Path: utzoo!utgpu!attcan!uunet!lll-winken!lll-tis!ames!think!bloom-beacon!tut.cis.ohio-state.edu!rutgers!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony)
Newsgroups: sci.electronics
Subject: Re: Flyback Transformers
Message-ID: <1052@unccvax.UUCP>
Date: 28 Jul 88 17:47:18 GMT
References: <14741@shemp.CS.UCLA.EDU> <3844@pdn.UUCP>
Distribution: na
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 71

In article <3844@pdn.UUCP>, larry@pdn.UUCP (Larry Swift) writes:
> So what's a flyback transformer, and how did it get its name?

	This comes from the darker ages of television engineering,
and unfortunately continues to this day.  The original "flyback"
term has to do with the topology and operation of the horizontal
sweep in a television receiver.

	Exceedingly basically, you have a switch (transistor, 
tube, whatever) strobed at the horizontal line rate.  This switch
is connected in series with an appropriate inductor. Now, assuming
you have no current initally in the inductor, and you close the
switch (the inductor connected to your handy-dandy perfect 
voltage source) you have the dreaded relationship

	v(t) = L (di/dt)  (has it been that long :-) :-)

	Neat. Now, the net.magnetics experts can show you that
the changing magnetic field inside a solenoid inductor can be
used (with a little help from some more nonlinear stuff, but who
cares at this point) to deflect the CRT electron beam and make
a raster.

	However, when we want this beam to "fly back" we want
it to do so instantaneously. We write the beam across the CRT at
a constant surface velocity (which is not the same as a constant
angular velocity on the deflection axis--why?? :-)) but would
like to start the next line instaneously after the end of the 
first.  In reality, you can't change the magnetic field or 
current in the inductor instantaneously, but for TV purposes,
fast enough is simply to open the bloody switch. The magnetic
field collapses damn fast, causing the terminal voltage across
the inductor to increase drastically, among other things, etc.

	If we make the switch the horizontal output transistor,
provide a damping diode to recover the negative excursion of
the inductor current (it will oscillate at some natural frequency
because there is a physical capacitance); make the inductor a
transformer coupled to a deflection yoke, and put a big fat
autotransformer coupled to the same transformer with a half-wave
rectifier (tripler, or other such ilk) you have a nice high voltage
supply made from the collapsing field (and rising terminal voltage,
multipled through the autotransformer) during retrace. The
high voltage rectifier doesn't conduct during scanning, so
no big deal.  Instead of the collapsing magnetic field doing
everything in the circuit in (because that voltage will flash
over somewhere, eventually), you've harnessed it and made the
accelerator voltage for the CRT. Hence, the "flyback" transformer.

	Now, in reality, we tune this turkey to ring at the
(third, fifth, seventh) harmonic, and we rearrange the windings
on the transformer so that (in the vacuum tube case) the damping
diode provides the necessary boost in plate supply voltage to
the horizontal output tube. As a consequence, we get all kinds
of other neat voltages from this transformer.  Virtually every
CRT monitor today gets not only acceleration voltage, but 
all other supply voltages from the "flyback" transformer. It
is a very efficient switching power supply.

	Anyone over the age of 25 will tell you what a drag it
is to change a flyback transformer in a consumer TV set.

	Really, folks, it's just like the points (whatever) in
your car, with a rectifier on the end :-) :-)

York David Anthony
DataSpan, Inc.

PS. There were some really crummy flyback designs out there, too.
How many of you have changed the transformer in an RCA CTC-10
through CTC-51 (the one where the plate cap for the 3A3 is 
moulded into the secondary winding.) I thought so. :-) :-)