Push-Pull Output Transformers - Part III, The Final Countdown:

Discussion in 'Amateur Radio Amplifiers' started by KD2NCU, Sep 28, 2017.

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  1. WA1GFZ

    WA1GFZ Ham Member QRZ Page

    I used the Fair-rite 2643102002 in my drain transformers or the slightly larger 2643101902 (More expensive also) Mouser a good source (Type 43 ferrite)
    The 2661102002 type 61 I used in my combiner output transformer.
    Check out W6PQL site for ideas. I think he used Wurth parts
    Mark, My brain is fixed on a 50 ohm load so didn't study your circuits closely. The problem with AC coupled power on the low Z side of a transformer is the stress on the caps. Looking back at the circuits I see what you mean.
    I sent you some TLT stuff to look at. I wonder if flux walking shows up as low frequency modulation on a carrier? I had the problem like that once when I blew a gate load resistor and the dive was way out of balance. IMD was nasty. I was lucky to catch it before I blew anything up. I think you just create distortion in the output as a way to try to reset the core. Yes saturation would happen in a severe case that could blow parts. I've been there. gfz
  2. KD2NCU

    KD2NCU Ham Member QRZ Page

    Greetings ON4LDY: The information above was not intended to answer your questions. I was just finishing up some analysis that I was discussing with WA1GFZ and others.
    I had sent you an email asking some additional questions. I was waiting to get some clarification about what you wanted to see. I think you have provided that information above now. Did you receive an email from me?
    In any case, I think I know what you are looking for now.
  3. KD2NCU

    KD2NCU Ham Member QRZ Page

    Greetings WA1GFZ: I was looking at your article again tonight, specifically figure 5 whereby you state "Figure 5 — This graph is the LTspice simulated phase currents in a DC shunt feed choke, showing the imbalance between phases that creates the feedback signal." I was perplexed by the bottom portion of this graph until I reversed the polarity (flipped the graph vertically) and it started to make perfect sense to me. Here's what it would look like if you had reversed the polarity of your measurement in the simulation for the bottom graph.
    Now when you flip the polarity of the bottom waveform, both waveforms look very much like a full wave rectified sinewave of current pulses that I predicted we'd see in the wires of the bifilar coil and coming into the center tap of the bifilar coil. And viewed this way, the peaks of both waveforms look pretty balanced to me. I can't explain why one cycle doesn't return all the way to zero at this point but the peaks definitely look reasonably even. Is this a surprise to you at all? Do you still consider these unbalanced? I'm surprised they are as balanced as they appear.
    This is so interesting I decided to post it on the thread rather than ask you via email so that others could see this as well.
    Could you explain a little bit about your simulation and how you got it to give you the graph in figure 5? Is figure 5 a direct graph of the currents in the bifilar coil provided by the simulation?
    This interesting in that it appears to be showing that the currents in the bifilar coil are definitely full wave rectified sine pulses as predicted by circuit analysis.
    But given that this is a simulation, do you have any idea why it's not perfectly balanced? Do you remember a few days ago discussing the magnetizing current in a transformer primary? IE: even in a one to one transformer, the primary current will be slightly larger because of the magnetizing current that exists only in the primary winding. In the case of the Bifilar Coil, the two windings alternate as primary. So in reality, one coil or the other has the magnetizing current and the other one doesn't. The two windings of the bifilar coil alternate or take turns as the primary. Is this what we are seeing in the simulation? Notice that they take turns having a slightly higher peak than the other guy.
    Very interesting! Looking forward to your response.
    Last edited: Oct 25, 2017
  4. KD2NCU

    KD2NCU Ham Member QRZ Page

    If we are seeing the magnetizing current, then this would say the magnetizing current is about 200 mA or about 3% of the signal current. This seems very reasonable as a ballpark number for the magnetizing current. Is LTSpice that smart to take into account the magnetizing current in a transformer or coupled coils?
  5. WA1GFZ

    WA1GFZ Ham Member QRZ Page

    That magnetizing current sure makes sense but I did not input any core information into LT spice because I don't know how. I only adjusted the coupling factor between the two coils and played with the inductance value. The inductance value has a big effect on broadband performance. I know the waveform really starts to degrade when the coupling goes below about 96%. I'm going to need to think about this because I can't think of a reason why everything isn't balanced. I even measured the signal when a third winding for feedback is added and see a good signal so I know it is real. Also look down near zero at the imbalance. I wonder if it has something to do with the bias source impedance. Simulation used the same part number FET with the same source resistance value. I think drive is balanced but will check carefully. Maybe I'll put a balun on the input. I'll have to reverse the output leads to see if it changes.
    Interesting question. gfz
  6. WA1GFZ

    WA1GFZ Ham Member QRZ Page

    Here is more from the Pitzalis dead sea scrolls. I think you will find interesting use of the 180 degree hybrid with the center tap loaded. I have seen this in Harris amplifiers where the DC Bus had an AC load. They used series beads on th eDC bus with a AC coupled load to ground. This one reads a bit easier. I found some simulations last night so will look closer later gfz

    Attached Files:

  7. WA1GFZ

    WA1GFZ Ham Member QRZ Page

  8. ON4LDY

    ON4LDY QRZ Member

    hello KD2NCU,

    Still no hint how you could tell me what will be your choice the next problem ?
    I think that after the theory, we need you to show us a pratical case.


    knwoing the frequency, the ouput power, the transistor,

    a) what core should be use for the bifilar trnasformer
    b) how to calculate the losses in the core if there is nearly no B field in it
    c) for what leakage inductance are we looking for
    d) how to calculate the number of turn to be winded
    c) ....

    answering all this questions with the corresponding physical expalnation is the key: go from the theory to the real practice.

    I propose this example to start :

    frequency : 40 MHz
    output power : 1.25 kW
    transistor : nxp 1k25
  9. KD2NCU

    KD2NCU Ham Member QRZ Page

    Hello ON4LDY
    I'm ready to work on this now.
    1. What is the publication you were referring to above?
    2. What DC voltage will you be connecting to the bifilar coil?
    3. Have you already decided on a final transformer? The bifilar coil provides the voltage and current to the primary winding of the final transformer and the materials and design of the bifilar coil depend on the impedance the bifilar coil will be looking into when connected to the primary winding of the final transformer. So if you tell me turns ratio of your final transformer and the impedance that you are connecting to the secondary of the final transformer then I will know what to work with.
    As an example: Suppose you are asking for 1.5 KW connected to a 50 ohm load. The voltage across the 50 ohm load is then 274 volts RMS.
    If your final transformer has an impedance ratio of 1:4, (turns ratio = 1:2) then the voltage on the primary will be 137 volts RMS or 194 volts peak.
    The bifilar coil has a 1:2 turns ratio so it doubles the DC supply voltage so in this case the DC supply voltage would need to be 97 volts DC in order to get 194 volts peak on the primary of the final transformer.

    The current of the 50 ohm load would be 274/50 = 5.5 amps RMS. The current in the primary winding of the final transformer would then be 11 amps RMS. Each coil of the bifilar coil will see 11 amps RMS or 15.6 amps peak. Each transistor will see twice this current during its conduction cycle or 31.1 amps peak for 1/2 cycle. In this case then, the transistor will be handling 31.1 amps peak and will see 97 volts peak when it starts to conduct and will see 194 volts peak during its off cycle. Is the transistor rated for this?

    In any case, I'll need to know the final load impedance, 50 ohms or whatever, and the turns ratio of the final transformer in order to design the bifilar coil. Or, I can help you determine what the final transformer needs to be as well if you need help with that. Coaxial cable transmission line transformers often have significantly lower losses than conventional wound transformers. They typically have turns ratios of 1:2, 1:3, 1:4 etc.
    Last edited: Nov 18, 2017
  10. ON4LDY

    ON4LDY QRZ Member

    My application is a little special and is not realy Radio ham related:

    transistor : 1k5 from NXP -> DC voltage = 50Vdc
    output impedance : 5 ohms (yes it is not 50 ohms...)
    final transformer turn ratio : made with two separate ferrite core and turn ratio 1:1 in order to have the correct impedance as specified by the transistor data sheet

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