For a flyback converter, 120 V AC in and 400 V - 2000 V out, is it necessary to provide galvanic isolation between the input and output?

First, from a safety point, would not having a direct connection provide any benefit?

And from a certification perspective, if a product were to be sold to the public, would having the output isolated be helpful?

The high voltage is used to produce the electric field in an ionization chamber to detect radiation.

The entire system would be enclosed in a plastic enclosure, with a LCD screen at the front with some buttons and showing some system information.

  • 2
    \$\begingroup\$ This is impossible to answer. You need to be a lot more specific about what you are making, how the user is protected from contact with the wiring, its expected use, etc. Put the details in your question and not in the comments. \$\endgroup\$
    – Transistor
    Mar 27, 2018 at 19:31
  • 1
    \$\begingroup\$ Is it linked to this and this questions ? \$\endgroup\$
    – zakinster
    Mar 27, 2018 at 19:38
  • \$\begingroup\$ As far as isolation goes, those would be questions for UL and ISO to answer. Once you start shipping, you will be 'married' to UL and ISO regulations. Count on product labels in English, Spanish and French. \$\endgroup\$
    – user105652
    Mar 27, 2018 at 19:59
  • \$\begingroup\$ unless you need to connect more than one it shouldn't matter from a circuit perspective \$\endgroup\$
    – dandavis
    Mar 27, 2018 at 20:04
  • \$\begingroup\$ How are you going to protect the public from the 400V to 2000V output of your circuit? \$\endgroup\$ Mar 29, 2018 at 17:09

3 Answers 3


Yes, of course galvanic isolation is valuable. Regardless of the voltages being used on the secondary side, it prevents the user from being shocked by the mains voltage in the event of some sort of fault.

Now, given that you are probably providing lots of isolation between the high secondary voltage and the end user, and provided that this isolation meets the safety requirements for "double-insulated" equipment, then galvanic isolation between primary and secondary is not strictly required.


The answer to this is:

It depends there are applications where provided you know the exact use its OK for the output of a circuit to not be isolated but you have to understand the exact load.

For example in a range of power supplies I developed we had a two module solution. The first module took mains in and produced a 400V (ish) output while controlling the shape of the input current to provide power factor correction (PFC).

The output of this module was not isolated and so potentially very dangerous if we did not have complete control over our load.

Fortunately we did the load was always one or more of our output modules and these were designed such that they did not care the input was not isolated because they provided the isolation required for the end user. Each module produced a different output voltage and power meaning we could rapidly provide custom power units just by selecting the output modules we needed and a suitable input module to provide the total power requirement.

Most switch mode power supplies work in this way except that the PFC and Output modules are usually part of the same product and not separate products which we would either put together in a single box or sell as modules to approved customers who we were confident knew how to use them. We would not have sold either of the modules to an end user because of the potential safety risk.

Similarly I used to design TVs when they still had CRT (Cathode Ray Tube) displays. These often need several kV to work and the CRT supply was usually not isolated. Every precaution was made however to ensure the TV as a whole was double insulated however.

However except in special circumstances such as this galvanic insulation is a must to avoid shock risk, and possible death, to users.


The question and other answers ignore the 2 most important criteria for AC product safety.

1) HIPOT Leakage current to ground ( 100% product testing ) - A much higher voltage is ramped (>3kVdc )between Input (Line AND Neutral shorted ) and ground to verify that leakage current is < 100uA - actual HIPOT tester level depends on IEC/UL/CSA/etc specs for 60s or 1s test level and DC avoids line filter ac current by design

2) Safety ground continuity ( pre-approved by design ) - the safety ground to chassis must not raise voltage more than 1V with 10A current. - if there is no ground connection, then insulation system must be redundant by at least 2 methods

  • There is NO REQUIREMENT for isolation between AC input and DC output with respect to Neutral.

    However it may be illogical to superimpose Line on HVDC, it is not forbidden to use neutral as common to the HVDC output. However the following must apply so the design must still pass HIPOT and function as expected.

  • The hot and neutral wires are interchangeable as far as the equipment is concerned.


  • Inter-System Ground Noise is never solved by a (Galvanic) Isolation transformer.

    These have absolutely no affect on this problem.

  • In some countries and locations, for rural and remote equipment, the HVAC is delivered without a Neutral return wire ( e.g. at 20kV) and relying on the earth for a power conductive path. See Single-wire earth return (SWER)

It does not have to be galvanically isolated from the mains neutral.

But it must be safe to the operator under any condition.

This includes; cable disconnect, power interruption, secondary arc, primary over-voltage, contaminated insulation with any humidity range.

  • While low %RH promotes voltage build up on insulation by stray HVDC fields and high %RH promotes leakage current to stray surface paths.

How you make it safe is not determined by galvanic isolation, rather by design of insulation system and protection.

This determined by controlling E fields and HiPot test verification of the system.

  • e.g. If 200kV can get in, it can probably get out so plan on ESD/HIPOT testing to all surfaces under worst case dust condtions above your required levels.

A shielded conductive spray and earth bonded is safer inside the plastic case. Then any dielectric breakdown will be safely shunted to earth ground via the inside connections. A Bleeder resistor must have the power dissipation to discharge the cap when the unit is disconncted from primary power when on such that any line filter will not cause a floating high voltage between any exposed surfaces and pin or have some other safety measure.

If you plan on using bushings or insulation, the power industry BIL ratings are for AC and not HVDC, so BIL200 can withstand 200kV lightning but not 200kV AC and much less than 200kV dc. This is why air gaps are rated ~500V to 3kV/mm depending on contamination and surfaces much less.

Plan on having an AM radio tuned off-channel to detect any unexpected and possibly silent static noise discharges from inadequate insulation until to have finished the design verification tests planned. (DVT)


For example I used 1cm thick nylon sheets above an open oil-filled 5MVA transformer tank with two(2) BIL200 rated insulation bushings for 400kV lightning protection in order to test up to 150kVdc. Then static discharges started on the wall 30 meters away with a mounted steel girder for show N tell of insulation components that was electrically floating from earth ground. It then started to tick quick loud like a clock, while I stood a few m away outside the cage. Even though the cage was earth grounded, it too had high voltage on the paint that felt like normal carpet ESD door-knob discharges when brushing against the paint. Even the 100 ton transformer tank paint was electrically charged, when touching it during testsm it gave off static discharge of ~1kV for on every surface of epoxy paint.

To test at 500kV or > 1Gv demands much better gas insulation or drone cleaned surfaces.

  • \$\begingroup\$ the person who voted (-1) is dangerously broadcasting ignorance by leaking a biased negative whim . They need to be more "grounded" with experience \$\endgroup\$ Apr 18, 2018 at 12:33

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