Sorry, you're absolutly correct about margin and I do have
margin built in but I didn't put that into the note. In my particular
application our legal requirement is Class A but we set Class B as a
design goal. If you meet that goal by .1 dB on a small sample size I
accept it without to much reservation. If you can't meet it you have
"some 'splaining to do Lucy". Tell me what the problem is, what the cost
of fixing it is and then we will consider whether we want to spend the
money to meet the design goal. Yes, it has happened that we accept
values above the design goal but never closer than 2 - 3 dB from the
legal limit. This applies not only to vendors but to our own engineering
staff. Even then the product only gets a provisional okay, meaning we
will take some small number unit until you demonstrate 3 dB or greater.
I think 6 dB is pretty excessive from a hard specification line. If I
was jumping around that much consistantly, I would want to look into
that variablility a little further to understand what it varies that
much. I would be happy with your 3 to 4 dB. Many large companies will
actually specifiy that amount if you're building for them rarely do I
see 6 dB (that should unleash a hoard of comments from the rest of the
forum)
Thanks for your thoughts and response.
Gary
> -----Original Message-----
> From: Mona Akerblom [SMTP:[email protected]]
> Sent: Monday, August 10, 1998 9:38 AM
> To: [email protected]
> Subject: spec'ing OEMs, Was: Re: Power Factor
>
> Another nice thing to specify to vendors is the MARGIN you expect the
> full
> system to have versus the limits when the OEM part is installed into
> your
> equipment.
>
> we try to have a 6dB margin for EMC, but that's often not attainable,
> so a
> 3-4dB margin is what we'll "live with". unfortunately, when you spec
> "must
> passs XXX when installed into the system", all you are asking for is
> 0.1dB
> margin, just enough to not fail. so much for any headroom to allow
> for unit
> to unit and test site to test site variations!!
>
> mona
>
> Gary McInturff wrote:
>
> > I might add that I specify that the power supply will meet all of
> the
> > requirements when installed into our equipment. That eliminate the
> > fights between the test configuration and results. The power supply
> > manufacturers are often using resistive loads to run their tests.
> That
> > can give dramatically different results because switchers have (or
> used
> > to at any rate) quite different characteristics when fully loaded
> and
> > lightly loaded. The equipment I run falls somewhere in between. This
> get
> > to be a really interesting issue when specifying the EMC compliance.
> > Make sure to specify that the supply works in your system and the
> > voltage range you expect out of it. Most folks run the conducted
> > emissions test to the CISPR limits for radiated. The FCC allows that
> but
> > states that if you accept those limits for radiated then you must
> > accept them for the conducted. That means your supply has to meet
> the
> > CISPR levels down to 150 kHz and 120 Vac - Not 450 KHz at 120 Vac.
> This
> > can be a very important point for your power supply vendors. They
> have
> > to start being careful as the frequency range of measurement goes
> down
> > at the same time the input frequency goes down and the load on the
> power
> > supply is not maximized. In fact I specify the voltage to be 100 Vac
> to
> > meet Japans line voltage. I think VCCI accepts the voltage at 120
> Vac,
> > but I demand 100 Vac.
> >
> >
> > -----Original Message-----
> > From: Rich Nute [SMTP:[email protected]]
> > Sent: Thursday, August 06, 1998 5:57 PM
> > To: [email protected]
> > Cc: [email protected]
> > Subject: Re: Power Factor
> >
> > Hi Scott:
> >
> > In answer to your questions:
> >
> > > 1. What is the standard that addresses power factor
> > correction?
> >
> > IEC/EN 61000-3-2.
> >
> > > 2. What is the range of acceptable power factors for
> ITE?
> >
> > The standard does not specify power factor. Instead, it
> > specifies the maximum harmonic current emitted by the
> > product. Limits are specified for each harmonic up to the
> > 39th harmonic in both absolute and relative units, depending
> > on the type of product.
> >
> > Some engineers may be able to correlate the harmonic limits
> > to power factor. But, power factor is NOT the measure of
> > compliance.
> >
> > > 3. How should I call out PFC in my power supply spec?
> >
> > Simple! We specify compliance to IEC 61000-3-2.
> >
> > As a general rule, rectifier circuits such as commonly used
> > in electronic equipment power supplies, whether linear or
> > switcher, will not comply with the harmonic current emission
> > requirements.
> >
> > As a general rule, products with PFC circuits on the front
> > end of SMPS will comply with the harmonic current emission
> > requirements with lots of margin.
> >
> > EN 61000-3-2 is applicable to products in the EU. It has
> > not been adopted elsewhere in the world.
> >
> > Best regards,
> > Rich
> >
> >
> -------------------------------------------------------------
> > Richard Nute Quality Department
> > Hewlett-Packard Company Product Regulations Group
> > San Diego Division (SDD) Tel : 619 655 3329
> > 16399 West Bernardo Drive FAX : 619 655 4979
> > San Diego, California 92127 e-mail: [email protected]
> >
> -------------------------------------------------------------
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