I have seen in most of the block diagrams of SMPS of isolated topologies, that the input AC voltage is first rectified and filtered to DC voltage. Then, it is passed through a high frequency switch to form a square pulse, which is then stepped up/down using a transformer, after which, it is again rectified and filtered, before giving it to the output. Also, a feedback is taken from the output, which is given to the controller, which gives the PWM to the high frequency switch.

My question is that, why are we rectifying the AC input in the beginning and then again inverting it to form a square pulse, before it is passed onto the transformer. Can we directly give the input from the AC mains to the transformer, and then rectify it.

  • \$\begingroup\$ The low mains frequency requires large, costly transformers. The main benefit of SMPS is that their high switching frequency allows for a very small transformer. \$\endgroup\$ – Unimportant Jan 31 '19 at 5:28
  • \$\begingroup\$ Actually in the name ac-dc, the real ac-to-dc conversion is made by the diode bridge and the filtering capacitor called the bulk capacitor. The downstream converter operating in a switching manner is a high-voltage dc-dc. You need to rectify the mains because its polarity changes along the 20-ms or 16.6-ms period. The high dc voltage then feeds the converter which requires a galvanic isolation before supplying the load, a PC for instance. This is the role of the transformer which does require high-frequency operation if you want a small size. \$\endgroup\$ – Verbal Kint Jan 31 '19 at 6:36

You got part of the answer in the comments: because mains frequency is lower than the operating frequency of the power supply. Up to a point, the higher the frequency, the smaller the magnetics can be. Typical mains frequencies are well below that point.

The other part of the answer is that the square pulse that is generated has a controlled duty cycle, so that the output voltage of the supply can be regulated.

It is possible to use a transformer at the mains frequency and rectify the output -- this is how the job was done just about everywhere until the 1970's, still in a lot of places until the end of the 20th century, and is still found in a few places now (ca 2019).


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