I am concerned about the output impedance of a transformer with a center tapped primary that will be driven in a class A push pull topology.
Will the part of the secondary that couples to the half of the primary that is not being driven, and is open circuit, appear in the output impedance as leakage inductance? If this does happen are there ways to design the transformer to prevent this?
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No. If half of the winding is not being driven part of the time it is not class A. At best it is class AB1 or AB2 or class B. Class A drives all of the driven windings all of the time.
Leakage inductance is the inductance that does NOT link the primary to the secondary. If a winding is not being driven or loaded, it's linkage to other windings is immaterial. However, winding capacitance may alter this picture because its reactance may present a significant load at higher frequencies.
If this does happen are there ways to
Audio transformers should be wound to limit leakage inductance and winding capacitance.Multi-filar windings, interlaced windings and other techniques are used.
Class A would always drive a DC component. If this is for your piezo pulser you don't really need class A in a push-pull topology. AB2 would be good enough, as much quiescent current as needed for the desired linearity but not more.
Leakage inductance is the part of the inductance that is "in series" with a winding and doesn't couple to any of the others. In a tightly wound ferrite core transformer at a few hundred kHz it won't be much.
You can get good pointers on how to build this stuff from ham radio literature. Look for articles that describe the construction of a transistorized shortwave power amplifier. The main difference in your case would be that yours would operate a good order of magnitude lower in frequency so the transformer must be changed accordingly. Also, since AFAIR you only need to provide a single cycle once in a while you might be able to provide a fast control for the quiescent current so you can turn it off during most of the time. This will prevent a lot (a whole lot) of power dissipation. But make it slow enough so the transducer doesn't "see" it and maybe highpass-filter it away.
Last but not least mind core saturation. At the power levels you described in your other post you'll need a pretty fat core. A pot core would probably easiest because you can get nice matching bobbin carriers for those. And don't send all that energy of one tiny capacitor anywhere, it might explode. Seriously, I had one turn its white ceramic into greenish glass with a loud bang.
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The pulse is 1000 Vp @.5 Ap into a mostly capacitive load. This is 500W of peak power. The output has to be a full cycle with both polarities. A class A amplifer would dissipate too much quiescent power. Class AB outside of push pull would by definition not fully amplify one of the sinewave polarities. It would be clipped.
Eliminating the bias current between pulses would cause a flux change in the transformer just like the intended signal would, causing an undesired output from the transformer.
Ham radio circuits is what I have looked at before deciding on Class B Push Pull. Their 600 meter circuits are close to what I need to do. At this wavelength they use class A using a tetrode or class B push pull with transistors or pentodes. The only difference between my class B push pull and theirs is they have a tank circuit to filter out the higher harmonics and I would use a low pass filter.
I have also been looking at avoiding the need for a low pass filter by buffering the output of a high voltage power amplifier such as one of these:
After Phil Allison's colorful reply I did realize that to the first approximation all the flux from any part of any primary would pass through all the windings of any secondary. I also realized it would pass through the windings of the open circuited other half of the center tapped primary and so this doubles the amount of voltage the pass transistors must withstand.
I have designed transformers for flyback power supplies. But I have never had to design a transformer for this much power and voltage. So I will leave the design of this one to Butler Winding:
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** A correctly operating *push-pull* circuit has no nett DC component in the primary.
Only single ended stages have that issue.
..... Phil
" Bullshit Artist"
** It is never "open circuited" in a Class A push- pull stage - you bullshit artist.YOU posted this: " I am concerned about the output impedance of a transformer with a center tapped primary that will be driven in a class A push pull topology. '
** You still have completely wrong ideas about how transformers work.Even a single ended class A stage applies twice the DC supply voltage to the driver device.
Piss off TROLL.
.... Phil
From a magnetics point of view they cancel but from a thermal point of view it all adds up (winding resistance). Gets warm.
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Yes, you either need push-pull or, with a single-ended stage, a resonant circuit. Push-pull is easier these days where we can buy cheap FETs, this was different in the tube days.
Not really. Jot down the current orientations in the primaries on sketch paper. Ideally they cancel out. But you'd have to remove and re-apply the bias gently in order not to cause a spurious pulse in the transducer because this canceling ain't 100% perfect, no transformer is ideal.
This is an age-old trick in ham radio, for CW (morse code) transmission. During long pauses and receive phases we'd remove the bias and that saved a whole lot of electricity. Plus it stretched the lifetime of the rather expensive tubes a bit.
If you'd scoot it a tad towards AB you'll see less harmonics.
To be honest I've never considered the Apex amps. Of course the main reason was price but also that all that nice super-power vanishes at a few hundred kHz when real-life conditions hit.
The FETs are going to be the least of your problems. That's why I always say the good old days are right now. You can buy nice 600-800V FETs that can stomach several amps for less than a buck. When I started out this was unthinkable.
Good choice. Companies like this should know their magnetics rather well, when the ferrite saturates, how hot it's allowed to get and so on. Plus they've got all the materials right there.
However, tell them to oversize the core by 50% or more. RF-transformers that are taylored to a glove-fit will get warm and in a physics or chemistry experiment setup like yours that's not desired because it (very slightly) changes the pulse. The extra cost should be miniscule in the grand scheme of things.
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Class A was a typo. It should have been class B there. I intended to drive the center tapped primary with class B not class A.
A push pull topology is sometimes driven with class AB amplifiers with a little bit of bias to reduce or eliminate cross over distortion.
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When I wrote about the fluxes I was describing why I realized I was wrong about the leakage inductance.
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Yes, but as I said earlier I don't think you'd see a difference between sinusoidal drive and full square pulsing. When you are done with this you could try it out by driving the thing hard from a pulse generator. But before you do maybe you could seal a bet with your scientist. Who knows, you could get some free beers out of it :-)
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" Bullshit Artist"
** When last I went shopping, I forgot my list.Then I found out the price of fish.
But no kettle could be found.
Capice ??
...... Phil
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