Recycled Ferrites III (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Fri, 19 Dec 2003 21:58:08 +1030
From: Matthew Smith <matt@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Recycled Ferrites III

Hi All

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.

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?

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.)

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia
http://www.kbc.net.au