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.

 

[D.A.(Tony)Stewart]

BTW when testing L&N from PE gnd, the floating case is not null when Y caps are balanced but 50% of the input line transient.

There is no need to connect secondary to the generator. If you connect secondary 0V to PE ground to case, is almost the same thing except now if your case is floating at nearly 50% of the input. (so your method is valid in testing this way assuming installation errors)

A 50 Hz 2-second burst is a thousand times less stressful than a 1us pulse with 1us risetime. Remember that Ic=CdV/dt, and human touch impedance is mostly capacitive.

The best solution is a Balun on the input which now acts as a differential LC filter with L&N shorted and then a CM filter for the flyback noise when operational.

I wonder why a 2s 50Hz burst at 4kVrms was used. hmm.