Re: Toridal Mains Transformer

Ross Herbert wrote:

> On Thu, 8 Sep 2005 19:32:59 +1000, "Phil Allison"
> <philallison@xxxxxxxxxx> wrote:
>
> >
> >"Ross Herbert"
> >
> >>
> >> OK, not strictly correct I admit.
> >
> >
> > ** Err - dead WRONG actually.
> >
> >
> >> Since the OP hasn't said whether he
> >> is using switching techniques to generate the 300V dc (after the
> >> isolation transformer)
> >
> >
> > ** The OP posted a bloody good hint:
> >
> > ".... followed by a bought in PFC module which can handle 85-260VAC in."
> >
> >
> >> I assumed he was simply wanting a linear dc
> >> supply, otherwise why include a toroidal transformer.
> >
> >
> >** Because most AC supply, PFC correctors generate DC and are non
> >isolating.
> >
> >
> >> He may simply
> >> need it for the isolation factor but that seems unneccessary.
> >
> >
> > ** Go read up on PFC regulators.
> >
> >> Linear supplies of the size generally found do not contribute very
> >> much at all in the way of power factor distortion
> >
> >
> > ** WRONG - most linears have PFs of 0.5 or worse.
> >
> >
> >
> >> and I haven't come
> >> across any linear power supply which incorporates PFC,
> >
> >
> > ** Since it is close to impossible.
> >
> >
> >> but you might
> >> know of one. PFC is generally not necessary on linear supplies but it
> >> is required on switch mode supplies due to the very short duration
> >> current spikes needed to top up the input filter capacitor on these
> >> devices.
> >
> >
> >** There is simply NO basic difference.
> >
> >Supplies with no isolation transformer eliminate a bit of series
> >esistance - ie the copper winding resistance of the transformer.
> >
> >A linear supply, using a toroidal or C-core tranny, has an IDENTICAL supply
> >current waveform to a SMPS.
> >
> >You simply have never investigated the matter.
> >
> >
> >
> >
> >............ Phil
>
> Most of what you say about linear supplies is accurate Phil, except
> for how they are usually used in the real world - and here I am not
> talking about piddling wall warts. You will be aware that the closer
> the loading approaches the design maximum (assuming proper component
> specification) the closer the pf will be to ideal, and that will be
> much higheer than 0.5.I doubt very much that linear power supplies are
> generally operated with no load and that is where the pf is worst.
>
> Actually Phil, Most PFC pre-regulators are included directly on the
> mains side ahead of the SMPS proper. The only thing between the PFC
> and the smps proper is the EMI/RFI filter and bridge rectifier and
> input filter capacitor. I have never seen a transformer used purely
> for isolation and then follow it with the PFC regulator ahead of the
> smps proper. The very fact that most PFC regulators are designed for a
> wide range ac input infers that they are intended to be directly
> interfaced to the mains supply. Check any of the IC manufacturers and
> this fact will be verified. Her's one to start with
>
> http://www.st.com/stonline/products/literature/an/4046.pdf

The kindest thing I can say is that I think the entire contents of the thread to
date simply whizzed over your head !

Btw, your statement that " The only thing between the PFC and the smps proper is
the EMI/RFI filter and bridge rectifier and input filter capacitor " is simply
staggeringly wrong !

Did you know that PFC stages actually produce *more* EMI than a typical smps
designed for the same load ?


Graham


.

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