Re: Recycled Ferrites III (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 22 Dec 2003 09:10:38 +1300
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Recycled Ferrites III (fwd)

Hi Matthew,

> Many thanks to those who have provided such helpful advice on my
> attempts at re-using ferrite cored transformers...
>
> Going through all the odds and sods that I have lying around, I have
> found a load of tiny little transformers removed from the flash boards
> of disposable cameras.  These have pretty good turns ratios - may need
> to watch insulation if I put more than the usual 1.5V into them.
>
> Looking at an alternative way to drive them (if you put more than 1.5V
> on the entire flash board, you get smoke after a very short time) with
> a suitably low component count.  I have half a rail of LT1171
> switching regulators - this is the 2.5A/100KHz version of the old
> LT1170.
>
> Thinking of hooking an LT1170 onto one of these little transformers
> (goodness knows how they will handle 100kHz).  The only topology in
> the Application Note which uses two windings is flyback.  However, I
> don't think that these transformers have a gapped core as a flyback
> corner should.

They are not flyback transformers. The converter topology they're
used in is a forward converter. They work by simple turns-ratio step-
up. You might find tracing out the circuit and analyzing its
operation a useful exercise.

   The flashgun converter does not operate at anything like 100kHz.
In fact, the frequency ramps up as the flashgun cap charges. I leave
it to you to figure out why from your circuit analysis. Having said
that, the ferrite grade used in the transformer is most likely 3C8 or
similar which is certainly useable for hard-switching at 100kHz.

> Can anyone tell me what would the effect be of running a flyback
> circuit with no core gap?  If the energy is stored in the gap, would
> the lack of gap mean that I wouldn't get anything out of the
> secondary?

Put simply, a converter without a gapped core will *not* be a flyback
circuit. However, not all circuits using a gapped transformer are
flyback circuits either. What characterizes a flyback converter is
that it is single-ended (i.e. not half-bridge, pushpull etc.), uses a
gapped transformer (really two chokes on a common core) and
incorporates a single rectifier at its output for current steering
(i.e. the switching transistor delivers energy to the core and gap,
never directly to the load). The output diode makes this happen.

> My proposed test circuit is VERY basic - LT1171, transformer,
> rectifier, capacitor.  I can either hook the feedback pin to ground to
> enable flyback mode or just leave it floating.  Maybe the latter would
> be best, since I would then really just have an oscillator driving the
> switch.  (The application note shows this as a legitimate way of doing
> things - this is how higher voltages are handled using an external
> switching element.)

I suggest you bone up on the various topologies to learn how they
work. Why is this useful? Because you'll be in a position to choose
the appropriate one for your application.

Malcolm