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As the question is asking, why is a push-pull converter named push-pull?

Also, according to Wikipedia: "The operation of the circuit means that both transistors are actually pushing, and the pulling is done by a low pass filter in general, and by a center tap of the transformer in the converter application. But because the transistors push in an alternating fashion, the device is called a push-pull converter." Anyone has any idea what does "pulling is done by a loss pass filter in general, and by a center tap of the transformer in the converter application" mean?

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    \$\begingroup\$ You have two questions, one in the topic about a circuit topology name, and a different one about interpreting Wikipedia text. Wikipedia is a free encyclopedia which everyone can edit. It does not mean you can always have content that is correct, understandable, or even making sense. It's not a genric electronics book, or power electronics book. \$\endgroup\$
    – Justme
    Mar 1, 2023 at 8:58
  • \$\begingroup\$ Yea sorry, that was suppose to be a side question. I had a brain fart and forgot to mention the main focus of this post. \$\endgroup\$
    – xrosaber
    Mar 1, 2023 at 11:18

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Form the same wikipedia article

The term push–pull is sometimes used to generally refer to any converter with bidirectional excitation of the transformer.

We need to let the word 'generally' be as general as we like.

To pick apart your specific query -

The operation of the circuit means that both transistors are actually pushing, and the pulling is done by a low pass filter in general, and by a center tap of the transformer in the converter application. But because the transistors push in an alternating fashion, the device is called a push-pull converter.

I would think of both transistors of a centre-tapped transformer converter pulling, with the centre-tap being the thing they're pulling against. But that's just me.

The reference to a low pass filter is a mystery to me. That is the only reference to a low pass filter in the article. A low pass filter is not needed to make a push-pull transformer converter work. I think an editor must have been over zealous with cut'n'pasting something not understood. I might just go in there and remove it myself. (done, let's see if anybody objects).

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  • \$\begingroup\$ I mean, it will have a low-pass characteristic, because a transformer is ultimately a low-pass network (set by inter-winding C and leakage, and their interaction with surrounding components), but it's so random to mention that here. Just one of those random jank Wikipedia edits, agreed. \$\endgroup\$ Mar 1, 2023 at 11:17
  • \$\begingroup\$ "I would think of both transistors of a centre-tapped transformer converter pulling, with the centre-tap being the thing they're pulling against. But that's just me." I also had a similar interpretation. What confused me as well was the reference to a loss pass filter. \$\endgroup\$
    – xrosaber
    Mar 1, 2023 at 11:25
  • \$\begingroup\$ Oh also, I think they mean in reference to an RF circuit, where any general phase-shift network (such as a 0/180° hybrid) can be used to marry two amplifiers out-of-phase, with equivalent function and benefits to a transformer at LF. They might not be low-pass though; they could be all-pass or etc. too. Low-pass is merely preferred because of attenuating harmonics above the intended carrier frequency. \$\endgroup\$ Mar 1, 2023 at 11:40
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Perhaps it's more understandable in historical context.

The term dates back the better part of a century, when only "N-type" (if you will) devices existed -- vacuum tubes. Since complementary devices didn't exist, there was no complementary totem-pole configuration that could be most symmetrically called "push-pull"; rather, this term was used for the center-tapped transformer coupled version. The term instead contrasts with the "single ended" configuration, which used a single winding on a transformer.

Note that a transformer was (almost*) always required because vacuum tubes have such high output impedances (~kΩ), compared to typical loads (speakers 4-16Ω, radio transmitters 50/75/300Ω, audio line / telephone 600Ω, etc.).

*There were some dabblings with OTL (output transformer-less) configurations, from time to time, but nothing really caught on, at least that's my understanding. Not having complementary devices, a totem-pole circuit always had to make some kind of compromise, as far as how to get enough gain to drive the high-side device (which is a cathode follower) and how to balance current flow (and distortion) in the low-side (which is common-cathode). The most prominent was probably by Philips: see The SPP Amplifier | audioXpress. As you can see, it's mainly made possible by the unusual extremely high voice-coil impedance.

Also it probably goes back even further as a mechanical term, but I'm afraid I don't know nearly enough mechanical engineering history to explore that. Offhand, for example: Push-Pull Linkage - Design and Technology Online The most direct analogy would be two tension links (ropes/belts/etc.) on a bell crank, which both strictly pull, but one can be said to push by the linkage.

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