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I'm reading up on SMPSs and related technology and have come across contradictory information, or so it seems to me.

Various sources on push-pull converters (for example, the English and German Wikipedia pages, but others, too) will tell you that they are good for transforming voltage efficiently when it comes to high powered applications with a couple of kW and more, and are thus used in SMPSs.

However, any overview on SMPSs I found gave me an AC / AC converter after an inverter / chopper, and then a rectifier after that, like in this picture from the English Wikipedia article:

SMPS block diagram

What I don't seem to understand is why convert to AC for the voltage translation, when push-pull converters apparently are well suited for DC-DC conversion (which is what happens once the input has bene rectified)?

Am I missing something here? Is there different cases where you would use one or the other? Is there a technical advantage to doing the voltage conversion in either DC or AC?

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You're not really missing anything.

A switching power supply converts DC to DC by first converting AC to DC, then to AC, then back to DC:

  • The input rectifier and filter converts the low-frequency AC (50-60 Hz) input into DC
  • The chopper converts the DC into high-frequency AC (usually several hundred kHz)
  • The output transformer steps the high-frequency AC up or down and passes it to the secondary
  • The output rectifier and filter converts the high-frequency secondary-side AC into DC

So yes, at it's heart, there is AC transformation going on. The frequency of the AC is made much higher to allow for a much smaller and less lossy transformer to be used in the conversion process.

A push-pull converter, as you've described in your question, is AC to AC at it's heart.

enter image description here

The action of the two switches chops the DC input bus into AC by applying the input to the transformer with alternating polarity. This AC waveform is converted to the secondary through the transformer, then rectified back into DC. Per the article,

The transistors are alternately switched on and off, periodically reversing the current in the transformer.

Periodic reversing of current = alternating current = AC, just to be explicit.

Note that the image shows only part of the power train. Here's a more complete representation of a push-pull converter:

enter image description here

This shows an output LC filter, which is needed to make proper DC on the output side.

Of course, transformers only work with AC. You cannot apply DC to a transformer and expect any output, since transformers only "work" by a change in primary current generating a change in flux which is coupled to the secondary and generates a change in voltage. Change = AC.

When it comes to DC, all you can really do without some intermediary AC stage (be it a transformer, an inductor, or even flying capacitors) is dissipate power to lower the voltage. This is what linear regulation is - dissipative.

Also, you can never increase DC voltage without some form of intermediate AC stage.

Note the word "dissipative" - that's why in the vast majority of cases (when the power levels are not trivial) a switching power supply will be more efficient than a linear solution.

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  • \$\begingroup\$ Thank you for your answer, but what I don't seem to understand is why the voltage conversion happens in AC instead of DC, when that adds an inverter and an additional rectifier stage plus an apparently less efficient voltage converter? I updated my question because it seems I wasn't clear enough on that. \$\endgroup\$ Apr 9 '13 at 1:21
  • \$\begingroup\$ Thank you very much for that update. Took me a while, but now it's much clearer to me. What eluded me (I'm a noob, forgive me) was that transformers always work with AC and that there's effectively a chopper in the push-pull converter. Very helpful! \$\endgroup\$ Apr 9 '13 at 22:14
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The link tells you in the first line (English version) that a push-pull convertor is a DC-to-DC conversion process. The drawing you have added is for an AC-to-DC convertor.

The wiki page also is featuring full H-bridge converters and they have their place definitely at the higher end of power conversion. They are a type of SMPS but they are not used in the majority of SMPSs.

Technically high power systems such as AC power grids and sub-stations use transformers suited for 50/60 Hz and there is no cost-effective technology to rival them because they are AC-AC convertors and with the voltages and currents involved, with the expected reliability they beat semi-conductors.

Semi-conductor converters can produce AC outputs if they use a transformer but they all require a DC input. If you look at your phone charger or PC power supply it converts the AC to a DC then uses a high frequency "chopper" (using the language of the picture you supplied) to feed to a transformer that provides safety isolation to the low voltage DC output.

In fact in your picture above, the input goes through an input rectifier and filter - this converts the power AC to a DC voltage.

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  • \$\begingroup\$ Thank you for your answer, but what I don't seem to understand is why the voltage conversion happens in AC instead of DC, when that adds an inverter and an additional rectifier stage plus an apparently less efficient voltage converter? I updated my question because it seems I wasn't clear enough on that. \$\endgroup\$ Apr 9 '13 at 1:20
  • \$\begingroup\$ @HannoFietz You can't convert DC voltage efficiently - a linear regulator does a great job when the watts are small but it converts a higher voltage to a lower one and all the volts it "drops" are wasted in heat. Conversely, you can't convert a voltage to a higher voltage without some form of switching or AC conversion. \$\endgroup\$
    – Andy aka
    Apr 9 '13 at 6:57
  • \$\begingroup\$ Thank you for the comment. Reading the updates to @Madmanguruman's answer made me realize that I didn't have some basics quite straight, yet. You can probably tell I'm a noob. You guys helped a lot, thanks. \$\endgroup\$ Apr 9 '13 at 22:22
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I'd like to summarize this noob's "a-ha" (or more like "d'oh") moments after reading Madmangurman's and Andy aka's answers.

I failed to understand two rather basic things:

a) Every transformer works in AC, because the alternating current is what induces the magnetic field that is the underlying working principle of the device.

b) A push-pull converter simply is not DC at the voltage conversion stage, rather a chopper-inverter and a rectifier are an integral part of it.

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