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I have started learning about DC-DC converters and switch mode power supplies (SMPS.) I know that the standard SMPS (attached fig. below) has an input rectifier, DC link capacitor, inverter, output transformer, output rectifier, output filter and load.

SMPS

Is it possible to replace the second half of the circuit with just a buck converter (or any other type of DC-DC converter) after the DC-link capacitor as shown below instead of going through an inverter, transformer and another rectifier? A buck converter is also a switched converter and would reduce the size of the filter elements because of the high frequency switching.

Buck Converter

Is there a particular reason why one is preferred over the other?

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  • \$\begingroup\$ Except that all SMPS don't need the input rectifier. That is only needed if the input voltage is AC, and is not needed when the input is DC, such from a battery. \$\endgroup\$
    – SteveSh
    Mar 5 at 17:41

3 Answers 3

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Isolation.

Your proposed circuit has the output directly connected to the mains with nothing in between.

Between the output terminals of your circuit, you will have the desired low voltage DC.

The output will be at mains voltage above the earth ground of your mains supply. Touching the (supposedly safe) low voltage output of your proposed converter would supply the full mains voltage to your body - that could kill you.

The normal circuit you showed has the transformer there to break the path between mains and the output. Touching the output will not allow current to flow through your body and back to the Earth ground of the mains - it is perfectly safe to touch the output of that circuit.

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Let step back to the definition. Basically, SMPS and DC converter are using same principle of PWM and filter to reduce DC voltage but SMPS use AC main power inlet.

The element you want to replace is switching transformer. it provide outout isolation from main voltage which practically safer when dealing with main voltage.

Second, transformer can reduce current ripple and transfer higher power. When you convert fromvery high to very low voltage without using switching transformer, the PWM duty cycle is very low but in draw very high current from DC input. Also, low duty cycle will cause high current ripple and required higher current rate inductor.

In other hand, if we using switching transformer, it will convert high-voltage low-current input to low-voltage high-current using winding, so it will output more power without drawing high current pulse. That why we prefer that transformer.

For the switching frequency, I think it not limited by tranformer but the skin effect which reduct current rate of transformer.

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When using a 3 phase supply, the power is > 100W and I believe is now required to be active PFC on the front end and not just a 6 diode bridge. This draws in phase sine current so the PF=1.0 and produces a high voltage DC on the Link-Cap.

You can then use that to convert to isolated AC or isolated DC. A multi-phase converter produces less ripple and EMI which for high powers is what is superior to your single phase Buck converter.

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