which add extra $$. You will also be able to use a proper scope with good sample rate, front-end, memory depth and user interface. Why risk your (or someone else's) life if it doesn't even save money?

author: Chasm
author_rank: Regular Contributor
timestamp: January 22, 2011, 08:17:15 am
content: The bottom line is this:If you do not know wtf you are doing there is a definite chance that you'll pay a high price.Only chance and circumstance will determine whether you pay in health and livelihood or new equipment.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2011, 03:20:45 pm
content: Quote from: alm on January 22, 2011, 07:44:04 amYou're right, just make sure the device has the correct fuse and don't touch the 'scope whilst the power is on. lol

author: scrat
author_rank: Frequent Contributor
timestamp: January 23, 2011, 05:24:01 pm
content: What's this, people insulting each other? The topic is very insteresting, but it's a discussion, let's calm down, please!

author: Unknown
author_rank: Unknown
timestamp: January 23, 2011, 05:47:01 pm
content: Quote from: Hero999 on January 22, 2011, 03:20:45 pmYou forgot to mention to run away after turning it on.