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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.
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.
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.
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.
