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I have planned to design a SMPS like the one in PC.In most smps,AC is rectified using bridge rectifier adn converted to high frequency AC and step down using high frequency transformer and bucked/boosted based on application.

Instead of going through these many process.why cant use a phase controlled rectifier and have required DC voltage.

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  • \$\begingroup\$ I think because you get a very low power factor and you need a large smoothing capacitor if you go from 220 V to 12 V with just rectification at 300 W+ power levels. But I'm not a power specialist. \$\endgroup\$ – Arsenal Dec 11 '17 at 15:07
  • \$\begingroup\$ What advantage do you think your approach would have? You add complexity to an already complex system. \$\endgroup\$ – PlasmaHH Dec 11 '17 at 15:14
  • \$\begingroup\$ Instead of going through these many process.why cant use a phase controlled rectifier and have required DC voltage. 1) there would be no mains isolation. 2) The power factor is very likely terrible. 3) At 300 W it might be a huge challenge to do the conversion to 12 V efficiently. 4) Transformers can have multiple winding/taps for 12 V, 5 V, 3.3 V etc. Like PlasmaHH I do not see the advantage, if it was a better solution for sure it would be used. \$\endgroup\$ – Bimpelrekkie Dec 11 '17 at 15:14
  • \$\begingroup\$ Clear XY problem. What do you plan on gaining by going from diode to SCR? \$\endgroup\$ – winny Dec 11 '17 at 21:12
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Among other things, switching noise in a standard SMPS is somewhat isolated from AC by the rectifier. If you use SCRs for voltage control, it's much harder to keep switching transients from propagating back out into the AC line and causing unacceptable RFI.

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That's how it was done for years, BEFORE the advent of smaller-cheaper-faster MOSFETs to do it with PWM. An SCR front-end doing voltage control results in a much larger power supply PU of output, plus more heat rejection that must be dealt with. Yes there are more parts and processes involved, but they are significantly smaller and more efficient.

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I have an ancient Tek analog storage oscilloscope that uses a power supply like this. It's not very common in the current century because it requires a lot of iron in the transformer.

Modern switching power supplies operate at high frequency so the circuit (particularly the transformer) can be made much lighter and cheaper.

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