Output Transformer Design
Output transformer design requires a book on this topic
alone. It sounds simple but it actually requires a lot of design considerations
to have an output transformer design that meets its application. One cannot have
an output transformer design that can do everything. That simply defies the law
of physics. You cannot have everything in output transformer design.
One thing that really misleads users out there is the use of
core types & materials, like C-core, double C-core, R-core, amorphous core,
nickel core, cobalt core, silver wire, gold wire, 6N or more N wires and such. Yes,
they make a difference, but if the techniques to properly utilize their unique
characteristics are not there, we might as well go back to EI core which could
be better if properly designed and implemented. It’s just a waste of materials.
We really got to match materials with proper technique and
application, only that will bring out the best from them. Over the years, I’ve
tested lots of output transformers and some are really just marketing gimmick
and nothing to shout about in quality.
Let’s discuss some aspects of output transformer design.
The core size, core thickness, number of windings, wire
size, and etc determines the output power of the output transformer. Having the
right size will greatly affect the quality of the output transformer. We need
to choose the right core size and thickness, matching the number of windings
and wire thickness to obtain the right output power capacity. The low frequency
response especially, will rely on this. If the core size is too small, the
output will be distorted and bass will be thin, worse if it is working at rated
power.
Forever increase of core size will not solve everything as
well. Having a larger core also signifies that the wires need to be lengthened as
the winding diameter increases with the size. Wire length increase will raise
the wire resistance and therefore creates additional voltage drop, meaning more
wire loss. It will also increase the windings surface area and therefore
increase the winding capacitance, causing poor high frequency response.
To counter wire resistance effect, wire gauge needs to be increased.
Increasing the size caused the space available to be lesser for windings and therefore
the core size need to be increased to fit in enough windings to get the right
ratio. This process goes on.
Other than the above, the kinds of magnet wires being used
will also affect the output transformer design. If one just look at the core
gauges for design, they the results will be very disappointing as one fails to
consider the insulation of the magnet wires (enamel) that will cause difference
frequency response and impedance change. Different thickness of the insulation/enamel
will generate different winding thickness and therefore varies the coupling
between the winding, for good or for bad, and that, needs to be taken into
consideration too.
What about the insulator layers in between the windings (if
one does use it of course!). To us, it is very important for the safety + reliability
perspectives and we manage to make it as transparent as possible. Not having
insulator layers will greatly reduce the lifespan of the transformer if not
done correctly. As previously blogged, different insulation materials have
different electrical characteristics. That will directly affect the frequency
response and coupling between layers. Too thin, is not good, too thick, is not
good as well. Better insulation material increases the magnetic coupling and
therefore increased the high frequency response, but that’s not always desirable
to get balanced frequency response.
Then, here comes the impregnation medium, varnish, lacquer,
wax, compound mixture and etc. All these will again change the already so
complex equation.
You want more? Okay, let’s assume materials are all settled.
Now we need to focus on the primary inductance. Output transformer low
frequency response depends on having adequate primary inductance. Higher
inductance equals to higher number of turns. Higher of turns also equals to
lower high frequency response, and vice versa. So, we need to go back to the
drawing board to ensure we have flat low to high frequency response. Panting
now?
We’re not done yet! Let’s get back to the core material.
Some will always like to have amorphous, nickel or cobalt core as their output
transformer core material. Well, all of them should be better as they’ve better
permeability ratio, which gives higher efficiency and better high frequency
sensitivity. Well, there is no perfect material in this world. Higher
permeability comes with lower flux density compared to Z11 iron core, almost
30% or less. So to get the same output power, one needs more core material to
gain enough low frequency response. There goes again to the wire size and
surface problem. So, sometimes we’d prefer to gain the advantage of special
core materials although still having the Z11 core by special winding techniques
and experience. This is much easier to be achieved.
Of course, we’re not against such materials but we’d like to
alert the users that all these considerations need to be taken before you order
your output transformer. For such core materials, be prepared to pay more for
the increase in size and sometimes due to law of physics, you got to lose some
to gain some others. There is no free lunch! Don’t be fooled if someone says
you can have it all, you can’t!
So, balancing all the aspects is crucial to achieve
excellent, not just good transformer design. Also, part of it will require
trial and error to get to a certain understanding and mastering of output
transformer design. Everyone can wind a transformer, but to wind a good transformer,
not everyone can manage it.
J&K Audio Design
21/12/2013
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