Author Topic: Why do we use inductive isolation?  (Read 305 times)

Offline Overlordcheese

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Why do we use inductive isolation?
« on: September 03, 2022, 07:06:53 PM »
Hello! I was recently trying to find a high speed and high power isolated gate driver when I came across this part : UCC5390ECQDWVRQ1. It uses "capacitive" isolation (which i have yet to research, but im sure you are aware of how these work), and decided to research what kind of isolation is best for this purpose, and I can't seem to find any reason as to why inductive isolation is better than this. Any advice?

Offline Mads Barnkob

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Re: Why do we use inductive isolation?
« Reply #1 on: September 03, 2022, 08:17:14 PM »
Hello! I was recently trying to find a high speed and high power isolated gate driver when I came across this part : UCC5390ECQDWVRQ1. It uses "capacitive" isolation (which i have yet to research, but im sure you are aware of how these work), and decided to research what kind of isolation is best for this purpose, and I can't seem to find any reason as to why inductive isolation is better than this. Any advice?

Gate drive transformers are best in terms of low complexity, low cost and high robustness. They do however lack precise control, cross conduction protection and other neat tricks that can be done with high side drivers.
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Offline Mike

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Re: Why do we use inductive isolation?
« Reply #2 on: September 03, 2022, 09:55:13 PM »
Mads summed it up really well. Those fancy capacitively isolated drivers can do amazing things and have a lot of advantages, with a couple of big downsides. The biggest issues is complexity. Notice on the datasheet that the isolated rails, the chip doesn't generate these, you need to supply them, this adds a lot more complexity than their example layout and application diagrams initially suggest.

This isn't probative, and can often be achieved with something as simple as a bootstrap diode and capacitor, but it is another complexity which can be mitigated entirely by using a form of gate drive that transfers signal and power in the same device (like a GDT).

The other big factor is basically the availability of community experience. If you wanted to create a design with a fancy isolated driver the number of people on forums such as this one with experience drops significantly, especially if you gave some esoteric issue with a particular chipset. In industry you can reach out to FAEs, but hobbiest rarely have access to such resources.

Offline klugesmith

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Re: Why do we use inductive isolation?
« Reply #3 on: September 04, 2022, 06:08:32 PM »
Am reminded of why wired Ethernet connections use inductive coupling.   
Each diff pair input or output has a little transformer, often built into the connector casually called a RJ45.
https://www.cuidevices.com/product/resource/crj010.pdf

The answer: because it's mandated by the Ethernet standard. 
Isolation (from magnet wire insulation) is tested at more than 1000 volts.
One educational article pointed out that an Ethernet device could have its digital ground tied to hot side of mains power, and data I/O would operate completely within spec.

Who knows what happens when we take the step to Power Over Ethernet?

Offline Hydron

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Re: Why do we use inductive isolation?
« Reply #4 on: September 04, 2022, 08:07:37 PM »
PoE mandates the same isolation as the rest of the Ethernet standard. You have to either use an isolated DC-DC converter in the powered product (PD), or have the whole PD floating and isolated from it's chassis.
On the side providing the power (PSE), you need the same, though one isolated supply can source power to multiple ports, as long as it's isolated from mains, chassis etc.

In a previous job I had a bit of fun trying to work out whether I could avoid the efficiency loss and expense of an isolated converter by connecting brushless motors directly to the ~50-57V that you get with the higher power PoE standards - looked like it might have been possible, but it would have been a pain in the ass to do and I left before it went anywhere.

Note that there is some signaling and detection involved in the PoE standards (there are at least 3) to avoid powering stuff that isn't designed to accept it. Of course the standards are classics product of a committee with conflicting interests and therefore quite stupid in places (to the point where bits just get totally ignored by most), and you also can run into "PSE"s which just throw the voltage on the line and can therefore burn out ports which are not designed to deal with it (e.g. have centre taps connected together) - ask me how I know!

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Re: Why do we use inductive isolation?
« Reply #4 on: September 04, 2022, 08:07:37 PM »

 


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