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Author Topic: 4:1 Baluns  (Read 9300 times)
G3TXQ
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Posts: 1516




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« Reply #15 on: June 02, 2010, 03:40:22 AM »

Tom,

I think you are missing my argument, or I am not getting it across very well  Embarrassed

I am saying the same thing as you about the flux in the individual chokes of a 4:1 Current Balun. I already did the experiment you propose and got EXACTLY the results expected for the different load conditions. I posted a link to those results in my original posting; here it is again:
http://www.karinya.net/g3txq/temp/4%20to%201%20current%20balun%205.png

I am trying to make two points - obviously not very clearly:

1) The CM voltages across the two chokes in a 4:1 Current Balun must equal the full balun input voltage **whatever the load configuration**. Unlike the case with a 1:1 current balun, there is NO load configuration which can produce zero flux in both cores of a 4:1 current balun.

2) A 4:1 Voltage Balun does not **primarily** operate as a flux-coupled transformer. Therefore comments such as:

Quote
The ferrite core of a transformer balun (the Ruthroff "voltage balun") sees all of the transmitted power, so it is easily overheated and saturated by high power.

Quote
the cores of voltage baluns must have relatively low loss, and they need fairly high permeability to support the flux needed to carry the power.

are misleading.

73,
Steve G3TXQ
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W8JI
Member

Posts: 9296


WWW

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« Reply #16 on: June 02, 2010, 07:43:41 AM »

Tom,

I think you are missing my argument, or I am not getting it across very well  Embarrassed

I am saying the same thing as you about the flux in the individual chokes of a 4:1 Current Balun. I already did the experiment you propose and got EXACTLY the results expected for the different load conditions. I posted a link to those results in my original posting; here it is again:
http://www.karinya.net/g3txq/temp/4%20to%201%20current%20balun%205.png

I am trying to make two points - obviously not very clearly:

1) The CM voltages across the two chokes in a 4:1 Current Balun must equal the full balun input voltage **whatever the load configuration**. Unlike the case with a 1:1 current balun, there is NO load configuration which can produce zero flux in both cores of a 4:1 current balun.

2) A 4:1 Voltage Balun does not **primarily** operate as a flux-coupled transformer. Therefore comments such as:

Quote
The ferrite core of a transformer balun (the Ruthroff "voltage balun") sees all of the transmitted power, so it is easily overheated and saturated by high power.

Quote
the cores of voltage baluns must have relatively low loss, and they need fairly high permeability to support the flux needed to carry the power.

are misleading.

73,
Steve G3TXQ

Where did you see those quotes? I know there are a few odd statements in Baluns and UN-UNs, but where did that come from??

I don't think we can say the core 100% conveys power is ANY transformer with interleaved windings. But where there is core magnetization, we must pay attention to core flux density and core loss tangent.

In other words when we need a core in a system, we have to be careful what we use no matter what the system.

Are those quotes from UnUns and Baluns??

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G3TXQ
Member

Posts: 1516




Ignore
« Reply #17 on: June 02, 2010, 01:04:30 PM »

Where did you see those quotes? I know there are a few odd statements in Baluns and UN-UNs, but where did that come from??

I don't think we can say the core 100% conveys power is ANY transformer with interleaved windings. But where there is core magnetization, we must pay attention to core flux density and core loss tangent.

In other words when we need a core in a system, we have to be careful what we use no matter what the system.

Are those quotes from UnUns and Baluns??
Tom,

Those quotes are not from UnUns and Baluns; they are from a publicly available tutorial on the web. The third quote was contained in a private eMail from the author to me:
Quote
The beauty of impedance transformation by series/parallel wiring of common mode chokes (the Guanella balun) is that they do NOT put differential flux in the core like a voltage balun.

The author appears not to frequent this forum, so I'm trying to be fair and preserve his anonymity; although at least one reader has already worked it out! My purpose in posting originally was not to embarrass the author - rather it was to test my own ideas in a wider forum at a point where he no longer wished to discuss the issue until I had "studied the fundamentals".

Here's my simplistic "take" on that last quote: I wind three identical common mode chokes - same wire, same turns, same cores - and I use two of them in a 4:1 Current Balun and the other one in a 4:1 Voltage Balun. Fig 1 and Fig 2 here:

http://www.karinya.net/g3txq/temp/4%20to%201%20current%20balun%207.jpg

The core flux and losses are related directly to the common-mode voltages Vcm1, Vcm2 & Vcm3. Then, trying different load-balance conditions:

Load floating: Vcm1 = Vcm2 = Vin/2, Vcm3 = Vin
Balanced load (point C ground): Vcm1 = 0, Vcm2 = Vin, Vcm3 = Vin
Unbalanced load (point A ground): Vcm1 = -Vin, Vcm2 = 2.Vin, Vcm3: N/A Balun Input short-circuited
Unbalanced load (point B ground): Vcm1 = Vin, Vcm2 = 0, Vcm3: N/A choke output short-circuited

So, taking as a reference the losses in the voltage balun with a floating load or balanced load, the losses in the current balun might be anywhere from half to 5 times that, depending on the load configuration. I realise that those extreme load configurations are unlikely to be encountered in practice, but I believe they serve to illustrate the principle.

Please don't think I'm advocating the use of a voltage balun over a current balun - I'm not! There are good reasons to choose the 4:1 current balun over the 4:1 voltage balun in many situations, but that quote is not one of them!

73,
Steve G3TXQ
 
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W8JI
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Posts: 9296


WWW

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« Reply #18 on: June 02, 2010, 04:12:29 PM »

I found the article. He is a genuinely nice fellow. I'll e-mail him myself.
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