Offline flyback SMPS needs conformal coat over via?

[cupoftea]

Hi,.
We are hipot testing an offline flyback PCB. The flyback is totally enclosed in
an earthed metal enclosure.
Unfortunately, on the secondary side, there is a via under a connector, and this via is
just 1.2mm away from a bit of the earthed enclosure. As such, this flashes over
and causes hipot fail, since the actual earth must not be connected for the hipot test.

Anyway, i spread some acrylic conformal coat over the via and left it to cure.
But when it had cured, the conformal coat had parted itself away from the via, and so the via
is still exposed, even though i pasted conformal coat right over it. As such, the board still fails flash test.
Do you have any ideas for ways in which i can get the conformal coat to cure and stay
covering the via? The via is just 0.3mm hole and 0.6mm diameter (round). We cant drill the via out as this would expose internal
ground plane and we would then just fail hipot on that.

 

[KlausST]

and this via is
just 1.2mm

sounds to me like a design mistake... that violates saftey regulations.

since the actual earth must not be connected for the hipot test.

FOR THE TEST maybe you don´t need to connet it to real EARTH GND .. but the high pot test has to be done against your devices´ earth connection.
I assume your earth wire needst to be connected according to the test setup.

Currently we don´t know what device it is what power supply and voltages you use, the intended use case of your device, the environmental specifications...
Thus ... to get more detaield answers you need to provide a lot more informations.

i spread some acrylic conformal coat over the via

I assume your sprayed coat does not count at all regarding safety regulations. Similarily: solder stop is not considered any protective isolation at all.

***
The first thing to do is to know the exact rules you need to follow
Then you need to do the PCB layout according the rules. Especially the safety distance and creepage distances.
Then you need to do the test.

--> a passed test does not mean the circuit is considered safe ... when you don´t follow the safety regulations (PCB layout) in first place.

This means, even if you pass the test, you might be held responsible for an electric shock, if you did not keep on mandatory minimum distances.

Klaus

Klaus

 

[FvM]

Purpose of conformal coating is to increase CTI (compariative tracking index) of PCB material or to reduce pollution degree (presumed it's applied correctly). It does not act as solid insulation. You can apply massive insulation layer e.g. silicone rubber.

 

[D.A.(Tony)Stewart]

The most common solution is a thin layer of film or silicone insulation or both. For UHV mica tape or Kapton Tape ( my choice)

Other types of tape might be possible.

CHoice of PE gnd will affect Hipot and must be used unless it is medical grade or isolated from human touch.

 

[cupoftea]

So, the top post continued as follows..

We are hipot testing an isolated offline flyback and getting unusual
findings..

So we have an isolated offline flyback in an earthed metal enclosure.
As is well known, when you hipot test from L&N to output, one
must have the earthed enclosure floating (not connected to earth).

Anyway, as discussed in top post, there is a secondary-side via which is just 1.2mm from an earth plate which
comes down and meets the PCB surface. Obviously this flashed over during 4kV
hipot test. (The earthed enclosure acted like a conduit for the clearance to
primary side, since primary side copper is 3mm away from the "earthed" enclosure, so this
via is only effectively 4.2mm away from primary side copper.)

Anyway, I coated over the via with conformal coat, and also taped over the earth plate,
so as to insulate the via. All was well, the flashover was solved.

So then I considered how to modify the 200 other boards which have the same problem..
I decided that I would omit the conformal coating, and just do the taping, so as to save time.
However, this resulted in subsequent hipot failure. I could clearly see the via flashing over to the
earth metal. -But what was strange, was that some connector legs (also secondary side, and maybe 2cm away from the offending via) which are 3mm away from the earth enclosure
also started flashing over aswell. These connector legs do not present a clearance violation, and were not
flashing over during the first flash test where it passed hipot.

So do you believe that when a hipot test instrument causes flashover, then the actual incidence
of flashover, makes the hipot instrument's voltage suffer high voltage spikes, over and above the
level that was set?...….and then this makes other regions of a PCB suffer flashover aswell...even though
they should not be flashing over, as they are not within clearance violation zones.

 

[D.A.(Tony)Stewart]

So, the top post continued as follows..



So do you believe that when a hipot test instrument causes flashover, then the actual incidence
of flashover, makes the hipot instrument's voltage suffer high voltage spikes, over and above the
level that was set?...….

No.

I bet you did not use Kapton tape or equivalent polyamide tape.

 

[cupoftea]

Thanks, we used yellow transformer tape.....it has been lying about for a while.....dont know what type it is to be truthful...but is the "transformer yellow" color.
I suspect the "gum" on it is slightly conductive, or has become so.

 

[FvM]

If tape is a solution, it must be applied so that the isolation distance is through the tape, not along the interface between PCB and tape, which isn't necessarily hermetically sealed. This can require to apply the tape around PCB edge.

Polyimide (Kapton) tape is transparent yellow-brown to light-brown, not yellow. Neverthelesses insulation tape adhesive should never become conductive.

 

[mtwieg]

Are you absolutely certain the flashover is between this via and the earthed enclosure?

Usually hipot test is done with the HV applied between protective earth and the mains (primary) side. If the flashover is happening through a secondary-side via, then that current must be passing through a primary-secondary insulator as well.

Or are you actually applying the hipot from secondary to PE?

 

[D.A.(Tony)Stewart]

Yellow PET film tape ought to good for 5kV when sealed between conductors without creepage contamination (?) or stretching.

Normally Hipot is done H||L to PE with secondary PE grounded if used. This stresses the low BDV high Z zones with feedthru capacitance even with slow ramped DC 10% higher than peak Vac. I once had a major PSU supplier who transitioned Mfg from SD, Cali. to Mex. and the gaps changed. They tested with secondary floating and passed. I tested with 2ndary PE grounded and failed catastrophically ( until I modified our Hipot probe with added R current limit of 100uA) I disqualified them until they cleaned up their act. (1k/month) with half a dozen failure zones.

If you use an AM radio, you should hear partial discharge PD (like distant lightning) on a quiet channel before any arc if you ramp slow enough with DC. e.g. 500~1kV/s.. The rep rate can vary from 0.01 pps to > 50 Hz until it punches thru any contaminant particle in the otherwise pure insulation. I doubt you will hear this on a clean unit unless flux residue is nearby. This is why air gaps are preferred in any PCB design for isolation.

 

[cupoftea]

Thanks, the hipot test we do is from primary (L&N) to secondary outputs, because that is meaningful in terms of its how people could get shocked in the actual product.
The whole product, pri and sec, is surrounded by the earthed enclosure, and everywhere , the pri and sec tracking should be 3mm away from the earthed enclosure at least...however, unfortunately, there is one secondary via which is just 1.2mm away from the earthed enclosure wall....and so when the hipot voltage is applied, we get flashover from pri to sec, obviously by way of this via flashing to earth , and then this same earthed enclosure flashing to primary.

During this hipot test, the earthed enclosure just floats, since the regs say it must, because in the installation, it may be forgotten to connect the enclosure to earth...so we have to take that into account. Also, in any installation, the impedance of the actual earth ground (soil) may be very very high (eg a clay or chalk area)

 

[mtwieg]

Thanks, the hipot test we do is from primary (L&N) to secondary outputs, because that is meaningful in terms of its how people could get shocked in the actual product.

To my knowledge, hipot is done both from Primary to Secondary and Primary to PE (at a lower voltage). I think this depends on the particular standard applied though.

and so when the hipot voltage is applied, we get flashover from pri to sec, obviously by way of this via flashing to earth , and then this same earthed enclosure flashing to primary.

You get flashover between primary, secondary, and PE? That makes no sense.

 

[cupoftea]

You get flashover between primary, secondary, and PE? That makes no sense.

Thanks, the pri to sec distance is 6mm all over the board...so no flahsover there...but....a sec via is 1.2mm from the earthed enclosure...and also, many pri tracks are just 3mm away from the (same) earthed enclosure........so in these cases, we only actualy have 4.2mm of clearance pri to sec.....which means its flashing over from pri to sec, but via the earthed enclosure.

 

[mtwieg]

Ok so no flashover directly from primary to secondary.

As for fixing the via flashover issue, it would help to have a picture of the layout there. Depending on that, there may be a couple options:
1. Put a cutout in the board between the via and PE
2. Backdrill the via a bit. That should give the conformal coating something better to adhere to.

 

[cupoftea]

1. Put a cutout in the board between the via and PE

Thanks, but sorry that doesnt work here as the clearance distance is still the same

 

[FvM]

As is well known, when you hipot test from L&N to output, one
must have the earthed enclosure floating (not connected to earth).

Can you tell which test specification requires/describes this setup?

As far as I understand, secondary to enclosure test voltage doesn't exceed 1500 V, in some cases not more than 500 V. If you get I higher voltage as indirect effect of the primary test, there's probably something wrong with the setup.

 

[cupoftea]

Can you tell which test specification requires/describes this setup?

Thanks, its because in a real installation, Earth may mistakenly not get connected up...also, in chalk or clay areas, the Earth ground doesnt provide a proper low Z "earth" connection. As such, all test specs , in all the standards, require earth to float during hipot test.
Its just like you have to assume that Line and Neutral may get accidentally switched. All The standards require it.

Also, and here is part of the problem here....

..The two schems show offine flybacks with Y capacitors.
One is obviouly showing the "correct" connection of Y caps for offline flyback. (correct as per EMC)
The other shows an inferior connection.
However, the "Correct" version (during a hipot test) shows a voltage of 4kV RMS from Earthed enclosure to secondary side.
The "Incorrect" version gives only 2kV RMS between secondary side and the earthed enclosure. As such, the "Incorrect" version
needs less clearance between earthed enclosure and secondary side on the PCB. And as such, it appears more favourable,
because it gives the PCB layout more room.

The well known way of reducing the hipot voltage between secondary and earth for the "correct" version is
to add TWO more Y2 caps as follows..(same value as the Mains to Earth Y caps)
One from secondary positive to Earth.
One from Secondary ground to Earth.

But its becoming a bit of a "Y cap christmas tree".

Have you any better solutions?


Y caps done "correctly"...

Y-caps-done-correctly hosted at ImgBB

Image Y-caps-done-correctly hosted in ImgBB

ibb.co

Y caps done "wrong"

Y-caps-done-wrong hosted at ImgBB

Image Y-caps-done-wrong hosted in ImgBB

ibb.co

--- Updated Monday at 3:13 PM ---

Y caps correctly in LTspice

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Y caps wrong in LTspice

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[cupoftea]

Here again the LTspice's but in zipped format

--- Updated Monday at 7:42 PM ---

Also, We all know that if you only have room for one Y cap on the pri side, then it cannot go in line or neutral.....since that would give an unbalanced common mode filter....so it must go to post rect pos or neg.
As we all know, if only one pri side y cap is used, and is taken to neutral (ie neutral to earth) , then the common mode emissions would be worse than if no pri side y cap at all.

 

[cupoftea]

Please may i ask this related qu on Y caps....

Hi, we have 2 Y caps in our offline isolated SMPS with earthed enclosure, both 1n5,
but one is Y1 (DK1E3EA102M86HAH01, 8th page) and the other is Y2 (DE2E3KY152, 42nd page)

Y caps

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/431/Lead%2CResin_Ceramic_Cap_CAT.pdf

We need them to be the same capacitance right up to 5600V, since we are 4kV flash testing, and of course,
they form a capacitive divider centred on the earthed enclosure, so we need to know they are same capacitance
whatever the voltage. But the datasheet doesn't say. Do you know?

 

[D.A.(Tony)Stewart]

Please may i ask this related qu on Y caps....

Hi, we have 2 Y caps in our offline isolated SMPS with earthed enclosure, both 1n5,
but one is Y1 (DK1E3EA102M86HAH01, 8th page) and the other is Y2 (DE2E3KY152, 42nd page)

Y caps

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/431/Lead%2CResin_Ceramic_Cap_CAT.pdf

We need them to be the same capacitance right up to 5600V, since we are 4kV flash testing, and of course,
they form a capacitive divider centred on the earthed enclosure, so we need to know they are same capacitance
whatever the voltage. But the datasheet doesn't say. Do you know?

I see things differently.

A floating enclosure will be a capacitive divider between the mismatched Y caps which could easily be 10% or more unless a human is touching between enclosure and PE ground, So add 1n to 10nF at any human contact point and measure current in each C added. But don't use an unrealistic 2s 50 Hz burst of 4kVac, use a realistic 1us pulse with a 1 us risetime, falltime can be 10us. Then plot it for 10 to 100 us.

(I do not recognize your test methods.)

If you don't like the results add a Schaffner Balun to the input.

Don't assume the impulse gen has a perfect ground add 100m of ground wire to it or 100 uH or more then measure secondary ground and all the human currents.
See any spikes over 25 mA? over 1A? ouch.