One thing that might be a problem is if the heat sink of the primary switching
transistor is grounded.
Then it might be difficult to reinforced isolate the primary switching
transistor as changing the isolation material will also influence the heat
conducting properties and thus the life span of the transistor.
Rich Nute <ri...@sdd.hp.com> on 09/29/99 04:50:43 AM
Please respond to Rich Nute <ri...@sdd.hp.com>
To: pe...@itl.co.il
cc: b...@anritsu.com, emc-p...@majordomo.ieee.org,
jo...@exchange.scotland.ncr.com (bcc: Rene Charton/TUV-Twn)
Subject: Re: Re: EN 60 950 and Denmark
Hi Peter:
> This is the exact deviation which I was referring to. One of my
> clients states that a distributor wishes the product to be safe for
> those establishments where earthing cannot be relied upon. Does
> this mean to provide double or reinforced insulation from hazardous
> live parts to earth? This may be a pain killer if the power supply is
> Apporved for Class I.
Not necessarily.
Today's designs of SMPS are typically double or reinforced
insulated. Even if certified as Class I, you have the option
of evaluating it for double- or reinforced-insulation and
declaring it as complying with double- or reinforced-insulation
for use in Denmark (and other Nordic countries where grounded
outlets may not be present in the electrical installation).
There are two insulations that must be considered:
primary-to-secondary
primary-to-ground
1. Primary-to-secondary.
In a typical SMPS, there are three such insulations that
must be considered:
transformer
opto-isolator
PWB spacings
Typical SMPS transformers are double-insulated. It is
very inefficient for a SMPS transformer to employ a
grounded shield. If the certifier did a good job, he
will have ascertained that the solid insulation, the
through-air insulation, and the surface insulations meet
the respective double-insulation requirements. In my
experience, most SMPS transformers do meet these
requirements.
Opto-isolators are almost always rated as double- or
reinforced-insulation.
Most PWB spacings are those for double-insulation. If
not, the secondary circuit and conductors must be
grounded as if they were protective conductors. So, I
believe you will find that most PWB spacings are
sufficient for double-insulation.
2. Primary-to-ground.
In a typical SMPS, there are two insulations that must
be considered:
PWB assembly and the metal chassis
PWB spacings across Y capacitors and similar spacings
Here is where you are likely to find spacings only
sufficient for Class I. Most power supply vendors will
likely balk if you should require Class II construction
between the circuits and ground.
But, you can easily get around MOST of the spacings by
interposing a solid insulating sheet between the circuits
and the grounded parts (e.g., chassis).
You can get around the Y capacitor spacings by asking for
double-insulated spacings on the PWB conductors.
Its really not too difficult. Its really easy if you spec your
power supply as double-insulated primary-to-ground and primary-
to-secondary. Almost zero cost.
By the way, we find that the weakest insulation in such designs
is the Y-cap lead spacing on the PWB. We experience arcs in
the range of 4-5 kV, well above the required 3 kV!
We do this on all of our products. We KNOW our products will
be used in situations without ground. So, we require our
power supply vendors to provide double-insulation as well as
grounding.
Best regards,
Rich
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