Hello Manfred,
Tuesday, May 16, 2017
What a totally awesome reply Manfred. I have read it several times and
believe I fully understand the how and why of it all, so that equals a
great explanation! I have ordered a couple of cores and 3 bobbins, so
if disaster strikes I can try again. Could a similar arrangement be
used for the output transformers of the amps themselves, which run
warm, if not hot? I don't think I linked directly to the amp
schematic, so here it is below. Quite how long it took you to do all
the below i don't know, but obviously not a five minute job....
Sincerest thanks for not only telling me how to further improve it,
but also why and sharing your thinking as you approached the task!
http://www.w1vd.com/137-500-KWTX.html
and for completeness, this is the LPF I run after either a single amp,
or the combiner
http://www.w1vd.com/LPF.html
Again, wonderful stuff, very helpful indeed!
> Chris,
> at that low frequency, indeed you can make the individual amps for 100
> ohm and put them in parallel, or for 25 ohm and put them in series.
> There should be no phasing problems, at such a low frequency. But you
> have to make sure that both amplifiers have extremely similar responses,
> in terms of gain curve mainly.
> The advantage of using a combiner is that it isolates one amp from the
> other, so the system is highly tolerant to differences between the
> individual amps, down to the point where one amp module can completely
> fail, and the other will continue limping along. Without a combiner, the
> failure of one module would probably cause the demise of the other too.
> I have been giving some thought to your transformer. First, I suggest
> NOT buying that giant toroid. Bigger isn't always better. A huge core
> has a lot of ferrite, that causes losses! That huge toroid has an
> enormous space for winding, which you would never take advantage of. A
> smaller core in a better design can provide far better performance. So,
> let's start optimizing this design:
> The first step is turning it into an autotransformer. The schematic you
> linked shows a conventional transformer, with separate primary and
> secondary, and both of them having one end grounded. That's very
> inefficient! There is no reason at all to use separate primary and
> secondary windings, if anyway they will be connected together! So, for a
> start I would change this into an autotransformer: 7 turns total, with a
> tap at 5 turns - or multiples of that, depending on the core used. This
> eliminates 36% of the wire, and strongly improves coupling, without any
> ill effect at all. Also transformer action is now required for only 571W
> instead of the full 2000W, which allows using a far smaller core, and
> far less total copper.
> In that autotransformer, the current coming from your amplifier enters
> the transformer through the tap, and splits in two parts: 5/7ths of the
> current flows "up" through the 2 turns and into the load, while 2/7ths
> flows "down" through the 5 turns and returns via ground. The voltage
> applied to the 5 turns induces 2/5ths of that voltage in the 2 turns. So
> the output voltage is 7/5ths of the input voltage, while the output
> current is 5/7ths of the input current, the transformer works at only
> 2/7ths of the total power (571W), and at the output you still get the
> full 2000W.
> There is an old adage: Engineering is a combination of material and
> brains. The more you use of one, the less you need of the other.
> Okay. Now lets try to come up with a good transformer for those 571W. I
> will write here as I attempt to design it, so you can learn how to do it.
> A good core shape is an RM or a pot core. They have bobbins (easy to
> wind), round center legs (even more easy to wind), a much shorter path
> length than a toroid, and they are available in suitable materials. The
> catch is that they aren't very large. So, let's take the largest RM core
> offered by FairRite, and see very dumbly how it works out.
> This core is available both in the 95 and the 98 materials. They are
> quite similar, but I like 95 better because of its flatter loss versus
> temperature curve. So, the chosen core would be the 6295420121.
> First let's find out how many turns we need. 2000W on 50 ohm is 316V.
> This core has a cross sectional area of 1.95cm². Its volume is 14.36cm³.
> How much power can we make it dissipate? That's a decision one has to
> take. I would say, 2W is fine for continuous use, some more is
> acceptable for intermittent use. So, at 2W to be on the safe side,
> 139mW/cm³ loss is acceptable. Looking at the material loss chart given
> by Fair-Rite, an acceptable maximum flux density at 136kHz seems to be
> 0.11T.
> Now we can use equation 4 on my page
> http://ludens.cl/Electron/Magnet.html
> to calculate the required number of turns:
> 316V /4.44 / .000195m² / 136000Hz / 0.11T = 24.4 turns
> That's the minimal requirement. Since we need multiples of 7 turns,
> let's use 28. So the recipe is 28 turns total, with a tap at 20 turns.
> We don't need to make the 28 turns of the same wire, since the top 8
> turns carry more than twice as much current as the lower 20 turns. To
> evenly distribute losses, it's better to distribute copper cross-section
> according to actual current flow.
> Also it's hard or impossible to wind very thick, stiff wire on such a
> bobbin, and on top of that thick wire suffers badly from skin effect. It
> follows that you should wind this transformer with several strands of
> thinner wire. That invites using a single size of wire, but using more
> strands for the 8 turn winding than for the 20 turns.
> For best coupling it would also be optimal to interleave primary and
> secondary layers. In this case you could first wind a layer with 10
> turns, then one layer with 8 turns, then a third layer with 10 turns,
> and interconnect the three layers properly so that the 8 turn layer ends
> up at one end of the other 20 turns. This scheme is still reasonably
> easy to do, but doesn't allow us to use optimal copper cross sections
> for each winding... Anyway, let's try a modification of it:
> The winding space on the bobbin is roughly 18*8mm. We can start from
> enamelled wire of roughly 0.7mm diameter (AWG #22), and wind 10 turns
> with two strands side-by-side. That's a total width of 14mm, which
> should fit in the 18mm bobbin space despite some slight kinks and
> imprecisions. Try to keep the winding centered, leaving some empty space
> at each side, because this reduces the risk of flashover between layers.
> Then wind two or three layers of Mylar or Kapton tape, cut just a tad
> wider than the bobbin, so that it seals well against the bobbin sides.
> Then comes the 8 turn winding. For simplicity let's use the same wire,
> but 4 strands instead of 2. Wind 4 turns, with the 4 strands nicely
> side-by-side. That will use up most of the bobbin width. Then wind one
> or two layers of Mylar or Kapton tape, threading the four wires through
> it, and then wind the other 4 turns. So this is a double-layer winding,
> with both ends coming out of the bobbin on the same side.
> Now wind another two or three turns of Kapton or Mylar tape, and then
> wind the topping layer of 10 turns of 2 strands of wire. Finish with
> another few layers of tape.
> The whole thing should be only around 5mm tall, fitting comfortably in
> that bobbin.
> Now the windings have to be interconnected. A lot of wires will be
> sticking out of the bobbin... The two 10-turn windings have one end on
> each side of the bobbin, while the 8-turn winding has both ends on the
> same side. First thing is to take one end of one 10-turn winding, and
> the opposite end of the other 10 turn winding, and join them. This can
> be done on top of the Mylar tape, at a place of the bobbin that will end
> up in one of the core's openings. Make the connection nice and short.
> Then the still free end of a 10-turn winding on the same side of the
> bobbin where both 8-turn ends come out, has to be joined to the CORRECT
> end of the 8 turn winding. The correct one is the ENDING, not the
> BEGINNING, assuming that you wound everything in the same direction. So,
> join that, fit the core (you can tape it together for now, later glue or
> clamp it), and the transformer is ready for testing.
> I would expect this to work pretty well, although it's not entirely
> optimized. We could have used more strands of a thinner wire, and
> interleave 3 primary with 2 secondary layers, for example. Anyway it's
> MUCH better than winding separate primary and secondary on a stack of
> large toroids, let alone a giant and non-optimally shaped toroid!
> Manfred
--
Best regards,
Chris mailto:chris@chriswilson.tv
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