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Topic: NoLoop galvanic isolator (Read 4168 times) previous topic - next topic

NoLoop galvanic isolator

Hi all,

I had a problem some time ago with a nasty ground-loop and that cost me the USB port on my old laptop. It took me a while to realize what had happened and it was a generic problem we all run into more often than we think. Time to solve this particular problem once and for all and make generic isolation for Serial and SPI ports.

The idea of the isolator is to use a standard chip (RF-coupling) to isolate the signal lines. A switching power transformer is added to have power from the primary side transferred to the secondary side. This powers the isolating coupler and has enough "bite" to power other stuff too.

The NoLoop Serial is pin-compatible with the standard FTDI breakout (Sparkfun/Adafruit/FTDI). The NoLoop SPI differs in that it has one input and three outputs, whereas the NoLoop Serial has two inputs and two outputs.

The power supply converter and isolation coupler can do level-shifting as well (from 5V->5V, 5V->3V3 and 3V3->3V3). It cannot do step-up conversion, though, primarily because the FTDI internal 3V3 LDO supply would not be suited for that anyway.

A more full description is available at my homepage http://www.vagrearg.org/content/noloop including the schematics. I also listed the first small batch on Tindie.

Serial version:
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SPI version:
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Re: NoLoop galvanic isolator

Reply #1
This looks like a really thoughtful design, Bertho (both layout and features included).  I really like that you included the isolated supply in the deisgn, as well as directional arrows and plenty of documentation on the bottom side of the board. 

The SN6501 is much more compact and elegant than my most recent switching supply design, based on the reference design in the ADM2483 datasheet - the selectable output is a really nice touch, too:)

Re: NoLoop galvanic isolator

Reply #2
[quote author="bearmos"]This looks like a really thoughtful design, Bertho (both layout and features included).  I really like that you included the isolated supply in the deisgn, as well as directional arrows and plenty of documentation on the bottom side of the board.[/quote]
The isolated power was already needed for the secondary's operation, so I just extended it a bit further and is really nice to have. It makes the device completely transparent when in line with the USB/Serial converter.

But, yeah, the features are the ones I'd been looking for myself. Especially the text on the back is readable (for me) without reading glasses. I also always have to check what signal is where and what is that direction now again... I guess many feel the same way..

Quote
The SN6501 is much more compact and elegant than my most recent switching supply design, based on the reference design in the ADM2483 datasheet - the selectable output is a really nice touch, too:)
I stumbled upon the SN6501 while looking for a suitable transformer. Googling the transformer gave me the chip. It is no perfect design though and has some ripple at higher loads, but that can be largely fixed with an external low ESR cap.

Re: NoLoop galvanic isolator

Reply #3
Looks good, could be handy. Is there much in the way of capacitive coupling between the two sides - e.g. if one side is bouncing up and down level wise at a fast rate, is there likely to be much coupling going back? I understand it's not really for that, but just curious if you happen to know.

Re: NoLoop galvanic isolator

Reply #4
[quote author="Sleepwalker3"]Looks good, could be handy. Is there much in the way of capacitive coupling between the two sides - e.g. if one side is bouncing up and down level wise at a fast rate, is there likely to be much coupling going back? I understand it's not really for that, but just curious if you happen to know.[/quote]
The isolation should be able to withstand quite a beating. However, larger differentials will degrade the silicon barrier faster. The 1kV version (as used) will hold, worst case, for 60 years at 150Vrms and about a day at the rated 1kVrms. That may sound worse than it actually is because having such differentials permanent present while fiddling around may have a large potential of zapping your fingers...

If you would take the secondary side and put a (floating) square-wave on it (so your secondary will vary wildly w.r.t. the primary), then you will see some common-mode coupling through capacitance. That would be the same for all types of isolation, though.

The common-mode transient protection is according to the datasheet about 25kV/us, so I guess you'd need quite a beating to actually influence anything. The capacitive coupling is about 2..4pF from the sides on the logic part. I think that the transformer will give you more problems here, but that is quite a low-impedance path, so it will swallow most of such common-mode changes before you will see it.

The actual isolation distance is 4mm on the board (covered in soldermask). The "official" distance for standard 230V AC is 6mm if you have it as a permanent differential. There is a SOIC-wide version that can accomplish this, but I opted for the higher density as it is not intended for mains operation as such. Just breaking the ground-loop(s) was the primary goal.

So, for all practical intends and purposes, you'd be quite safe. But do not put your life at risk to prove me wrong ;-)

Re: NoLoop galvanic isolator

Reply #5
Yeah I was more thinking along the lines of Isolated DC-DC supplies utilised in drive systems.  I'm rather surprised if the transformer part would also withstand 1kV for a day, that would be quite an achievement for a small part with close windings. Typical '1kV' Isolated DC-DC converters are often, in the fine print (see attached typical data sheets), rated for that level for 1 second and/or have another rating for 1 minute. Feeding an output stage where you have a large inductive load switching at typically 7 - 20kHz, rise/fall times perhaps the 10's of nS in some cases and usually a DC Bus voltage of at least 150V DC and not uncommonly 600V DC, presents quite some challenges. But I am comparing Apples to Oranges, it just interested me, as isolating two levels is one thing, isolating something that spiking or changing rapidly, is quite another - not that the intended use of the NoLoop would be used in that situation.

Obviously this is quite different to what's presented here, but the floating supply there made me wonder how that would compare.

Re: NoLoop galvanic isolator

Reply #6
Very nice board. It's inspired me to put together a USB one using Analog's USB isolator chips when I get a chance.

Re: NoLoop galvanic isolator

Reply #7
[quote author="Sleepwalker3"]Yeah I was more thinking along the lines of Isolated DC-DC supplies utilised in drive systems.  I'm rather surprised if the transformer part would also withstand 1kV for a day, that would be quite an achievement for a small part with close windings. Typical '1kV' Isolated DC-DC converters are often, in the fine print (see attached typical data sheets), rated for that level for 1 second and/or have another rating for 1 minute.[/quote]
You are completely right. The transformer I use is not rated for high voltages and certainly not for continuous high-voltage differentials. But that was not the point of the design.

Quote
Feeding an output stage where you have a large inductive load switching at typically 7 - 20kHz, rise/fall times perhaps the 10's of nS in some cases and usually a DC Bus voltage of at least 150V DC and not uncommonly 600V DC, presents quite some challenges. But I am comparing Apples to Oranges, it just interested me, as isolating two levels is one thing, isolating something that spiking or changing rapidly, is quite another - not that the intended use of the NoLoop would be used in that situation.
Yes, having a "real" galvanic separation that is made permanently to withstand a true beating is a different ballgame.

If you need to have certified separation, then you need a transformer that is rated for exactly that purpose. It will have a shield between the two windings. Also, it will be a lot bigger.

Quote
Obviously this is quite different to what's presented here, but the floating supply there made me wonder how that would compare.
The supplies you link are made to the specification of high(er) isolation. They are also cast in epoxy for better isolation (no immediate moisture threat). Using them in a USB powered applications can be a bit of a problem. There is a rather high quiescent load for most of them. But more importantly, they are rather large and expensive. There are many good uses for these PSUs. I have used them in RS485 systems for isolation and there you require 1.5kV isolation to be on spec all the time.

But as you mention yourself, it is a far cry from my hobby induced isolator to protect my PC from yet another USB blowout to a far away communication system that needs to work on guaranteed specification. The price is also accordingly ;-)

Re: NoLoop galvanic isolator

Reply #8
Yeah, I was comparing Apples to Pineapples. Your unit looks good for the intended application, I was going a bit far off topic I think.