Peter,
I am pleased to give you greater details below as required.
"Peter L. Tarver" <peter.tar...@sanmina-sci.com>
05/11/03 11:40 PM
Please respond to
<peter.tar...@sanmina-sci.com>
To
<emc-p...@ieee.org>
cc
<raymond...@omnisourceasia.com.hk>, <c...@netscreen.com>, "'Rich Nute'"
<ri...@sdd.hp.com>
Subject
RE: Class 1 AC/DC adapter
Let's look at these what ifs.
> From: ChengWee Lai
> Sent: Tuesday, November 04, 2003 4:38 PM
>
> Raymond might be able to convince Safety agency
> to accept PE continuity test
> with 1500Vac Hipot at the production line.
> However in the Scenerio with end
> customer, it could mean business or no business.
>
> Question regarding:
>
> Whether customer can accept his adaptor with only
> 1500Vac hipot tested where
> other power supplies can withstand 3000Vac?
The only clear answer is some form of redesign is necessary.
Simple solutions work best. For any reasoned advice, there
are too many unknowns about the physical construction of the
power supply. See the below for more on this.
> Would that affect his customer Safety testing,
> because his customer unit
> with his power supply only meet 1500Vac hipot
> after heating test?
Only Raymond and his customers can answer this.
> Or what if Safety agency require his customer
> unit to do grounding test to
> accessible metal part?
OK, but why? The discussions have centered around an SELV
output, making the end product Class III. There would have
to be some specific about the final application to justify
this. We have no information to base such an assumption on.
> Regards,
> Chengwee
As with many discussions on the list, practical issue
discussions often get side tracked by the hypothetical and
theoretical.
More to the point, Raymond hasn't given us specifics on the
physical orientation of parts in the power supply to any
great degree. Proprietary information aside, we know:
K1) the insulation between earthed parts and mains
connected parts is somewhat better than Basic, but not
Reinforced
K2) the EST potential where breakdown occurs (though I've
discarded the e-mail with the identified potential)
K3) the breakdown is suspected (or possibly confirmed) to
occur to the earthed board that is used as an EMC shield and
which also electrically connects to an SELV output
What we don't know (or I've lost track of):
DK1) if the power supply is a linear or switched mode type
(we can assume it's a SMPS, since EMC shielding was
mentioned, but it would be nice to have confirmation)
Your assumption is correct - it is a SMPS that is commonly for digital and
electronics products.
DK2) what mains connected part(s) are involved in the
dielectric breaking down (transformer winding terminations,
leads or bodies of filtering, surge suppression or other
components and how they are mounted)
According to Rich's suggestion, the PCB mounted AC receptacle contributes the
breakdown. From the signs of breakdown, there were two places of breakdown
between the mains copper tracks and earth copper track of the receptacle where
have less than 5 mm spacing.
DK3) what area or specific portion of the earthed parts are
involved in breaking down (at solder connection points for
lead wires, the copper on the shielding board)
DK4) if the supply uses an appliance inlet, has a
nondetachable power supply cord or if it a direct plug-in
type and how the mains circuit is brought to the power
conversion element(s) of the primary circuit
The power is brought in via a detachable power cordset.
With some of this additional information, a more practical
solution may be possible, rather than talking around the
specifics.
A WAG or two:
One issue that hasn't come up is whether or not the copper
on the shielding board is facing the mains/primary circuitry
or not. Based on the application and the general
discussion, this board appears to be separate and
independent of the mains/primary circuit board. If this is
true, it seems logical that the board is single-sided. If
the board is single-sided and the copper is facing the
mains/primary circuit, has anyone considered flipping the
board over and relying on the base laminate to provide
Supplementary insulation to a clearance?
The earthing plane is a separate single-sided fibre glass pcb. The copper
side is not facing to the mains circuitry.
If the first WAG is unusable, why not add a 0.4mm thick
insulating sheet between the earthed board and the portion
of the primary circuit involved in breaking down? This will
effect margins and pricing, but sometimes one must bite the
bullet to get into the market or meet a customer's time
constraint.
As explained by Rich, the class 1 AC receptacle is designed for the hi-pot of
1,500 Vac. It cannot withstand 3,000 Vac together with associated pcb tracks.
Probably why the designer has to design it in that way. Such design gives
challenge at final stage testing in production and incoming QC.
Regards,
Peter L. Tarver, PE
ptar...@ieee.org
