Re: Ferrite Cores - Where's the Gap? (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Wed, 14 Apr 2004 12:14:00 +1200
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Ferrite Cores - Where's the Gap? (fwd)

Hi Matthew,

On 8 Apr 2004, at 14:47, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Thu, 08 Apr 2004 08:08:27 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: Ferrite Cores - Where's the Gap? (fwd)
>
> Malcolm
>
> >       HV and EHT supplies are not always flyback supplies. For
> > instance, many of the so-called flyback supplies built by people in
> > this circle are actually self-oscillating forward (often pushpull)
> > converters. Sorry if this sounds like pure pedantry but this is an
> > engineering forum of sorts and it is helpful to get accurate
> > descriptions of equipment to facilitate long-distance
> > troubleshooting.
>
> This is certainly helping me!  I've been trying to learn my SMPS stuff
> from a Linear Technology application note.  It's pretty comprehensive
> and more easy to get to grips with than most text books (which you
> can't get round here anyway), but doesn't cover everything.
>
> My original question related to the type of transformer used for TV
> line-out.  I have just taken one apart and confirm the absence of any
> type of spacer.  Does this mean that it is wrong to assume that the
> line-out from a TV is actually a flyback transformer?

The first question has to be: does it just look like a TV LOPT or was
it actually taken out of a TV? If the latter, I have yet to see a TV
which does not employ a flyback topology for its LOP stage and the
transformer must therefore be gapped somehow. If it did not actually
come out of a television and doesn't have any gapping, it would not
have been used in a flyback type supply (hence, the supply must have
been a forward converter in some form).

> Can you confirm, therefore, what topology you consider to be a
> single-ended (switched DC) supply to be - which looks to all intents
> and purposes like flyback topology - but without the core gap?  In
> other words, take the gap out of a "flyback" transformer, and what do
> we get?

A topology which does not store (a useful amount of) energy in the
core. Forward topologies allow secondary current to flow while the
switch (ing transistor) is passing current through the primary
winding. Such supplies are rife - the cap charger in your flashgun is
most often this type of supply. All the one-shot cameras I've
examined have one and use the same basic circuit. Incidentally, the
design in those is rather clever and, as one might expect from the
use of a single 1.5V alkaline battery giving about 20 flashes of 5 -
7J, quite efficient.

    Given that a flyback supply only allows current to flow in one
winding at a time, by noting the sense of the windings together with
the direction of the output diode and taking into consideration the
polarity which must be present across each winding at any given
instant (in conjunction with the presence or absence of core
gapping), you can determine which topology is being employed.

Malcolm

> If popular usage of a term is causing confusion, maybe it is best that
> we get our terminology right and start to call things what they really
> are!  (Either that, or give up and go into politics.)
>
> Does anyone know if there's a "SMPS design for Dummies" ;-?  I think I
> could do with it...
>
> Lastly, to save wearing out my calculator/brain, I'm going to be
> putting all the main formulae from the application note into a
> spreadsheet - I'll post it to my site if it would be of any interest
> to anyone.
>
> Cheers
>
> M
>
> --
> Matthew Smith
> Kadina Business Consultancy
> South Australia
> http://www.kbc.net.au
>
>
>