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We need a galvanic isolation of 10 kV in our flyback powersupply. We faced some problems in choosing the optocoupler in terms of the price. We thought maybe it could be possible if we connect two optocouplers to increase the isolation, but there are some doubts:

  1. Is it possible to connect two optocouplers?
  2. How should they be connected?

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    \$\begingroup\$ A problem I see is where do you get the supply to power the second OC LED from. That supply must also be isolated. \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Commented Mar 31, 2023 at 6:49
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    \$\begingroup\$ No, on top of what Lorenzo said above, they will not share equally. There are optocouplers up to many tens of kV on the market. Do yourself a favor and get the right component for the job. \$\endgroup\$
    – winny
    Commented Mar 31, 2023 at 6:54
  • \$\begingroup\$ I second what @winny said. Unless you have a very peculiar technical requirement, get a properly rated part. Isolation specs are safety related. Especially for high power supplies, I wouldn't want penny-pinching in anything safety-related that has to enter the market. Not only for the risk of lessening the safety of the equipment, but if you plan of getting some certification, like UL-listing, you might spend more in the end. BTW, since you said "we" I assumed you are performing a professional design and that's not some kind of hobby, one-off thing. \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Commented Mar 31, 2023 at 7:00
  • \$\begingroup\$ Anyway the argument doesn't even make sense even if you could find a suitable way to power the intermediate cicuit. Say a voltage of 9kV causes the first part to arc. Who is to say that the second part does not get hit with 7kV as a result? In other words there is no reason to assume that isolation ratings sum when used in series. \$\endgroup\$
    – danmcb
    Commented Mar 31, 2023 at 7:34
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    \$\begingroup\$ In this case, I recommend you use a primary-side regulated (PSR) flyback controller which samples the reflected voltage on an auxiliary winding and can reach +/- 5% in regulation accuracy. There is no optocoupler and the transformer, alone, has to sustain the high voltage. If you need better figures then a downstream regulator can help unless you need a large amount of current of course. There are plenty of PSR controllers to choose from these days. \$\endgroup\$
    – Verbal Kint
    Commented Mar 31, 2023 at 7:55

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Two optos will not share voltage properly, so you would need additional balancing components, not to mention the provision of a floating intermediate supply.

You must get a single isolator rated for the job. That means rated for continuous service operation at 10 kV, which also means it will probably be rate for short time survival of much higher transient voltages.

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