How to work out Hi-Pot test voltage etc, for offline SMPS?

[Faringdon]

Hi,
As we know, Offline SMPS’s get tested for high voltage withstand between their primary and secondary sides. The test involves the application of a certain voltage, for a certain time, and during that time, the leakage current between primary and secondary must stay below x milliamps.

How the test voltage, length of test time, and leakage current is calculated is completely unknown.
Its not possible to calculate these things, and if you want to know such things, then you must purchase expensive standards documents. The number of standards documents that an engineer would have to purchase in relation to an offline SMPS development, is a great many…and no engineer could possibly afford to buy all these standards. The company that an engineer works for will not lend out the standards documents to any engineer, -because if a product gets shipped without passing one of the standards, (which they so often do) then the company doesn’t want people knowing about it…so keeps their own standards documents to themselves.

Sometimes the “Hi-Pot” test voltage is AC at 50Hz, and sometimes its DC.

Its impossible to calculate what [DC test voltage /Test Time /Leakage current] corresponds to what [AC test voltage /test Time /Leakage current].

But , ….supposing you do know that your product must pass "2500VAC for 1 minute, and the leakage current must stay below 3mA for that one minute"…

Then you pick up a Hi-pot tester and set it to 2500VAC….and you count to 60 seconds and hope that it doesn’t flash red (fail) at you. Even if it doesn’t flash red, you still don’t know whether your product really passes or not, because the Hi-pot testers you get never have the leakage current threshold written on them, and they are usually so old that they have no datasheet. So fingers crossed and goodness help you.

Then you get another offline SMPS to test, and it again is [2500VAC for one minute and 3mA]…..but at this company they only have a DC Hi-pot tester, instead of an AC one. So of course, you simply calculate what [DC/time/mA] corresponds to what [AC/time/mA]….

…except you don’t, because there’s no way of calculating that.

So you use your sense of logic…..and realise that you must apply a DC voltage of 2500 x SQRT(2).

But for what time?....well, less time than the AC tester that’s for sure, since the AC tester isn’t always at its peak level. Surely an equivalent length of time would be [0.63 * one minute], ie 38 seconds(?) This is because the average of a sine is [0.63 x peak]. Again, you would have to assume that the current threshold would be 3mA…….and lucky you if your DC hi-pot tester allows you to set the current threshold, most don’t, and you’ve no idea what it is.
Whilst you are fumbling about, wondering about this, the other (time served) Engineers in the company declare to the boss, that you are obviously a 2-bit idiot, and that you should spend the rest of your working life doing nothing more than replacing dirty solder-tip-wiper-sponges on the lab benches, and scrubbing the conveniences whilst you're re-filling their solder water bottles.

I mean, it isn’t as if the regulatory hi-pot [Voltage/time/current] settings make any particular sense for offline SMPS. For example, take an SMPS with a Boost PFC front end, with its output capacitor bank…….there is virtually no chance that a typical 50us transient (Line_Neutral) is going to lift the voltage of that capacitor bank up more than a few 10’s of volts anyway……and certainly not up to 1kV. So why the voltage level has to be up to 2500VAC is anyone’s guess. Its appreciable that there should be a length_of_test_time, because high voltage events are cumulative in the damage that they do, so its good to apply the test voltage for sufficient time.
Though, suppose a 2.5kV voltage spike came to the offline SMPS between line and earth. Then , if the offline SMPS output was earthed, then there could be a flashover due to the isolation barrier seeing the full 2500V. [remembering mains cable stray L meaning that for the short transient duration v(neatral)-not-equal-to-v(earth)]…..but no…..there couldn’t be…..the secondary earth would be at neutral potential (and obviously earth potential) and so once again, this transient would simply get quenched to <<1kV by the PFC output caps.
So why we always test to 2500VAC is anyone’s guess.

How long do you think the Hi-pot mystery will continue for?

[jonpaul]

Very old and well know,

https://www.hipot.pl/files/Standard_Referenc_Chart_July_2017.pdf

http://www.rodl.com/

see Hypot tester mfg app notes and specs

j

[uer166]

How long do you think the Hi-pot mystery will continue for?

_{Exactly as long as it takes to pull your head out of your own a$$.}

But seriously: every single supposed "unknown" is clearly stated in the appropriate IEC standard (how many volts, for how long, DC or AC, and if you can substitute one for the other.). Just spend the ~30 Euros or so on evs.ee for it. If that is "expensive" I'm not sure you should be in the business if touching any SMPS. The threshold of a hipot tester is easily adjustable, so your second question makes zero sense. A red light on a tester does not mean you failed the test, it just means that it passed a certain current threshold. It is up to the manufacturer to specify the leakage spec most of the time (up to limits given in the appropriate standards).

[uer166]

take an SMPS with a Boost PFC front end, with its output capacitor bank…….there is virtually no chance that a typical 50us transient (Line_Neutral) is going to lift the voltage of that capacitor bank up more than a few 10’s of volts anyway……and certainly not up to 1kV.

Is your DC link not isolated from PE?? A line-PE transient would lift the entire bank common-mode to PE by many kV during a transient event.. Obviously a Line-Neutral transient wouldn't, but that's only half of the modes!

[Gyro]

How long do you think the Hi-pot mystery will continue for?

_{Exactly as long as it takes to pull your head out of your own a$$.}

But seriously: every single supposed "unknown" is clearly stated in the appropriate IEC standard (how many volts, for how long, DC or AC, and if you can substitute one for the other.). Just spend the ~30 Euros or so on evs.ee for it. If that is "expensive" I'm not sure you should be in the business if touching any SMPS. The threshold of a hipot tester is easily adjustable, so your second question makes zero sense. A red light on a tester does not mean you failed the test, it just means that it passed a certain current threshold. It is up to the manufacturer to specify the leakage spec most of the time (up to limits given in the appropriate standards).

I don't know if you know, but Far...Treez never buys standards! He actually seems to prefer ignorance and wild speculation.

[Faringdon]

Is your DC link not isolated from PE??

Thanks, AYK, if its Boost PFC, then into the DC link caps.
The DC link "ground", AYK, is alternately connected to line then neutral (every 10ms) due to the mains input rectifier bridge.
The Neutral is connected to Power_Earth at the building "incoming".
So no, the DC link caps are not isolated from Power_Earth.

Very old and well know,

https://www.hipot.pl/files/Standard_Referenc_Chart_July_2017.pdf

Thankyou so much! Thats the best ive seen for a while.
However, i am not being funny, but its out of date...the new safety standard is IEC62368, and this doesnt appear in that document.


EDIT *******************************
IEC62368 seems to be here...
https://webstore.iec.ch/preview/info_iec62368-1%7Bed3.0.RLV%7Den.pdf
...but i havent been sucesful in the answering of the top post with it.
END OF EDIT ****************************

From the above replies, i believe that we agree that mains transients between Line and Neutral cannot have a bad impact, as they would get quenched by the boost PFC output capacitor bank....no mains transient could raise a boost PFC output cap bank above 1kV...so we are looking at transients where Line and neutral both somehow "dis-attach" from earth, and get a common mode voltage of several kV w.r.t. earth.
How this happens transiently, for 50us or so,  is a bit of a mystery.
....i appreciate it could happen for many minutes at a time...but that is much more than a transient, and very little circuitry or layout, or protection features , etc,  could protect against that.

[Gyro]

Thankyou so much! Thats the best ive seen for a while.
However, i am not being funny, but its out of date...the new safety standard is IEC62368, and this doesnt appear in that document.

Buy it then.

[Faringdon]

Hi,
Do we see people not putting MOVs in PFC'd offline supplies?
Because a MOV will die after a few transients and crowbar the mains input fuse.....and the SMPS will inevitably get tossed.
So just dont fit the MOV. (only fit it for the approvals testing)....and let the PFC output cap bank squench the transient instead....bypass the boost inductor and sense resistor with bypass diodes ready for that transient.
Do people do this....to help the SMPS last a long lifetime?
Surely it makes sense?

[Gyro]

Do people fit components for approval and then remove them? Not honest and professional people, no. 

[Faringdon]

Thanks, thing is, there is an environment freindly thing to not having a MOV, and relying on the caps....as AYK, MOVs die and take the power supply  fuse out.
A  PFC output cap bank will squench the vast majority of "real" transients with ease. (as opposed to lab generated ones).
(if not all transients that will ever be seen by a PSU...apart from PSUs in special locations)

We have PSUs sent to us where they are using bidi SMC TVS across mains input to life-long suppress transients..but they will just blow up.

[Gyro]

Ok, so design it to operate correctly without a TVS or MOV and persuade/demonstrate to your customer that it will meet spec without them. Do it properly rather than cheating.

[f4eru]

So just dont fit the MOV. (only fit it for the approvals testing)....

HAHA, good one.
Yeah. Don't.

[jonpaul]

yet another FTTS Farrington Treez Time Sync, 

question was answered long ago.

But Farrington is not seriously seeking solutions

Jon

[Faringdon]

In these posts, i depict...
1....How MOVs die and crowbar the fuse, meaning the PSU is binned......we have no alternative to MOVs and the regs force us to use them.....even though, as above.........there are other ways.....eg large cap banks. Does anyone have a "No-MOV" alternative?

You dont see MOVs in (proper) military power supplies....because they are not subject to civilian standards in real terms...because the military dont want their PSUs blowing unecessarily. Apart from military PSUs in countries which dont make their own PSUs generally (eg civilian use), because their "engineers" think MOVs are the be-all-and-end-all, because the civilian regs say so.

[Gyro]

I don't know who you think you're arguing with. Nobody here is insisting that you use MOVs. 

If you don't want to use MOVs then design / re-design the board to meet customer spec requirements without them and get their approval / agreement. Just don't come here spouting dishonest ideas about fitting them for approval and then removing them!

If you can't agree things with your 'customer' then that's between you. Nothing to do with us.


P.S. What "regs" are forcing you to use MOVs, I've never seen one (it's presumable it's not one that you've actually bought!). Please provide a reference and quote.